Rapid Antigen Tests (RATs), commonly used for detecting COVID-19, offer varying degrees of accuracy. Their effectiveness depends on several factors, including the timing of the test relative to symptom onset and the viral load present.
Accuracy Varies Depending on Several Factors
RAT accuracy isn't a single number. Studies show that RATs are most accurate when used within the first week of symptom onset. A study from Cochrane found an average accuracy of 82% during this period for confirmed cases. However, the accuracy decreases as the infection progresses. Furthermore, a negative result doesn't guarantee the absence of infection, especially early in the infection process. The FDA notes that antigen tests are less accurate than molecular tests (like PCR tests) and may miss the virus early in infection.
- Sensitivity: The ability of a RAT to correctly identify those with the virus. Studies show variability, with lower sensitivity potentially leading to false negatives. A study reported that for 177 positive COVID-19 cases, only 145 (81.9%) were accurately identified as positive using RATs https://www.cidrap.umn.edu/covid-19/researchers-report-covid-home-tests-accurate-same-tests-given-clinician.
- Specificity: The ability of a RAT to correctly identify those without the virus. A study illustrates this with an example: out of 100 people without COVID-19, a test with 98% specificity would correctly identify 98 as negative https://health.anu.edu.au/news-events/news/how-accurate-your-rat-3-scenarios-show-it%E2%80%99s-about-more-looking-lines.
- Timing of Testing: Accuracy is highest when testing early in the course of illness. A high viral load increases the chance of a positive result.
- Test Technique: Incorrect test administration can significantly affect results. Following the instructions carefully is critical for accurate results https://healthmatch.io/blog/your-technique-can-impact-your-rat-test-accuracy-how-to-know-if-youre.
RAT Usage in Australia
The high prevalence of RAT use is evident in Australia, where 93% of those who tested positive for COVID-19 in 2023 used a RAT to confirm their diagnosis https://www1.racgp.org.au/newsgp/clinical/what-are-the-most-reliable-rapid-antigen-tests and https://www.tga.gov.au/products/covid-19/covid-19-tests/post-market-review-antigen-and-rapid-antigen-tests. This highlights the widespread reliance on RATs despite variations in their accuracy. One concerning study showed complete failure of one Canadian RAT to detect COVID-19 protein at any concentration.
