Rapid antigen tests have become an important tool for detecting COVID-19 infections quickly and conveniently. However, questions remain about just how accurate these tests are. This article will examine how rapid antigen tests work, factors that impact their accuracy, and how accurate they are compared to PCR tests.
How do rapid antigen tests work?
Rapid antigen tests, sometimes called rapid diagnostic tests, detect viral proteins called antigens. They work by using antibodies that are designed to bind to specific antigens on the surface of the SARS-CoV-2 virus. The tests can provide results in as little as 15 minutes.
To perform a rapid antigen test, a nasal or throat swab is taken. This sample is applied to the test strip, which contains the antibodies. If SARS-CoV-2 antigens are present, they will bind to the antibodies on the test strip and generate a positive result. This is indicated by a colored line on the test strip. If no antigens are detected, the test will be negative.
What factors impact the accuracy of rapid antigen tests?
There are several factors that can influence the accuracy of rapid antigen tests:
- Viral load – Rapid tests are less accurate when viral load is low. They perform best when viral load is at its peak, which is usually around the time that symptoms first begin.
- Sample collection – Taking the nasal/throat swab correctly is imperative for getting an accurate result. Self-collected samples may be less accurate than those collected by a healthcare worker.
- Variant strains – Some data indicates that rapid tests may be slightly less sensitive to new variants such as Omicron and its subvariants, though more research is needed.
- Duration of infection – Rapid tests are most accurate 2-7 days after symptom onset or exposure. Accuracy may decline outside of this time window.
- Proper usage – Reading test results at the correct time window and following all instructions impacts accuracy.
- Environmental factors – Temperature and humidity can influence performance if tests are stored incorrectly.
These factors help explain why real-world accuracy of rapid tests is highly variable. However, overall rapid tests remain useful as an initial screening tool when interpreted carefully.
How accurate are rapid tests compared to PCR tests?
Polymerase chain reaction (PCR) tests are considered the gold standard for COVID-19 detection. Evaluating rapid tests against PCR provides insights about accuracy:
- Sensitivity – This measures how often a test correctly generates a positive result when SARS-CoV-2 is present. Rapid tests have moderately high sensitivities of 80-90% on average, compared to 95-100% for PCR.
- Specificity – This measures how often a test correctly gives a negative result when no SARS-CoV-2 is present. Rapid tests have very high specificities around 99%.
- Positive predictive value (PPV) – This measures how likely a positive test reflects a true positive result. PPV is influenced by the underlying prevalence of disease. When COVID rates are low, more positive rapid tests will be false positives.
- Negative predictive value (NPV) – This measures how likely a negative test reflects a true negative result. The NPV of rapid tests remains high, around 99%, meaning negative rapid test results are usually accurate.
Based on these metrics, rapid antigen tests have moderately high accuracy compared to PCR when used properly. Their high specificity means positive results can be trusted. Negative results may need confirmatory testing in some situations due to the potential for false negatives.
How accurate are rapid tests in asymptomatic people?
Most data on rapid test accuracy comes from symptomatic individuals. Some evidence suggests rapid tests may be less sensitive for detecting COVID-19 infections in people without symptoms.
One meta-analysis found that rapid tests detected only 58% of asymptomatic infections that were confirmed PCR-positive. However, other studies have reported higher sensitivity in the 70-80% range.
Reduced accuracy in asymptomatic individuals is likely due to lower average viral loads. However, those with high viral loads can still test positive even without symptoms. Repeating rapid tests over several days improves detection in asymptomatic cases.
How accurate are self-administered rapid tests?
Many rapid antigen test kits are designed for self-use at home. Studies show that consumer-collected nasal swabs generate similar results to those performed by healthcare staff if instructions are followed correctly. However, mistakes in sample collection and test execution are more likely with self-tests.
The FDA requires at-home rapid tests to demonstrate at least 90% agreement with PCR results in studies. Despite slightly lower real-world accuracy, the FDA granted emergency use authorization to many self-administered rapid tests due to their usefulness for routine screening.
Can rapid test results be trusted?
No diagnostic test is perfect, but rapid antigen tests can provide reliable results under certain conditions:
- Positive results are highly accurate, particularly when viral load is high in symptomatic people.
- Negative results are fairly accurate in most cases, but negative results may need confirmed with PCR testing if COVID-19 symptoms are present or a close contact is positive.
- Testing multiple times over several days improves detection rates and can identify early infections.
- Understanding the test’s limitations for accuracy in asymptomatic cases is important.
- Reading test results within the correct time window specified is critical for test accuracy.
While PCR remains the gold standard, rapid antigen tests have acceptable accuracy for widespread daily screening. Combining rapid testing with other mitigation measures can help reduce COVID-19 transmission.
Conclusion
Rapid antigen tests are a useful tool for detecting COVID-19 infections due to their quick results, ease of use, and reasonably high accuracy compared to PCR tests. Real-world performance is impacted by several variables including proper usage, viral load, symptom status, and testing frequency. While limitations exist, rapid tests can play an important role in screening and isolation strategies when results are carefully interpreted.