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Which of the Following is Not True of Saliva

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Which of the Following is Not True of Saliva

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Open Access

Peer-reviewed

Research Article

  • Published: May v, 2021
  • https://doi.org/10.1371/periodical.pone.0250202

Abstract

Diagnosis of whatever infectious disease is vital for opportune treatment and to prevent broadcasting. RT-qPCR tests for detection of SARS-CoV-2, the causative agent for COVID-nineteen, are ideal in a hospital environment. However, mass testing requires cheaper and simpler tests, particularly in settings that lack sophisticated mechanism. The near common current diagnostic method is based on nasopharyngeal sample collection, RNA extraction, and RT-qPCR for amplification and detection of viral nucleic acids. Here, we testify that samples obtained from nasopharyngeal swabs in VTM and in saliva can be used with or without RNA purification in an isothermal loop-mediated amplification (LAMP)-based assay, with lx–93% sensitivity for SARS-CoV-2 detection as compared to standard RT-qPCR tests. A series of simple modifications to standard RT-LAMP published methods to stabilize pH fluctuations due to salivary acerbity resulted in a significant comeback in reliability, opening new avenues for efficient, depression-price testing of COVID-19 infection.

Citation:
Uribe-Alvarez C, Lam Q, Baldwin DA, Chernoff J (2021) Low saliva pH can yield false positives results in unproblematic RT-LAMP-based SARS-CoV-2 diagnostic tests. PLoS ONE 16(five): e0250202. https://doi.org/10.1371/journal.pone.0250202

Editor:
Ruslan Kalendar, Academy of Helsinki, Finland

Received:
December 15, 2020;
Accepted:
Apr 2, 2021;
Published:

May 5, 2021

Copyright:

© 2021 Uribe-Alvarez et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in whatever medium, provided the original author and source are credited.

Data Availability:
All relevant data are within the paper and its Supporting data files.

Funding:
This work was supported by Cadre Grant P30 CA006927 and an appropriation from the country of Pennsylvania to the Fox Chase Cancer Centre.

Competing interests:

The authors have declared that no competing interests be.

Introduction

The years 2019 to 2021 will be remembered for the coronavirus-affliction 2019 (COVID-19) pandemic. The disease toll in the earth has surpassed 142.5 million cases and more than 3 one thousand thousand deaths and does non seem to take decreased its rate of contagion in the past months [1]. In the US alone more than 31.7 million cases and over five hundred and lx eighty thou deaths take been reported [1]. Example fatality ratio in the US is around ane.8% (number of deaths/number of confirmed cases), in developing countries such as Brazil, the fatality ratio is around 2.7%, while in Mexico information technology goes as loftier as 9.2%. Elementary, inexpensive, and authentic diagnostic techniques are of utmost importance to isolate infected individuals and irksome down the transmission of the affliction, prevent oversaturation of health care facilities and benumb morbidity and mortality. Accordingly, the scientific community has made remarkably quick strides to develop diagnostic tools, either for use in specialized wellness centers or for point-of-care community outposts [2].

Testing for SARS-CoV-2, the causal agent of COVID-nineteen, is usually based on detecting proteins (viral antigens or host antibodies) or viral nucleic acids. Antibody detection tests betoken if the person has been infected by SARS-CoV-2 and has generated IgG and/or IgM antibodies. These tests are performed in claret serum or plasma and, while cheap and easy to administer, practice non indicate if the infection is active [3, four], as it tin can take from i to three weeks afterward exposure to produce enough antibodies to be detected [5, 6]. Antibiotic titers vary in patients; those that present milder symptoms or who are asymptomatic usually have relatively depression antibody titers that disappear a few weeks afterwards infection, while patients with more astringent symptoms generally present higher antibody titers that may be detected two or three months after infection [seven]. When performed in the correct stage of infection antibody test sensitivity may be effectually xc% and results may exist obtained in as little as 15 minutes [3, 8].

Viral load-based tests detect viruses present in the host and can exist either antigen-based, detecting specific fragments of viral proteins, or PCR-based, amplifying viral RNA. Dissimilar serological tests, these tests indicate if the patient has an active infection regardless of their allowed response. Immunochromatographic antigen tests can yield results in 15 minutes. However, reported results range from 100% (based on seven samples) to 32% accuracy (based on 106 positive RT-qPCR samples) [4, 8–10]. The current FDA-recommended method to determine COVID-19 infections is based on reverse transcription quantitative polymerase chain reaction (RT-qPCR). This approach to virus detection amplifies specific sequences from viral SARS-CoV-ii RNA constitute in a given sample. Depending on the manufacturer, the nature and volume of the sample, and the oligonucleotides, RT-qPCR tests can find as few every bit 242 SARS-CoV-2 RNA copies/mL [eleven] or from 1 to 10 genomic copy equivalents per reaction [12].

At that place are three issues regarding standard RT-qPCR that make it less than ideal for large scale testing. First, the tests are ordinarily performed using nasopharyngeal (NP) samples suspended in virus transport medium (VTM). As the sampling method is unpleasant, requires specialized swabs, and is difficult to self-administrate, saliva sampling has been considered equally an alternative source of specimens [thirteen–15]. Second, extraction of RNA from the samples is boring and adds considerable time and expense to the assay. And 3rd, RT-qPCR tests generally require expensive kits and admission to an expensive thermocycler that may not exist available in all settings. We sought to address all iii of these problems to develop a faster, less expensive, and more accessible testing platform for detection of SARS-CoV-2 RNA from patients.

Methodology

Patient samples

Residual samples were retained in a de-identified fashion with no link to patient identifiers. These remnant diagnostic swab samples from Fox Chase Cancer Center, Jeanes Hospital, and Temple University Infirmary patients were stored in VTM at -lxxx °C after testing in the Fox Chase Molecular Diagnostics Laboratory. Saliva samples were obtained from good for you, consenting adult volunteers and stored at -80°C after pH measurments and RT-LAMP testing.

The SARS-CoV-2 diagnostic test used in the Fox Chase Molecular Diagnostics Laboratory extracts RNA from patient nasopharyngeal samples in VTM using a Qiagen QIAamp Viral or a Perkin Elmer chemagen Viral 300 kit, followed by RT-qPCR in an ABI QuantStudio 12K Flex instrument using the ThermoFisher TaqPath COVID-nineteen Philharmonic Kit, that tin can notice at least 10 copies of virus per reaction. SARS-CoV-ii is stable and tin can exist detectable by RT-qPCR and LAMP in both VTM and saliva for 7 to 25 days at a range of 4 to 30°C [thirteen, sixteen].

Protocols for nasopharyngeal (NP) and saliva samples

Direct assay.

100X of inactivation buffer (0.5 M of TCEP-HCl, 0.1 M EDTA< pH 8, plus 1.fifteen N of NaOH, 0.one% μL of NP-10 and 5% of SDS in MQ water, pH 8 with NaOH) was added to care for the samples using the direct analysis. Limit detection curves were fabricated using diferent dilutions from the TaqPath COVID-xix RNA control, A47814 ThermoFisher Scientific. Samples were immediately vortexed, pulse-centrifuged, incubated at 95°C for 5 minutes and centrifuged 30s at 5,000 xg to precipitate the proteins in the VTM and saliva samples. The improver of detergents and heating ensures killing the virus. one.0 μL of this supernatant was added to a previously fix 10 μL RT-LAMP reaction.

RNA atmospheric precipitation assay.

Nucleotides present in the sample were precipitated using silica beads [17]. Briefly, NP samples in VTM or saliva were added to an Eppendorf tube containing a solution with 100X inactivation buffer, and RNAsecure (25X) [18]. The addition of the RNAsecure (Beta-mercaptoethanol mix), irreversibly denatures RNAses by reducing disulfide bonds therefore protecting RNA. For saliva samples, 1 microliter of proteinase K (MEB 8107S) one:10 dilution was added per 250 μL reaction [18]. Samples were vortexed, pulse-centrifuged, incubated at 55°for 15 min and 95°C for v minutes, and centrifuged 30s at 5,000 xg to precipitate the unwanted protein. Treated samples were transferred to a new tube, taking intendance to avoid carry over of the precipitate. We added 0.35 mL of RNA bounden solution (6M of NaI, 2% Triton-100 and 10 mM HCl) and 5 μL of glass milk/silica gel 1:1 w/v in 10 mM Tris-HCl pH 8 and i mM EDTA pH 8, per 0.75 mL of sample and left at room temperature during fifteen–20 minutes shaking advisedly past inversion every two minutes. Samples were centrifuged 1 min at maximum speed in a microcentrifuge. The supernatant was discarded in x% bleach and the pellet was done with 80% EtOH without dislodging it. Samples were centrifuged for 1 min at xiii,000 xg, so ethanol was discarded and tubes were dried at 55°C for one minute. Samples were resuspended in 9 μL of preheated 1x inactivation buffer and used for the RT-LAMP assay or kept at -eighty°C. 3 μL of this sample were added directly to a previously set upward 10 μL RT-LAMP reaction.

RT-LAMP reaction

Reactions were prepare according to the WarmStart LAMP Kit (NEB). Get-go the LAMP main mix was added to the PCR tubes to avoid contamination. We used two or three sets of oligos for each assay: NEB Gene N-A, HMS Assay 1e, NEB orf1a-A oligonucleotides, and an actin command (ACTB) for saliva samples. Primers were designed for specific genes from the genome of the SARS-CoV-2.

LAMP primers.

Primers main mix was prepared as described in [nineteen]: 32 μM of each inner primer (FIP/BIP), 4 μM of each outer primer (F3/B3), 8 μM of each loop primer (LF/LB) were mixed in a 100 μL last volume. Primer sequence was obtained from [20] for NEB Factor N-A and NEB orf1a-A oligonucleotides, from [17] for HMS Assay 1e and from [21] for ACTB oligonucleotides.

The reaction mixture was 1 μL of oligonucleotide mix, 5 μL of WarmStart®
Colorimetric Master Mix, 3 or 1 μL of RNA template and nuclease free h2o to attain a terminal volume of x μL. After the reaction mixture was prepared, tubes were vortexed and centrifuged. Reaction mixtures were color pinkish or red. The presence of carried over silica beads in the sample did not affect the pH or the final SARS-CoV-2 result. Samples were incubated for 30 min at 65°C in PCR tubes in a Bio-Rad thermocycler. Absorbance was measured in a nanodrop device at 448 and 570 nm.

Statistical assay

Paired t-tests were used for comparison between the 448/570 absortion ratio of positive and negative samples used to make up one’s mind the critical threshold value; and between the absorbance ratios registered at each concentration of SARS-CoV2 compared to a sample with no SARS-CoV2 in order to establish the RT-LAMP limit of detection. All hypothesis tests were two-sided with a v% type I error. Sensitivity, specificity, positive predictive value and negative predictive value with 2-sided verbal 95% confidence intervals were computed to assess the operating characterstics of the direct assay and the RNA atmospheric precipitation analysis compared to the standard clinical (i.due east. with RNA purification) RT-qPCR examination. Statistical analyses were completed using GraphPad Prism half-dozen.0 or 7.0 (La Jolla, CA, USA).

Results

Limit of detection for RT-LAMP based methods to detect SARS-CoV-2 viral RNA

The “aureate-standard” examination for detecting SARS-CoV-ii infection uses nasopharyngeal (NP) samples collected in virus transpot medium (VTM) followed past RNA extraction and viral gene distension and detection past RT-qPCR [22, 23]. We sought to simplify all 3 parts of this procedure.

Warm Start colorimetric RT-LAMP (New England Biolabs, M1800L) assays can discover viral RNA using a unmarried temperature, can be completed in one-half an hour, and can provide a colorimetric readout, obviating the demand for a thermocycler or a device capable of real-fourth dimension fluorescence measurements. We began by determining if an RT-LAMP-based approach might be a suitable substitute for RT-qPCR. To compare the limit of detection of both tests, we spiked known quantities of SARS-CoV-2 viral RNA (Dilutions from the TaqPath COVID-nineteen RNA control, A47814 ThermoFisher Scientific) directly into a previously setup RT-LAMP reaction with Beak Gene North-A oligonucleotides [nineteen]. We then monitored the reaction over a 30-minute menstruation for changes in the medium acerbity which would tranlsate in a medium color shift from pink to yellow (Fig 1A). Nosotros measured the maximal absorbance of phenol red at 448 nm (yellow) in acidic conditions and at 570 nm (carmine) in bones conditions [24] (Fig 1B). We quantified the absorbance of the samples at these wavelengths and used the ratio betwixt 448/570 to ready the critical value that was used equally a threshold to make up one’s mind if a sample was positive or negative. Initial samples were pink/red in color; samples that lacked SARS-CoV-2 RNA, had no medium acidification and the 448/570 ratio remained below two at all times. Spiked positive samples amplified SARS-CoV-2 RNA switching colors from cerise to yellowish and increasing the value of the 448/570 ratio. Samples with a ratio above our threshold value 2 were considered positive (Fig 1C and 1D).

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Fig 1.

Disquisitional value threshold determination for RT-LAMP tests for SARS-CoV-ii detection.

A) NEB WarmStart LAMP kit pH is monitored using the pH indicator phenol scarlet. In acrid media phenol crimson has a yellow color and as the pH rises it turns to orange, red and finally pink. Addition of a new complementary nucleotide (dNTP) to a new synthesized Dna chain will form a phosphodiester bond between the α phosphate of the 3’ hydroxide of the pentose acidifying the medium and therefore turning the reaction color from cherry (basic) to yellow (acidic). B) Representative absorption spectrum from a negative and positive SARS-CoV-ii spiked sample using the Pecker kit LAMP. The absorption spectrum for the negative sample is shown in a blackness line and the positive sample is shown in red line. Measurements were taken at 2 absorption maximum points, one in yellow (λ = 448 nm) and one in red (570 nm). C) Box plots correspond the absorbance values of positive viral RNA-spiked samples and negative samples at 448 and 570 nm (due north = 20) D) The quotient of 448/570 nm of negative and positive samples was used to set the crisitcal value threshold at 2. Box plots correspond the values between positive and negative SARS-CoV2 spiked samples. Paired t-exam of northward = 20 **** P <0.0001.

https://doi.org/x.1371/journal.pone.0250202.g001

Having established the basic analysis and critical value, we side by side used iii different primer sets–Nib Gene N-A [xx], Neb orf1a-A [18] and HMS Assay 1e (As1e) [17]–to evaluate if they could find SARS-CoV-2 sequences. As in the previous assay, we added a known concentration of viral RNA direct into the RT-LAMP reaction tube and recorded the absorbance spectrum of the sample after a 30-minute incubation at 65°C (Fig 2, S1 Fig). NEB Gene N-A and HMS Analysis 1e primers detected as few as 2 copies of viral RNA per reaction (Fig 2A–2C), while the NEB orf1a-A primers were less efficient, with a detection limit of 12.5 copies per reaction (Fig 2A and second).

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Fig 2.

RT-LAMP assay limit of detection.

Known concentrations of SARS-CoV-2 virus diluted in inactivation buffer were used to fasten previously set up RT-LAMP reactions with oligonucleotide pairs NEB Gene N-A (N-A), HMS Assay 1e (As1e) and NEB orf1a-A (orf1a-A). Command samples with no virus were also monitored. LAMP tests were incubated at 65°C for 30 minutes in a Bio-Rad thermocycler and the resulting reaction was (A) imaged and the absorbance at 448 and 570 nm was measured. The absorbance quotient between 448/570 nm was used to distinguish positive versus negative samples for (B) oligonucleotide NEB Gene N-A, (C) oligonucleotide HMS Assay 1e and (D) oligonucleotide NEB orf1a-A. Statistics: Paired t-exam between the absorbance ratio registered at each concentration of SARS-CoV-two compared to a sample with no SARS-CoV-2. Values correspond the hateful ± South.D. of n = 3 for each concentration **** P <0.0001.

https://doi.org/x.1371/journal.pone.0250202.g002

Comparison of straight
versus
RNA-purified methods for detection of spiked SARS-CoV-ii in VTM

SARS-CoV-2 viral RNA purification adds time, complexity, and increases the cost of the assay. We adjacent sought to determine if the RT-LAMP reaction to detect artifitially spiked SARS-CoV-2 samples was imporved when RNA was extracted.

For the direct assay experiments, VTM was spiked with SARS-CoV-ii and inactivated with 100x inactivation buffer. The inactivation buffer has a terminal concentration of 0.05% SDS to help solubilize the virus membrane, rendering the virus non-infectious, every bit well as providing additional protection for RNA and Deoxyribonucleic acid [25–27]. According to the direct assays protocol, we transferred 1 μL of the treated sample (Fig 3A–3C) to PCR tubes containing the RT-LAMP reaction media previously prepared with primers for Bill Gene N-A (A, D), HMS Assay 1e (B, E) and Nib orf1a-A (C, F) and incubated it thirty minutes at 65°C. In the direct assay (no RNA-precipitation), the limit of detection was 769 copies/μL (769 copies per reaction) with NEB Cistron N-A and NEB orf1a-A primers, and 385 copies/mL (385 copies per reaction) when using the HMS Assay 1e primers (Fig 3A–3C, S2A Fig).

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Fig 3.

Limit of detection for SARS-CoV-2 spiked in VTM.

VTM medium was spiked with different concentrations of SARS-CoV-two. Samples treated with Inactivation buffer and RNA secure (Direct assay: A, B, C) and RNA precipitated samples following the HMS Assay (RNA precipitation Analysis: D,E,F) were added in the LAMP reaction with samples for (A,D) oligonucleotide Nib Factor N-A, (B,East) oligonucleotide HMS Assay 1e and (C,F) oligonucleotide NEB orf1a-A and incubated xxx minutes at 65°C. Each dot represents an individual experiment, n = three. Critical value threshold was set up at 2 (blood-red line).

https://doi.org/10.1371/periodical.pone.0250202.g003

To make up one’s mind if sensitivity was sacrificed by omitting RNA purification, we tested the RT-LAMP assay with a protocol where the viral RNA was isolated. We modified an inexpensive silica dewdrop-based (“glassmilk”) method to isolate nucleic acids known as the HMS Assay [17, 28] (Fig 3D–3F, S2B Fig). 0.75 mL of VTM were spiked with diferent number of copies of SARS-CoV-2. We added the 100x inactivation solution supplemented with a 25x non-enzymatic RNase inhibitor (RNAsecure™) and proteinase K [18]. Nosotros added an additional incubation footstep at 55°C for 15 min, and so equally in the direct analysis, samples were incubated at 95°C for five min and centrifuged at 5,000 xg for 30s. Protein pellet in these samples was considerably smaller than the obtained in the Direct Assay. Later on NaI RNA precipitation, we resuspended the smaple in 9 μL of 1X inactivation buffer. Three μL of RNA precipitated ssample were added to previously fix upwardly RT-LAMP reactions containing primers for Neb Factor Northward-A (Fig 5A and 5D), HMS Assay 1e (Fig 5B and 5E) and Actin (Fig 5C and 5F). Samples were incubated thirty minutes at 65°C. Using this protocol, we found the limit of detection to be 192 copies/reaction when using the NEB Cistron N-A primers. When using the HMS Assay 1e primers, the limit of detection was 385 copies/reaction. When using the NEB orf1a-A primers, nosotros successfully detected 1538 and 385 copies/reaction, only one sample unexpectedly gave a negative reading at 769 copies/reaction. Thus, the addition of an RNA purification step that increases the testing fourth dimension for virtually forty min, just yielded a 2 to a five-fold increase in sensitivity depending on the oligonucleotides used.

Directly
versus
RNA-purified methods for detection of SARS-CoV-2 in nasopharyngeal patient samples

We tested both methods in a blind randomized assay of 29 positive and 30 negative patient-derived samples that were previously analyzed by RT-qPCR (Fig 4, Tables 1 and 2, S3 and S4 Figs).

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Fig 4.

Detection of SARS-CoV-2 in clinical nasopharyngeal samples.

NP patient samples in VTM were tested using the LAMP Straight Analysis (A,B,C) or the RNA precipitation Analysis following the HMS Assay (D,E) to detect SARS-CoV-two using the (A,D) NEB Gene N-A, (B,E) HMS Assay 1e and the (C) NEB orf1a-A oligonucleotides. Positive and negative samples are paired with Table 1 (Direct Assay) and Table ii (RNA precipitation Analysis). (F) Two by two tabular array showing positive and negative samples detected past the Straight Assay and the RNA atmospheric precipitation Assay. Each dot represents an private experiment. Critical value threshold was ready at 2 (red line). Samples 1 to 29 are true positives by RT-qPCR. Samples from xxx to 59 are true negatives by RT-qPCR.

https://doi.org/x.1371/journal.pone.0250202.g004

The Direct Assay (i.due east., without RNA purification) successfully detected 17/29 positive samples when using the NEB Cistron Due north-A oligonucleotides (Fig 4A), 21/29 positive samples when using the HMS Assay 1e oligonucleotides (Fig 4B), and xiv/29 positive samples when using the NEB orf1a-A oligonucleotides (Fig 4C). The samples that scored positive for all three genes where those that had the lowest Ct values (≤20) determined by RT-qPCR, indicating a loftier number of viral RNA copies (Table 1, No symbol). The seven samples that were positive for 2 out of 3 genes, were considered positive and correlated with the mid-range Ct values (Tabular array 1, c). The v samples that gave one out of three genes positive were repeated with the Beak Gene North-A or the HMS Analysis 1e oligonucleotides and in but i case (27, 9C7) the sample showed positive detection (Table 1, b). The seven samples with the highest Ct’s (≥24) which corresponds to a lower viral load, gave negative results for all oligonucleotides (Table one, a). Overall, the uncomplicated method had a 65.5% (95% confidence interval (45.7, 82.one)) sensitivity as compared to the standard clinical (i.e. with RNA purification) RT-qPCR test. No imitation positives were detected.

RNA atmospheric precipitation prior to the RT-LAMP (Fig 4D and 4E, Table 2, S4 Fig) detected 25/29 positive samples for the NEB Gene North-A oligonucleotides and 26/29 positive samples for or the HMS Assay 1e oligonucleotides. We did non use Nib orf1a-A oligonucleotides due to the above-noted inconsistency in previous tests and the results obtained by others [17]. The samples that gave positive results with merely one primer set were those with the highest Ct values, indicating a low number of viral RNA copies (Table 2, c). Samples with indeterminate results were repeated and amplification was confirmed (Fig 4D and 4E). No false positives were detected, and method sensitivity increased to 93.1% (95% confidence interval (77.ii%, 99.two%)) indicating that concentrating the sample and precipitating RNA substantially enhances functioning. Yet, this protocol required boosted time and more easily-on manipulation for the sample precipitation, which should be considered versus the cost of an RNA atmospheric precipitation kit.

Comparison of direct
versus
RNA-purified methods for detection of SARS-CoV-2 in spiked saliva

Saliva-based tests do not require a certified swab, VTM, or a skilled worker to take samples. However, when using saliva, nosotros found that the RT-LAMP examination worked well for saliva samples with a neutral to basic pH (up to vii.0–7.4), just acidic saliva (less than pH 6.8) gave faux positive results. To address this problem, nosotros increased the pH of the inactivation buffer from viii.5 to 11. The increment in pH did not touch on the RT-LAMP examination results when using basic saliva, however, nosotros noted that the ratios between the readings at 447/570 nm were consistently lower as compared to the original low pH buffer.

Nosotros tested the straight (Fig 5A–5C, S5A Fig) and the RNA precipitation assays (Fig 5D–5F, S5B Fig) with known concentration of copies of SARS-CoV2 in saliva with the new high pH buffer and found the limit of detection for both, the Beak Gene N-A and the HMS Analysis 1e oligonucleotides to be 769 copies/reaction for the direct method and 386 copies/reaction for the RNA precipitation Analysis.

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Fig 5.

Limit of detection for SARS-CoV-2 spiked in saliva.

Saliva samples were spiked with unlike concentrations of SARS-CoV-2. Samples treated with Inactivation buffer and RNA secure (Direct Assay: A, B, C) and RNA precipitated samples (RNA atmospheric precipitation Assay: D,E,F) were added in the LAMP reaction with oligonucleotides (N-A: A,D) Neb Cistron-N-A, (As1e: B,E) HMS Assay 1e, (C,F) and Actin and incubated thirty minutes at 65°C. Positive threshold was set at 2 (red line). ♦ saliva sample pH seven.4, ♢ saliva sample pH 6.7. Values correspond the mean ± S.D. of north = 3 for each concentration.

https://doi.org/10.1371/journal.pone.0250202.g005

Nosotros then assayed mock samples by diluting l μL of randomly chosen NP-positive and -negative patient-derived samples in both acidic and alkaline saliva samples (Fig vi, Table 3, S6 Fig). The testing group included 10 positive samples and 6 negative samples. The direct analysis detected vii/x positive samples when using both the NEB Gene N-A and the HMS Assay 1e oligonucleotides (Fig 6A and 6B). The three non-detected samples had the highest Ct values when measured by RT-qPCR (Table 3). The RNA precipitation analysis detected 9/10 positive samples for both Bill Factor N-A and the HMS Assay 1e oligonucleotides (Fig 6D and 6B). Actin-based primers were used equally a positive command (Fig 6C and 6F) [21]. Nosotros detected ninety% of the samples but values were lower than in the samples that used VTM directly making the apply of a spectrophotometer obligatory. Sensitivy of these methods in saliva was lx.0% (95% confidence interval (26.2%, 87.8%)) and 90.0% (95% confidence interval (55.5%, 99.8%)) respectively, while showing no simulated positives.

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Fig 6.

Detection of SARS-CoV-2 in clinical nasopharyngeal samples in virus transport medium diluted in saliva.

Nasopharyngeal patient samples in VTM were diluted in saliva and tested using the LAMP Direct Assay (A,B,C) or the RNA atmospheric precipitation Assay (D,E,F) to detect SARS-CoV-2. (G) Two by 2 table showing true positive and negative samples detected by the Direct Assay and the RNA precipitation Assay. Each dot represents an individual experiment. Critical value threshold was set up at two (red line). Samples 1 to 10 are true positives past RT-qPCR. Samples from eleven to 16 are true negatives by RT-qPCR. Positive and negative samples are paired with Table three.

https://doi.org/10.1371/periodical.pone.0250202.g006

Discussion

In an attempt to choose a robust and easy-to-perform examination for SARS-CoV-two for utilize in signal-of-care settings, we tested several published RT-LAMP assay methods. We introduced modifications to simplify the procedure while maintaining high sensitivity and reliability. These modifications included changes in the pH of the RT-LAMP buffers, omission of an RNA purification stride.

In accordance to Huang et al. [29], RT-LAMP was able to discover upwardly to two copies of directly spiked SARS CoV-ii RNA per reaction showing it is every bit reliable equally RT-qPCR and any variation on positive or negative results may come from sample treatment and RNA isolation and stabilization methods. Using the Direct assay, we were able to detect the SARS CoV-two RNA in 385 or 769 copies per reaction for the HMS Assay 1e and for the NEB Gene North-A, and the Bill orf1a-A oligonucleotides respectively. The precipitated method was more sensitive detecting betwixt 192 and 385 copies per reaction for the NEB Gene North-A, and for the HMS Analysis 1e and the Nib orf1a-A oligonucleotides respectively. These number of copies detected are comparable to those reported past others, and slightly lower than the sensitivity obtained using RNA column purification plus qRT-PCR that goes down to 10–15 copies per reaction (RT-qPCR Ct’s ≤ 37) [12, 17, 29] and is considerably more expensive. Consistent with other reports, direct spiked RT-LAMP test without RNA purification detects from 50 to 400 copies per reaction [17, eighteen, thirty, 31] and Ct values below 24–26. Adding a column RNA purification footstep to ther RT-LAMP increases sensibility to 10–30 copies per reaction [13, 19, 32], which was lower than what we detected. The HMS Analysis (glass milk precipitation method) reports ane–2 copies per μl of an initial 500 μL sample [17], by modifying the methos adding detergents and an RNA protective amanuensis we were able to detect 0.6 to 1.54 copies per μl of an initial 250 μL sample which corresponds to samples with RT-qPCR Ct values of ~29.

The RT-LAMP assay has several potential advantages over qRT-PCR methods. Outset, RT-LAMP amplifies Dna fragments at a constant, modest temperature, obviating the need for a thermo-cycler. RT-LAMP also typically has higher Deoxyribonucleic acid yields than mutual PCRs, since in that location is no bind, amplify, and release cycle [33]. Finally, during nucleic acid synthesis, the binding of each nucleotide to the DNA growing strand releases a proton, acidifying the medium [34] (Fig 1A). Therefore, product aggregating during RT-LAMP may be evaluated using mutual pH sensitive dyes such as phenol blood-red, which changes from a pinkish/red tone at pH viii to yellow as pH acidifies (Fig 1A) [34]. Other inexpensive pH sensitive dyes such as cresol ruddy, neutral red and grand-cresol purple take also been used to runway Dna amplification [34].

Straight detection of SARS-CoV-2 in the absence of RNA purification was possible, merely the sensitivity was reduced from 93% to 65% when compared to assays where RNA was first purified. In practical terms, this relative lack of sensitivity may be adequate in certain circumstances, equally the test is inexpensive and like shooting fish in a barrel to perform it may exist applied multiple times if required, and tends to requite fake negative only for low-titer samples that likely correlate with less transmissibility and/or less severity of disease [35–37]. The negative predictive value for the directly test method is 75.0% (95% conviction interval (58.8%, 87.3%)), which means that fifty-fifty if this test is negative, in that location is nonetheless a 25% chance of being sick, and it’southward straight correlated with the virus load institute in the patient. The negative predictive value for the precipitation method is 93.viii% (95% conviction interval (79.two%, 99.2%)), which means that if the examination is negative, there is nevertheless a six% chance of being sick. For both methods specificity and positive predictive values were 100%.

The utilise of saliva samples in place of NP samples would represent a 3rd potential improvement. Saliva is easy to obtain and nosotros establish that it can exist kept at ambience temperature for periods up to 30 days [13, 15, xvi]. This feature may circumvent the so-chosen refrigeration barrier, described for sub-Saharan regions, or any location that lacks adequate refrigerating facilities. We found that acidic saliva samples complicate testing every bit the assay readout is based on acidification equally a result of Deoxyribonucleic acid amplification and giving a high number of simulated positives. Nosotros successfully addressed this issue by simply increasing the initial pH of the inactivation buffer from 8.5 to 11. Even if it’southward a dramatic modification in the initial pH, this buffer is diluted 100X in the saliva sample and further diluted to use in the RT-LAMP reaction. A higher pH helps maintain the neutral pH and color of the reaction buffer containing phenol red but all the same allows medium acidification and color change when there is a DNA amplification reaction. Other groups take avoided the pH outcome by using fluorescent labels to follow Dna distension which slightlty increases the price and needs specialized equipment for detection [32, 38].

We were not able to get saliva samples from sick patients, but we tested the nasopharyngeal samples using saliva as a vehicle. The direct detection method sensitivity was 60% (95% confidence interval (26.2%, 87.8%), which is also considerably lower than the 93% (95% confidence interval (55.5%, 99.8%)) sensitivity of the assays where RNA was first purified. In practical terms, this relative lack of sensitivity may be acceptable in certain circumstances, as the test is cheap and piece of cake to perform it may be applied multiple times if required, and tends to give false negative merely for depression-titer samples that likely correlate with less transmissibility and/or less severity of disease [35–37]. The negative predictive value for the directly examination method is sixty%, which means that even if this test is negative, there is still a 40% run a risk of SARS-CoV2 infection. The negative predictive value for the atmospheric precipitation method is 90%, which means there would only be a ten% gamble of being sick. For both methods specificity and positive predictive values were is 100%.

Nosotros envision a few additional modifications that might make the RT-LAMP-based cocky-testing procedure more suitable for point of care use. First, the enzymes (contrary transcriptase and
BstI
Dna polymerase), the buffer and the phenol red needed for the RT- LAMP reaction can exist acquired in lyophilized form, allowing storage at room temperature [39, 40]. 2nd, pocket-size transfer loops could conceivably be used in identify of micropipeting devices, allowing a reasonably accurate transfer of microliter volumes in settings that lack sophisticated equipment [41]. Tertiary, pooled testing has been successfully demonstrated for RT-LAMP-based SARS-CoV-2 detection, farther reducing costs while increasing output [forty].

Supporting information

S1 Fig. LAMP assay limit of detection.

Direct SARS-CoV-2 virus was diluted in inactivation buffer and LAMP using oligonucleotide pairs NEB Gene Northward-A (N-A), HMS Assay 1e (As1e) and NEB orf1a-A (orf1a-A). Controls using no virus were also monitored. LAMP tests were incubated in PCR tubes at 65°C for thirty minutes in a Bio-Rad thermocycler and the resulting reaction was imaged.

https://doi.org/ten.1371/journal.pone.0250202.s001

(PDF)

S2 Fig. Limit of detection for SARS-CoV-2 spiked VTM.

VTM medium was spiked with dissimilar concentrations of SARS-CoV-ii. 1 μL of samples treated with A) the Direct Assay: Inactivation buffer and RNA secure and the B) RNA atmospheric precipitation Analysis were added in the LAMP reaction with oligonucleotides Neb Factor North-A (Due north-A), HMS Analysis 1e (As1e) and NEB orf1a-A (orf1a-A) and incubated 30 minutes at 65°C and the resulting reaction was imaged.

https://doi.org/10.1371/journal.pone.0250202.s002

(PDF)

S3 Fig. Straight assay of SARS-CoV-2 in clinical nasopharyngeal samples in VTM.

NP patient samples in VTM were tested using the LAMP direct analysis to observe SARS-CoV-2 with oligonucleotides Neb Cistron N-A (Due north-A), HMS Assay 1e (As1e) and Bill orf1a-A (orf1a-A) and incubated 30 minutes at 65°C and the resulting reaction was imaged. Positive and negative samples are paired with Table ane.

https://doi.org/10.1371/journal.pone.0250202.s003

(PDF)

S4 Fig. Detection of SARS-CoV-2 in clinical NP samples in VTM precipitating using the RNA atmospheric precipitation assay.

NP patient samples in VTM were precipitated with the HMS modified method and then tested with LAMP to detect SARS-CoV-2 with the NEB Cistron N-A (N-A) and HMS Assay 1e (As1e) oligonucleotides. Samples were incubated 30 minutes at 65°C and the resulting reaction was imaged. Positive and negative samples are paired with Table 2.

https://doi.org/10.1371/periodical.pone.0250202.s004

(PDF)

S5 Fig. SARS-CoV-2 limit of detection in saliva samples using the LAMP assay.

Two saliva samples, of pH 6.7 and 7.4 were was spiked with different concentrations of SARS-CoV-ii. Samples treated with A) the Direct Assay B) the RNA precipitation analysis were tested with the NEB Gene North-A (N-A) and HMS Assay 1e (As1e) and Actin oligonucleotides. The resulting reaction was imaged after a 30 infinitesimal incubation at 65°C.

https://doi.org/10.1371/journal.pone.0250202.s005

(PDF)

S6 Fig. Detection of SARS-CoV-2 in clinical NP samples in VTM diluted in saliva.

NP patient samples in VTM were diluted in saliva in a i:5 ratio and tested using the (A) Direct Assay and the (B) RNA precipitation assay. LAMP test were washed for tested with the Nib Gene N-A (N-A) and HMS Assay 1e (As1e) and Actin oligonucleotides. Positive and negative samples are paired with Tabular array 3.

https://doi.org/10.1371/journal.pone.0250202.s006

(PDF)

Acknowledgments

We give thanks Johnathan Whetstine for loan of a RT-PCR auto and colleagues on 4W for donating RNA purification kits.

The content is solely the responsability of the authors and does not necessarily represent the official views of the Natonal Intitutes of Health.

References

  1. one.
    Dong East, Du H, Gardner L. An interactive web-based dashboard to track COVID-19 in existent fourth dimension. Lancet Infect Dis. 2020;twenty(5):533–4. Epub 2020/02/23. pmid:32087114

    • View Article
    • PubMed/NCBI
    • Google Scholar
  2. ii.
    CDC. Centers for Disease Control and Prevention. Coronavirus Disease 2019 (COVID-xix) 2020. https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/alphabetize.html.

    • 3.
      Zainol Rashid Z, Othman SN, Abdul Samat MN, Ali UK, Wong KK. Diagnostic performance of COVID-xix serology assays. Malays J Pathol. 2020;42(one):13–21. Epub 2020/04/29. pmid:32342927.

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 4.
      Guglielmi One thousand. Fast coronavirus tests: what they can and can’t practice. Nature. 2020;585(7826):496–viii. Epub 2020/09/18. pmid:32939084.

      • View Commodity
      • PubMed/NCBI
      • Google Scholar
    • 5.
      Long QX, Liu BZ, Deng HJ, Wu GC, Deng K, Chen YK, et al. Antibiotic responses to SARS-CoV-2 in patients with COVID-19. Nat Med. 2020;26(six):845–eight. Epub 2020/05/01. pmid:32350462.

      • View Commodity
      • PubMed/NCBI
      • Google Scholar
    • vi.
      Beeching NJ, Fletcher TE, Beadsworth MBJ. Covid-xix: testing times. BMJ. 2020;369:m1403. Epub 2020/04/x. pmid:32269032.

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 7.
      Long QX, Tang XJ, Shi QL, Li Q, Deng HJ, Yuan J, et al. Clinical and immunological cess of asymptomatic SARS-CoV-ii infections. Nat Med. 2020;1200–1204. Epub 2020/06/twenty. pmid:32555424.

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 8.
      Kontou PI, Braliou GG, Dimou NL, Nikolopoulos G, Bagos PG. Antibody Tests in Detecting SARS-CoV-2 Infection: A Meta-Assay. Diagnostics (Basel). 2020;ten(5). Epub 2020/05/23. pmid:32438677

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • ix.
      Scohy A, Anantharajah A, Bodeus Thou, Kabamba-Mukadi B, Verroken A, Rodriguez-Villalobos H. Low performance of rapid antigen detection test as frontline testing for COVID-19 diagnosis. J Clin Virol. 2020;129:104455. Epub 2020/06/03. pmid:32485618

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 10.
      Pekosz A, Parvu 5, Li Grand, Andrews JC, Manabe YC, Kodsi S, et al. Antigen-Based Testing but Non Real-Time Polymerase Concatenation Reaction Correlates With Astringent Acute Respiratory Syndrome Coronavirus 2 Viral Culture. Clin Infect Dis. 2021. Epub 2021/01/23. pmid:33479756.

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 11.
      Wang X, Yao H, Xu X, Zhang P, Zhang One thousand, Shao J, et al. Limits of Detection of vi Approved RT-PCR Kits for the Novel SARS-Coronavirus-ii (SARS-CoV-two). Clin Chem. 2020;66(vii):977–ix. Epub 2020/04/14. pmid:32282874

      • View Article
      • PubMed/NCBI
      • Google Scholar
    • 12.
      TaqPath Covid-19 Philharmonic Kit. Instructions for Use, Multiplex Real time RT-PCR exam intended for the qualitative detection fo nucleic acid from SARS-CoV-2. AppliedBiosystems. ThermoFisher Scientific. Catalog Number A47814. Publication number MAN0019181. 2020.

      • 13.
        Vogels CBF, Watkins AE, Harden CA, Brackney D, Shafer J, Wang J, et al. SalivaDirect: A simplified and flexible platform to enhance SARS-CoV-two testing capacity. Med. 2020;2:ane–18. http://doi.org/ten.1016/j.medj.2020.12.010 pmid:33521748

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 14.
        Nagura-Ikeda M, Imai Chiliad, Tabata S, Miyoshi M, Murahara N, Mizuno T, et al. Clinical evaluation of self-nerveless saliva by RT-qPCR, direct RT-qPCR, RT-LAMP, and a rapid antigen test to diagnose COVID-xix. J Clin Microbiol. 2020;Aug 24;58(ix):e01438–twenty. Epub 2020/07/09. pmid:32636214.

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 15.
        Wyllie AL, Fournier J, Casanovas-Massana A, Campbell Thou, Tokuyama M, Vijayakumar P, et al. Saliva or Nasopharyngeal Swab Specimens for Detection of SARS-CoV-2. N Engl J Med. 2020;383(xiii):1283–6. Epub 2020/08/29. pmid:32857487

        • View Commodity
        • PubMed/NCBI
        • Google Scholar
      • 16.
        Ott IM, Strine MS, Watkins AE, Kick M, Kalinich CC, Harden CA, et al. Just saliva: stability of SARS-CoV-2 detection negates the need for expensive drove devices. medRxiv. 2020. Epub 2020/08/15. pmid:32793924

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 17.
        Rabe BA, Cepko C. SARS-CoV-2 detection using isothermal distension and a rapid, cheap protocol for sample inactivation and purification. Proc Natl Acad Sci U S A. 2020;117(39):24450–eight. Epub 2020/09/x. pmid:32900935.

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • eighteen.
        Lalli MA, Langmade SJ, Chen X, Fronick CC, Sawyer CS, Burcea LC, et al. Rapid and extraction-costless detection of SARS-CoV-2 from saliva by Colorimetric Contrary-Transcription Loop-Mediated Isothermal Distension. Clinical Chemistry. 2021;67(2):415–24. https://doi.org/10.1093/clinchem/hvaa267 pmid:33098427

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 19.
        Lamb LE, Bartolone SN, Ward East, Chancellor MB. Rapid detection of novel coronavirus/Astringent Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) past contrary transcription-loop-mediated isothermal amplification. PLoS One. 2020;15(6):e0234682. Epub 2020/06/13. pmid:32530929

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 20.
        Zhang Y, Odiwuor North., Xiong J., Dominicus L., Ohuru Nyaruaba R., Wei H., et al. Rapid Molecular Detection of SARS-CoV-ii (COVID-19) Virus RNA Using Colorimetric LAMP. medRxiv. 2020. https://doi.org/10.1101.2020.02.26 pmid:20028373.

        • View Article
        • PubMed/NCBI
        • Google Scholar
      • 21.
        Anahatar MN, McGrath G.E.K., Rabe B.A., Tanner Northward.A., White B.A., Lennerz J.M.M., et al. Clinical assessment and validation of a rapid and sensitive SARS-CoV-2 examination suing reverse-transcription loop-mediated isothermal distension. Open Forum Infect Dis. 2020;ofaa631.

        • View Article
        • Google Scholar
      • 22.
        CDC. Centers for Disease Control and Prevention. Division of Viral diseases. CDC 2019-Novel Coronavirus (2019-nCoV) Real-Time RT-PCR Diagnostic Panel. Catalog # 2019-nCoVEUA-01 2020. world wide web.fda.gov/media/134922/download.

        • 23.
          CDC. Centers fro Disease Command and Prevention. Influenza Division. CDC Flu SARS-CoV-2 (Flu SC2) Multiplex Analysis. 2020. www.fda.gov/media/139743/download.

          • 24.
            Barbosa J. Indicators, Acid-Base. Encyclopedia of Analytical Science: Elsevier. Science Direct. Journals & Books.; 2005.

            • 25.
              Goldenberger D, Perschil I, Ritzler M, Altwegg M. A uncomplicated “universal” DNA extraction procedure using SDS and proteinase K is uniform with direct PCR amplification. PCR Methods Appl. 1995;4(6):368–lxx. Epub 1995/06/01. pmid:7580932.

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 26.
              Qamar Due west, Khan MR, Arafah A. Optimization of conditions to extract high quality DNA for PCR analysis from whole blood using SDS-proteinase Thousand method. Saudi J Biol Sci. 2017;24(7):1465–9. Epub 2018/ten/09. pmid:30294214

              • View Commodity
              • PubMed/NCBI
              • Google Scholar
            • 27.
              Rose K, Mason JO, Lathe R. Hybridization parameters Revisted: Solutions containing SDS. BioTechniques. 2018;33(ane). http://doi.org/10.2144/02331st01.

              • View Article
              • Google Scholar
            • 28.
              Vogelstein B, Gillespie D. Preparative and analytical purification of DNA from agarose. Proc Natl Acad Sci U S A. 1979;76(2):615–9. Epub 1979/02/01. pmid:284385

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 29.
              Huang We, Lim B, Hsu CC, Xiong D, Wu Westward, Yu Y, et al. RT-LAMP for rapid diagnosis of coronavirus SARS-CoV-2. Microb Biotechnol. 2020;13(iv):950–61. Epub 2020/04/26. pmid:32333644

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 30.
              Dao Thi VL, Herbst K, Boerner K, Meurer M, Kremer LP, Kirrmaier D, et al. A colorimetric RT-LAMP analysis and LAMP-sequencing for detecting SARS-CoV-2 RNA in clinical samples. Sci Transl Med. 2020;12(556). Epub 2020/07/29. pmid:32719001

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 31.
              Thompson D, Lei Y. Mini review: Recent Progress in RT-LAMP enabled COVID-nineteen detection. Sensors and Actuators Reports. 2020;two(1):100017. https://doi.org/10.1016/j.snr.2020.100017.

              • View Commodity
              • Google Scholar
            • 32.
              Wang D. Ane-pot Detection of COVID-19 with Real-fourth dimension Contrary-transcription Loop-mediated Isothermal Amplification (RT-LAMP) Assay and Visual RT-LAMP Analysis. bioRXiv. 2020:2020.04.21.052530. https://doi.org/10.1101/2020.04.21.052530.

              • View Article
              • Google Scholar
            • 33.
              Gadkar VJ, Goldfarb DM, Gantt South, Tilley PAG. Real-time Detection and Monitoring of Loop Mediated Amplification (LAMP) Reaction Using Self-quenching and De-quenching Fluorogenic Probes. Sci Rep. 2018;viii(1):5548. Epub 2018/04/05. pmid:29615801

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 34.
              Tanner NA, Zhang Y, Evans TC, Jr. Visual detection of isothermal nucleic acrid amplification using pH-sensitive dyes. Biotechniques. 2015;58(two):59–68. Epub 2015/02/06. pmid:25652028.

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 35.
              Liu Y, Yan LM, Wan 50, Xiang TX, Le A, Liu JM, et al. Viral dynamics in mild and severe cases of COVID-19. Lancet Infect Dis. 2020;20(half-dozen):656–7. Epub 2020/03/23. pmid:32199493

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 36.
              Magleby R, Westblade LF, Trzebucki A, Simon MS, Rajan Grand, Park J, et al. Impact of SARS-CoV-ii Viral Load on Risk of Intubation and Bloodshed Among Hospitalized Patients with Coronavirus Disease 2019. Clin Infect Dis. 2020:Jun 30:ciaa851. Epub 2020/07/01. pmid:32603425

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 37.
              Singanayagam A, Patel M, Charlett A, Lopez Bernal J, Saliba V, Ellis J, et al. Duration of infectiousness and correlation with RT-PCR wheel threshold values in cases of COVID-19, England, January to May 2020. Euro Surveill. 2020;25(32). Epub 2020/08/15. pmid:32794447

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 38.
              Yu Fifty, Wu S, Hao X, Dong 10, Mao L, Pelechano Five, et al. Rapid Detection of COVID-19 Coronavirus Using a Reverse Transcriptional Loop-Mediated Isothermal Amplification (RT-LAMP) Diagnostic Platform. Clin Chem. 2020;66(seven):975–7. Epub 2020/04/22. pmid:32315390

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 39.
              Chen HW, Ching WM. Evaluation of the stability of lyophilized loop-mediated isothermal distension reagents for the detection of Coxiella burnetii. Heliyon. 2017;three(10):e00415. Epub 2017/10/24. pmid:29057336

              • View Commodity
              • PubMed/NCBI
              • Google Scholar
            • twoscore.
              Carter C, Akrami G, Hall D, Smith D, Aronoff-Spencer E. Lyophilized visually readable loop-mediated isothermal contrary transcriptase nucleic acid amplification test for detection Ebola Zaire RNA. J Virol Methods. 2017;244:32–8. Epub 2017/03/01. pmid:28242293

              • View Article
              • PubMed/NCBI
              • Google Scholar
            • 41.
              Jacobs JA, De Brauwer EI, Cornelissen EI, Drent M. Accuracy and precision of quantitative calibrated loops in transfer of bronchoalveolar lavage fluid. J Clin Microbiol. 2000;38(six):2117–21. Epub 2000/06/02. pmid:10834963

              • View Article
              • PubMed/NCBI
              • Google Scholar

            Which of the Following is Not True of Saliva

            Source: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0250202

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