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Created July 6, 2022, last updated over 2 years ago.
Collection: 153, Score: 555, Trend score: 0, Read count: 2310, Articles count: 10, Created: 2022-07-06 00:01:49 UTC. Updated: 2022-07-06 03:40:15 UTC.Notes
Early in the COVID pandemic, diagnostic testing relied entirely on precise-but-expensive PCR testing. Late in 2020 the Lateral Flow Testing techniques, already widely used for home-pregnancy tests and similar, were developed for SARS-CoV-2 antigens, leading to COVID-19 Rapid Antigen Tests (RATs).
While cheap, scalable and able to give a result in 10-15 minutes, they were initially seen mostly as a supplement to PCR testing, with less accuracy. Although true that RATs have lower sensitivity than SARS-CoV-2 PCR – most licensed-RATs have sensitivity 80-95% – today this is both less important, and possibly even a strength of RATs over PCR.
Early in the pandemic the role of testing has primarily about diagnosis, in those either symptomatic or pre-symptomatic. Viral presence was practically assumed to be synonymous with contagion. Today with over half a billion cumulative COVID cases worldwide and counting, along with access to effective vaccines and antivirals, it is often more useful to know whether an individual is infectious or not at a discrete moment in time.
Growing research over the last 12 months shows that adequately-sensitive RATs are effective at identifying infectious individuals, even if the high-sensitivity of PCR testing identifies viral particles in those who are infected but otherwise non-infectious (either pre-infectious, or post-infectious with ongoing viral shedding).
PCR positive results with cycle thresholds (ie. number of thermal cycles of RNA replication required before fluorescence is detected) above 25-30 have good correlation with being non-infectious (ie. unable to culture virus). Adequately approved & validated RATs (by FDA, TGA, MHRA, etc.) have very high sensitivity at CT less than this 25-30 range, depending on the study and specific manufacturer.
The bottom line...
An adequately-validated RAT, when correctly performed, is likely a sensitive indictor of individual infectiousness at that specific moment in time. The reliability of a negative RAT will be improved if using the same manufacturer and technique as a RAT previously positive test, and more so if there are several subsequent negative RATs.
Daniel Jolley
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Collected Articles
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Various RATs investigated showed 99% sensitivity in identifying contagious COVID patients (PCR Ct < 30).
pearl -
The Lancet. Microbe · Sep 2021
Comparative performance of SARS-CoV-2 lateral flow antigen tests and association with detection of infectious virus in clinical specimens: a single-centre laboratory evaluation study.
Lateral Flow Devices / RATs show good correlation with low cycle threshold PCR results, and subsequent viral culture, indicating excellent sensitivity for identifying contagious individuals.
pearl -
The Lancet. Microbe · Apr 2021
Comparison of seven commercial SARS-CoV-2 rapid point-of-care antigen tests: a single-centre laboratory evaluation study.
SARS-CoV-2 rapid antigen test positivity correlates well with viral loads typically seen in the infectious period. RATs may be useful tools to discriminate between infectious and non-infectious individuals.
pearl -
Although labelled as 'false' negative, negative RATs associated with positive PCR results, were in this study all in low viral-concentration patients, determined by a high PCR cycle threshold (ie. number of thermal cycles of RNA replication required before fluorescence is detected).
High PCR CT / calculated low-viral-concentration, correlates well with low/non-infectivity of the individual. This does not mean that they may not become infectious in the future, but rather means that at that point in time they are unlikely to be infectious.
summary -
Reverse transcriptase-polymerase chain reaction (RT-PCR), the reference laboratory method of confirmed SARS-CoV-2 diagnosis, though requiring equipment, is time-consuming. There is a crucial demand for rapid techniques such as antigen detection test during the pandemic. This study assessed whether a rapid antigen detection (RAD) test was an effective and essential method for the early diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the COVID-19 pandemic. The probability of public screening at home and the application of RAD during the novel SARS-CoV-2 outbreak were also topics of interest. ⋯ We concluded that RAD could be a quick and feasible method to identify individuals infected with SARS-CoV-2 from non-contagious individuals during the COVID-19 outbreak. A RAD test was an effective and essential method for the early diagnosis of SARS-CoV-2 during the COVID-19 pandemic.
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Cochrane Db Syst Rev · Mar 2021
Review Meta AnalysisRapid, point-of-care antigen and molecular-based tests for diagnosis of SARS-CoV-2 infection.
Accurate rapid diagnostic tests for SARS-CoV-2 infection could contribute to clinical and public health strategies to manage the COVID-19 pandemic. Point-of-care antigen and molecular tests to detect current infection could increase access to testing and early confirmation of cases, and expediate clinical and public health management decisions that may reduce transmission. ⋯ Antigen tests vary in sensitivity. In people with signs and symptoms of COVID-19, sensitivities are highest in the first week of illness when viral loads are higher. The assays shown to meet appropriate criteria, such as WHO's priority target product profiles for COVID-19 diagnostics ('acceptable' sensitivity ≥ 80% and specificity ≥ 97%), can be considered as a replacement for laboratory-based RT-PCR when immediate decisions about patient care must be made, or where RT-PCR cannot be delivered in a timely manner. Positive predictive values suggest that confirmatory testing of those with positive results may be considered in low prevalence settings. Due to the variable sensitivity of antigen tests, people who test negative may still be infected. Evidence for testing in asymptomatic cohorts was limited. Test accuracy studies cannot adequately assess the ability of antigen tests to differentiate those who are infectious and require isolation from those who pose no risk, as there is no reference standard for infectiousness. A small number of molecular tests showed high accuracy and may be suitable alternatives to RT-PCR. However, further evaluations of the tests in settings as they are intended to be used are required to fully establish performance in practice. Several important studies in asymptomatic individuals have been reported since the close of our search and will be incorporated at the next update of this review. Comparative studies of antigen tests in their intended use settings and according to test operator (including self-testing) are required.
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The COVID-19 pandemic has highlighted the need for rapid, cost effective and easy-to-use diagnostic tools for SARS-CoV-2 infections that can be used in point of care settings to limit disease transmission. ⋯ The results indicate that the rapid antigen tests, especially the Panbio tests may be a valuable tool to detect contagious persons during the ongoing pandemic.
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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can spread from symptomatic patients with COVID-19, but also from asymptomatic individuals. Therefore, robust surveillance and timely interventions are essential for the control of virus spread within the community. In this regard the frequency of testing and speed of reporting, but not the test sensitivity alone, play a crucial role. ⋯ Numerous antigen tests are available for SARS-CoV-2 testing and their performance to detect infectious individuals may vary. Head-to-head comparison along with cell culture testing for infectivity may prove useful to identify better performing antigen tests. The antigen test analyzed in this study is easy-to-use, inexpensive, and scalable. It can be helpful in monitoring infection trends and thus has potential to reduce transmission.
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Diagnostics is crucial for a prompt identification of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infected patients, their isolation and treatment. Real-time PCR is the reference method for the diagnosis of SARS-CoV-2 infection; however, the unprecedented increase in the number of infections worldwide calls for faster and easy methods that do not require skilled personnel and special equipment. Rapid antigen tests have been developed and used as first line screening. ⋯ The level of agreement between the two tests was poor, k = 0.164. The Ag test performs well in the presence of high viral loads, whereas lower levels are missed. Considering the poor sensitivity of the method, real-time PCR remains the gold standard as front line screening for SARS-CoV-2 infection.
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