Covid antigen test
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COVID-19 Antigen Test: An Overview
Introduction to COVID-19 Antigen Tests
COVID-19 antigen tests are designed to detect specific proteins from the SARS-CoV-2 virus, providing a rapid diagnostic tool for identifying active infections. These tests are particularly useful in various settings due to their quick turnaround time and ease of use.
Performance and Accuracy of Antigen Tests
Sensitivity and Specificity
The sensitivity of COVID-19 antigen tests, which measures the ability to correctly identify those with the infection, varies significantly. Studies have shown that the sensitivity ranges from 30.2% to 80% in symptomatic individuals and from 28.6% to 77.8% in asymptomatic individuals1 2 4 9. Specificity, which measures the ability to correctly identify those without the infection, is consistently high, often exceeding 99%2 4 7.
Factors Influencing Test Performance
The performance of antigen tests is influenced by several factors, including the timing of the test relative to symptom onset, the viral load in the sample, and the specific brand of the test used. Sensitivity is generally higher in the first week after symptom onset when viral loads are higher2 4. Additionally, tests used according to manufacturer instructions tend to perform better2.
Use in Symptomatic vs. Asymptomatic Individuals
Antigen tests are more accurate in symptomatic individuals, with higher sensitivity observed in this group compared to asymptomatic individuals1 2 4. For symptomatic patients, the sensitivity can be as high as 80% in the first week of symptoms2. In contrast, the sensitivity in asymptomatic individuals is lower, often around 54.7%2 4.
Repeat Testing and Prevalence Impact
Repeat Testing
Modeling suggests that repeat antigen testing can increase sensitivity compared to a single test, although empirical data to support this is limited1 3. Repeated testing, especially in high-prevalence settings, can help mitigate the lower sensitivity of single tests by catching infections that might be missed initially3.
Prevalence and Predictive Values
The prevalence of COVID-19 in the community affects the predictive values of antigen tests. In high-prevalence settings, the positive predictive value (PPV) is higher, meaning that a positive test result is more likely to be a true positive. Conversely, in low-prevalence settings, the negative predictive value (NPV) is higher, meaning that a negative test result is more likely to be a true negative3 8.
Practical Applications and Recommendations
Point-of-Care and Laboratory-Based Testing
Antigen tests are valuable for point-of-care testing due to their rapid results, which can be available in approximately 15 minutes1 4 7. They are particularly useful in settings where timely molecular testing is not feasible, such as in rural or resource-limited areas5 10.
Recommendations for Use
The Infectious Diseases Society of America (IDSA) recommends using FDA-authorized antigen tests with high specificity and moderate sensitivity, especially when rapid RT-PCR or laboratory-based NAAT is not available1. For symptomatic individuals, confirmatory testing with RT-PCR is advised if the antigen test result is negative, and for asymptomatic individuals, if the antigen test result is positive4 8.
Conclusion
COVID-19 antigen tests offer a rapid and accessible means of diagnosing SARS-CoV-2 infections, particularly in symptomatic individuals and in settings where timely molecular testing is not feasible. While their sensitivity varies, their high specificity makes them a useful tool in the ongoing management of the COVID-19 pandemic. Repeat testing and awareness of community prevalence can further enhance their utility in controlling the spread of the virus.
Sources and full results
Most relevant research papers on this topic
The Infectious Diseases Society of America Guidelines on the Diagnosis of COVID-19: Antigen Testing.
Rapid RT-PCR or laboratory-based NAAT remains the testing method of choice for diagnosing SARS-CoV-2 infection, but rapid lateral flow assays can be used for point-of-care diagnosis when molecular testing is unavailable or logistically challenging.
Systematic ReviewRapid, point‐of‐care antigen tests for diagnosis of SARS‐CoV‐2 infection
Rapid, point-of-care antigen tests for diagnosing SARS-CoV-2 infection show potential for increasing access to testing and reducing transmission, but more research is needed.
Systematic Review
Very Rigorous Journal
Highly CitedDIAGNOSTIC STRATEGIES FOR ENDEMIC CORONAVIRUS DISEASE 2019 (COVID-19): RAPID ANTIGEN TESTS, REPEAT TESTING, AND PREVALENCE BOUNDARIES.
Repeated testing of COVID-19 rapid antigen tests can improve performance and extend prevalence boundaries, while avoiding contagion in packed spaces.
Systematic ReviewPerformance of an Antigen-Based Test for Asymptomatic and Symptomatic SARS-CoV-2 Testing at Two University Campuses — Wisconsin, September–October 2020
The Sofia SARS Antigen FIA test is a low-cost, rapid test for SARS-CoV-2 detection, but its sensitivity and positive predictive value are lower than real-time RT-PCR.
Observational StudyHighly CitedDiagnostic Accuracy of Rapid Antigen Tests for COVID-19 Detection: A Systematic Review With Meta-analysis
Rapid antigen tests (RAT) are effective for early COVID-19 detection, particularly in developing nations with limited resources and laboratory equipment, but negative samples may need confirmation using molecular tests.
Meta-AnalysisTesting for SARS-CoV-2 (COVID-19): a systematic review and clinical guide to molecular and serological in-vitro diagnostic assays
This review highlights the importance of understanding molecular and serological diagnostic assays for SARS-CoV-2 and COVID-19, as they inform crucial healthcare decisions and public health strategies.
Systematic ReviewHighly CitedCOVID-19 Antibody Tests and Their Limitations
Current COVID-19 antibody tests have limitations, requiring improvements in assay formats, viral antigen selection, and human antibody isotopes for improved sensitivity and specificity.
Highly CitedEvaluation of Abbott BinaxNOW Rapid Antigen Test for SARS-CoV-2 Infection at Two Community-Based Testing Sites — Pima County, Arizona, November 3–17, 2020
The BinaxNOW rapid antigen test offers quick results and lower costs for identifying SARS-CoV-2 infections, aiding in rapid infection identification and isolation.
Observational StudyHighly CitedLow performance of rapid antigen detection test as frontline testing for COVID-19 diagnosis
Abstract Background Ensuring accurate diagnosis is essential to limit the spread of the SARS-CoV-2 and for the clinical management of COVID-19. Although real-time reverse transcription polymerase chain reaction (RT- qPCR) is the current recommended laboratory method to diagnose SARS-CoV-2 acute infection, several factors such as requirement of special equipment and skilled staff limit the use of these time-consuming molecular techniques. Recently, several easy to perform rapid antigen detection tests were developed and recommended in some countries as the first line of diagnostic. Objectives The aim of this study was to evaluate the performances of the Coris COVID-19 Ag Respi-Strip. Study design. We performed a comparison study by testing nasopharyngeal samples with RT-qPCR and antigen rapid test. Results 148 nasopharyngeal swabs were tested. Amongst the 106 positive RT-qPCR samples, 32 were detected by the rapid antigen test, given an overall sensitivity of 30.2%. All the samples detected positive with the antigen rapid tests were also positive with RT-qPCR. Conclusions Highest viral load is associated with better antigen detection rates. Unfortunately, the overall poor sensitivity of the COVID-19 Ag Respi-Strip does not allow using it alone as the frontline testing for COVID-19 diagnosis.
Observational StudyHighly CitedDiagnostic Accuracy of Rapid Antigen Test Kits for Detecting SARS-CoV-2: A Systematic Review and Meta-Analysis of 17,171 Suspected COVID-19 Patients
Rapid antigen test kits show 99.4% specificity and 68.4% sensitivity in detecting SARS-CoV-2, making them a viable alternative for developing nations and rural areas.
Meta-Analysis
Rigorous JournalHighly Cited