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      Rapid antigen tests for SARS-CoV-2

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      , AB, , BHSc, , MD
      CMAJ : Canadian Medical Association Journal
      Joule Inc.

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          Rapid, point‐of‐care antigen and molecular‐based tests for diagnosis of SARS‐CoV‐2 infection

          Background Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) and the resulting COVID‐19 pandemic present important diagnostic challenges. Several diagnostic strategies are available to identify or rule out current infection, identify people in need of care escalation, or to test for past infection and immune response. Point‐of‐care antigen and molecular tests to detect current SARS‐CoV‐2 infection have the potential to allow earlier detection and isolation of confirmed cases compared to laboratory‐based diagnostic methods, with the aim of reducing household and community transmission. Objectives To assess the diagnostic accuracy of point‐of‐care antigen and molecular‐based tests to determine if a person presenting in the community or in primary or secondary care has current SARS‐CoV‐2 infection. Search methods On 25 May 2020 we undertook electronic searches in the Cochrane COVID‐19 Study Register and the COVID‐19 Living Evidence Database from the University of Bern, which is updated daily with published articles from PubMed and Embase and with preprints from medRxiv and bioRxiv. In addition, we checked repositories of COVID‐19 publications. We did not apply any language restrictions. Selection criteria We included studies of people with suspected current SARS‐CoV‐2 infection, known to have, or not to have SARS‐CoV‐2 infection, or where tests were used to screen for infection. We included test accuracy studies of any design that evaluated antigen or molecular tests suitable for a point‐of‐care setting (minimal equipment, sample preparation, and biosafety requirements, with results available within two hours of sample collection). We included all reference standards to define the presence or absence of SARS‐CoV‐2 (including reverse transcription polymerase chain reaction (RT‐PCR) tests and established clinical diagnostic criteria). Data collection and analysis Two review authors independently screened studies and resolved any disagreements by discussion with a third review author. One review author independently extracted study characteristics, which were checked by a second review author. Two review authors independently extracted 2x2 contingency table data and assessed risk of bias and applicability of the studies using the QUADAS‐2 tool. We present sensitivity and specificity, with 95% confidence intervals (CIs), for each test using paired forest plots. We pooled data using the bivariate hierarchical model separately for antigen and molecular‐based tests, with simplifications when few studies were available. We tabulated available data by test manufacturer. Main results We included 22 publications reporting on a total of 18 study cohorts with 3198 unique samples, of which 1775 had confirmed SARS‐CoV‐2 infection. Ten studies took place in North America, two in South America, four in Europe, one in China and one was conducted internationally. We identified data for eight commercial tests (four antigen and four molecular) and one in‐house antigen test. Five of the studies included were only available as preprints. We did not find any studies at low risk of bias for all quality domains and had concerns about applicability of results across all studies. We judged patient selection to be at high risk of bias in 50% of the studies because of deliberate over‐sampling of samples with confirmed COVID‐19 infection and unclear in seven out of 18 studies because of poor reporting. Sixteen (89%) studies used only a single, negative RT‐PCR to confirm the absence of COVID‐19 infection, risking missing infection. There was a lack of information on blinding of index test (n = 11), and around participant exclusions from analyses (n = 10). We did not observe differences in methodological quality between antigen and molecular test evaluations. Antigen tests Sensitivity varied considerably across studies (from 0% to 94%): the average sensitivity was 56.2% (95% CI 29.5 to 79.8%) and average specificity was 99.5% (95% CI 98.1% to 99.9%; based on 8 evaluations in 5 studies on 943 samples). Data for individual antigen tests were limited with no more than two studies for any test. Rapid molecular assays Sensitivity showed less variation compared to antigen tests (from 68% to 100%), average sensitivity was 95.2% (95% CI 86.7% to 98.3%) and specificity 98.9% (95% CI 97.3% to 99.5%) based on 13 evaluations in 11 studies of on 2255 samples. Predicted values based on a hypothetical cohort of 1000 people with suspected COVID‐19 infection (with a prevalence of 10%) result in 105 positive test results including 10 false positives (positive predictive value 90%), and 895 negative results including 5 false negatives (negative predictive value 99%). Individual tests We calculated pooled results of individual tests for ID NOW (Abbott Laboratories) (5 evaluations) and Xpert Xpress (Cepheid Inc) (6 evaluations). Summary sensitivity for the Xpert Xpress assay (99.4%, 95% CI 98.0% to 99.8%) was 22.6 (95% CI 18.8 to 26.3) percentage points higher than that of ID NOW (76.8%, (95% CI 72.9% to 80.3%), whilst the specificity of Xpert Xpress (96.8%, 95% CI 90.6% to 99.0%) was marginally lower than ID NOW (99.6%, 95% CI 98.4% to 99.9%; a difference of −2.8% (95% CI −6.4 to 0.8)) Authors' conclusions This review identifies early‐stage evaluations of point‐of‐care tests for detecting SARS‐CoV‐2 infection, largely based on remnant laboratory samples. The findings currently have limited applicability, as we are uncertain whether tests will perform in the same way in clinical practice, and according to symptoms of COVID‐19, duration of symptoms, or in asymptomatic people. Rapid tests have the potential to be used to inform triage of RT‐PCR use, allowing earlier detection of those testing positive, but the evidence currently is not strong enough to determine how useful they are in clinical practice. Prospective and comparative evaluations of rapid tests for COVID‐19 infection in clinically relevant settings are urgently needed. Studies should recruit consecutive series of eligible participants, including both those presenting for testing due to symptoms and asymptomatic people who may have come into contact with confirmed cases. Studies should clearly describe symptomatic status and document time from symptom onset or time since exposure. Point‐of‐care tests must be conducted on samples according to manufacturer instructions for use and be conducted at the point of care. Any future research study report should conform to the Standards for Reporting of Diagnostic Accuracy (STARD) guideline. How accurate are rapid tests, performed during a health‐care visit (point‐of‐care), for diagnosing COVID‐19? Why is this question important? People with suspected COVID‐19 need to know quickly whether they are infected, so that they can self‐isolate, receive treatment, and inform close contacts. Currently, COVID‐19 infection is confirmed by sending away samples, taken from the nose and throat, for laboratory testing. The laboratory test, called RT‐PCR, requires specialist equipment, may require repeat healthcare visits, and typically takes at least 24 hours to produce a result. Rapid point‐of‐care tests can provide a result ‘while you wait’, ideally within two hours of providing a sample. This could help people isolate early and reduce the spread of infection. What did we want to find out? We were interested in two types of rapid point‐of‐care tests, antigen and molecular tests. Antigen tests identify proteins on the virus, often using disposable devices. Molecular tests detect the virus’s genetic material, using small portable or table‐top devices. Both test the same nose or throat samples as RT‐PCR tests. We wanted to know whether rapid point‐of‐care antigen and molecular tests are accurate enough to replace RT‐PCR for diagnosing infection, or to select people for further testing if they have a negative result. What did we do? We looked for studies that measured the accuracy of rapid point‐of‐care tests compared with RT‐PCR tests to detect current COVID‐19 infection. Studies could assess any rapid antigen or molecular point‐of‐care test, compared with a reference standard test. The reference standard is the best available method for diagnosing the infection; we considered RT‐PCR test results and clinically defined COVID‐19 as reference tests. People could be tested in hospital or the community. Studies could test people with or without symptoms. Tests had to use minimal equipment, be performed safely without risking infection from the sample, and have results available within two hours of the sample being collected. Tests could be used in small laboratories or wherever the patient is (in primary care, urgent care facilities, or in hospital). How did studies assess diagnostic test accuracy? Studies tested participants with the rapid point‐of‐care tests. Participants were classified as known to have – and not to have ‐ COVID‐19, by RT‐PCR in all studies. Studies then identified false positive and false negative errors in the point‐of‐care test results, compared to RT‐PCR. False positive tests incorrectly identified COVID‐19 when it was not present, potentially leading to unnecessary self‐isolation and further testing. False negatives missed COVID‐19 when it was present, risking delayed self‐isolation and treatment, and spread of infection. What we found We found 18 relevant studies. Ten studies took place in North America, four in Europe, two in South America, one in China and one in multiple countries. Nine studies deliberately included a high percentage of people with confirmed COVID‐19 or included only people with COVID‐19. Fourteen studies did not provide any information about the people providing the samples for testing and 12 did not provide any information about where people were tested. None of the studies reported includedsamples from people without symptoms. Main results Five studies reported eight evaluations of five different antigen tests. Overall, there was considerable variation between the results of the antigen tests in how well they detected COVID‐19 infection. Tests gave false positive results in less than 1% of samples. Thirteen evaluations of four different molecular tests correctly detected an average of 95% of samples with COVID‐19 infection. Around 1% of samples gave false positive results. If 1000 people had molecular tests, and 100 (10%) of them really had COVID‐19: ‐ 105 people would test positive for COVID‐19. Of these, 10 people (10%) would not have COVID‐19 (false positive result). ‐ 895 people would test negative for COVID‐19. Of these, 5 people (1%) would actually have COVID‐19 (false negative result). We noted a large difference in COVID‐19 detection between the two most commonly evaluated molecular tests. How reliable were the results of the studies? Our confidence in the evidence is limited. ‐ Three‐quarters of studies did not follow the test manufacturers’ instructions, so may have found different results if they had. ‐ Often, studies did not use the most reliable methods or did not report enough information for us to judge their methods. This may have affected estimates of test accuracy, but it is impossible to identify by how much. ‐ A quarter of studies were published early online as ‘preprints’ and are included in the review. Preprints do not undergo the normal rigorous checks of published studies, so we are uncertain how reliable they are. What are the implications of this review? Studies provided little information about their participants, so it is not possible to tell if the results can be applied to people with no symptoms, mild symptoms, or who were hospitalised with COVID‐19. Accurate rapid tests would have the potential to select people for RT‐PCR testing or to be used where RT‐PCR is not available. However, the evidence currently is not strong enough and more studies are urgently needed to be able to say if these tests are good enough to be used in practice. How up‐to‐date is this review? This review includes evidence published up to 25 May 2020. Because new research is being published in this field, we will update this review soon.
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            Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study

            Background Scarce data are available on what variables affect the risk of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the development of symptomatic COVID-19, and, particularly, the relationship with viral load. We aimed to analyse data from linked index cases of COVID-19 and their contacts to explore factors associated with transmission of SARS-CoV-2. Methods In this cohort study, patients were recruited as part of a randomised controlled trial done between March 17 and April 28, 2020, that aimed to assess if hydroxychloroquine reduced transmission of SARS-CoV-2. Patients with COVID-19 and their contacts were identified by use of the electronic registry of the Epidemiological Surveillance Emergency Service of Catalonia (Spain). Patients with COVID-19 included in our analysis were aged 18 years or older, not hospitalised, had quantitative PCR results available at baseline, had mild symptom onset within 5 days before enrolment, and had no reported symptoms of SARS-CoV-2 infections in their accommodation or workplace within the 14 days before enrolment. Contacts included were adults with a recent history of exposure and absence of COVID-19-like symptoms within the 7 days preceding enrolment. Viral load of contacts, measured by quantitative PCR from a nasopharyngeal swab, was assessed at enrolment, at day 14, and whenever the participant reported COVID-19-like symptoms. We assessed risk of transmission and developing symptomatic disease and incubation dynamics using regression analysis. We assessed the relationship of viral load and characteristics of cases (age, sex, number of days from reported symptom onset, and presence or absence of fever, cough, dyspnoea, rhinitis, and anosmia) and associations between risk of transmission and characteristics of the index case and contacts. Findings We identified 314 patients with COVID-19, with 282 (90%) having at least one contact (753 contacts in total), resulting in 282 clusters. 90 (32%) of 282 clusters had at least one transmission event. The secondary attack rate was 17% (125 of 753 contacts), with a variation from 12% when the index case had a viral load lower than 1 × 106 copies per mL to 24% when the index case had a viral load of 1 × 1010 copies per mL or higher (adjusted odds ratio per log10 increase in viral load 1·3, 95% CI 1·1–1·5). Increased risk of transmission was also associated with household contact (3·0, 1·59–5·65) and age of the contact (per year: 1·02, 1·01–1·04). 449 contacts had a positive PCR result at baseline. 28 (6%) of 449 contacts had symptoms at the first visit. Of 421 contacts who were asymptomatic at the first visit, 181 (43%) developed symptomatic COVID-19, with a variation from approximately 38% in contacts with an initial viral load lower than 1 × 107 copies per mL to greater than 66% for those with an initial viral load of 1 × 1010 copies per mL or higher (hazard ratio per log10 increase in viral load 1·12, 95% CI 1·05–1·20; p=0·0006). Time to onset of symptomatic disease decreased from a median of 7 days (IQR 5–10) for individuals with an initial viral load lower than 1 × 107 copies per mL to 6 days (4–8) for those with an initial viral load between 1 × 107 and 1 × 109 copies per mL, and 5 days (3–8) for those with an initial viral load higher than 1 × 109 copies per mL. Interpretation In our study, the viral load of index cases was a leading driver of SARS-CoV-2 transmission. The risk of symptomatic COVID-19 was strongly associated with the viral load of contacts at baseline and shortened the incubation time of COVID-19 in a dose-dependent manner. Funding YoMeCorono, Generalitat de Catalunya. Translations For the Catalan translation of the abstract see Supplementary Materials section.
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              Panbio antigen rapid test is reliable to diagnose SARS-CoV-2 infection in the first 7 days after the onset of symptoms

              Highlights • The new antigen tests for COVID19 are an indispensable tool in the control of the pandemic due to their adequate sensitivity and specificity. • The implementation of the point of care technique in primary care is feasible and has good results. • Less than 5 days of evolution of the onset of symptoms and CT less than 27 in the PCR would define the results of the antigen techniques.
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                Author and article information

                Journal
                CMAJ
                CMAJ
                9711805
                CMAJ : Canadian Medical Association Journal
                Joule Inc.
                0820-3946
                1488-2329
                29 March 2021
                03 March 2021
                : 193
                : 13
                : E447
                Affiliations
                Harvard Medical School (Liu), Boston, Mass.; Unity Health Toronto (Liu), St. Michael’s Hospital, Toronto, Ont.; Institute of Biomedical Engineering (Arora), University of Oxford, Oxford, UK; Department of Community Health Sciences (Arora), University of Calgary, Calgary, Alta.; Department of Pathology and Laboratory Medicine (Krajden), University of British Columbia; BC Centre for Disease Control (Krajden), Vancouver, BC.
                Author notes
                Correspondence to: Mel Krajden, Mel.Krajden@ 123456bccdc.ca
                Article
                193e447
                10.1503/cmaj.202827
                8099168
                33658245
                da409a8d-67f8-47b8-92c2-a55e213963fe
                © 2021 Joule Inc. or its licensors

                This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY-NC-ND 4.0) licence, which permits use, distribution and reproduction in any medium, provided that the original publication is properly cited, the use is noncommercial (i.e., research or educational use), and no modifications or adaptations are made. See: https://creativecommons.org/licenses/by-nc-nd/4.0/

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