How Accurate Are COVID-19 Rapid Antigen Tests?
At the onset of the pandemic, getting tested for COVID-19 typically required visiting a healthcare institution, a diagnostic laboratory, or a specialized test center. In the process, many people had to wait in long lines and wait at least a few days to get their results. Especially in the face of more transmissible variants and upsurge of positive cases, the advent of at-home tests has been welcome news. For the first time during the pandemic, at-home tests enabled people to test themselves whenever, wherever, as often as they need, and get their results in minutes. Along with the comfort and convenience they offer to wide populations, these tests also help reduce the burden on healthcare institutions, sources, and healthcare personnel.
However, as the research advances, questions remain regarding the accuracy of at-home tests, especially relative to PCR testing. Here’s a brief look at what we know about the accuracy of at-home tests and what you can do to maximize the accuracy of your test.
Remind Me, What’s an At-Home COVID-19 Test and How is it Different from a PCR Test?
Polymerase chain reaction (PCR) testing refers to a laboratory method which uses matching primers to copy and amplify specific segments of a viral genome. By generating millions of copies of a small amount of genetic material, PCR testing helps determine whether a pathogen such as SARS-CoV-2 is present in each nasal or throat sample. Although they are very sensitive, PCR tests require specialized personnel, as well as laboratory equipment and technology. The samples need to be sent to a laboratory for analysis, and the tests may take up to several days to return results.
Unlike PCR tests which detect viral genetic material, at-home tests are rapid chromatographic immunoassays that qualitatively detect SARS-CoV-2-specific surface or nucleocapsid proteins, or antigens, present in a nasal or saliva specimen. Like PCR tests, at-home tests can only detect a current infection. In other words, they cannot inform you on a previous infection or asses your level of immunity against SARS-CoV-2.
Advantages of At-Home Testing over PCR Testing
However, while PCR testing is a complex procedure that typically requires laboratory technology, equipment, and personnel; at-home tests come with a ready-made kit with a reagent containing SARS-CoV-2-specific antibodies. That’s all you need to conduct the test in one package. In fact, at-home tests are deliberately designed for self-administration. They are easy to transport, to store, to use, and to interpret. They can be conducted by anyone, anywhere, whenever needed. What’s more, the results only take 15 to 30 minutes to deliver, so the entire process can start and be completed at the same place and in minutes.
At-home tests are available regardless of your vaccination status and whether you have symptoms. You can purchase them without prescription online, or in pharmacies, and retail stores.
Should You Trust the Results of an At-Home COVID-19 Test?
The overall accuracy of a diagnostic test is typically shaped by its sensitivity and specificity. Sensitivity of a diagnostic test refers to its ability to correctly detect the pathogen in people who are infected. A low sensitivity may result in false-negative results that report infected people as healthy. On the other hand, specificity refers to the ability of a diagnostic test to correctly identify the people that are not infected by that specific pathogen. Accordingly, a low specificity may lead to false-positive results that report healthy people as infected. A test is accurate if it combines a high level of sensitivity and a high level of specificity.
In terms of specificity, at-home tests prove to be highly reliable. In March 2021 a scientific review of several studies demonstrated that rapid antigen tests like at-home tests correctly reported a positive COVID-19 result in 99.6 percent of people. Therefore, receiving a false-positive result is very unlikely and if you test positive on an at-home test, it is very likely that you have an active infection with SARS-CoV-2.
However, as at-home COVID-19 tests detect antigens instead of viral genetic material and as these tests do not involve an amplification process, they require relatively more of the virus in your sample to deliver a positive result than a PCR test, resulting in relatively lower levels of sensitivity. There is a chance that an at-home tests may give false-negative results if the viral load is not as high, such as when they are used on asymptomatic patients or at the very early onset of an infection.
Therefore, while a negative result on an at-home test indicates that it is unlikely that you have COVID-19 at that time of the testing, it does not rule out the possibility altogether. After an at-home test, your healthcare provider may perform additional PCR testing to rule out or confirm your diagnosis. Especially if you have COVID-like symptoms that persist or worsen despite testing negative on an at-home test, it is a good idea to repeat the test a few days later or to follow up with PCR test.
Recent Studies
Indeed, recent studies have demonstrated that performing serial rapid antigen tests may overcome their relative drawback in sensitivity. In one study, researchers tested infected college students and employees every three days and found that rapid antigen tests successfully detected 98% of infections, which indicates a level of sensitivity equivalent with PCR testing.
Moreover, while they might not be as sensitive as PCR tests, at-home tests prove to be extremely effective in the detection of people who have high enough viral loads to transmit the virus. In other words, the results of these tests are excellent indicators of a person’s infectiousness. Thus, even if you get a false-negative result, it is very likely that your viral load is not high enough for you to spread the virus.
In fact, several experts argue that rapid antigen tests such as at-home tests may be more useful than PCR tests for the determination of infectiousness. As PCR tests can detect very low levels of viral load, they may continue to give positive results even after the end of the infectious period of the disease, which makes it difficult to differentiate between infectious and non-infectious patients. Especially as we begin to reopen public spaces and return to our daily activities, consistent information on whether we are infectious or not, is a particularly valuable insight.
Conclusion
Altogether, there is growing evidence to suggest that while PCR testing might be better from a clinical perspective, rapid antigen tests such as at-home tests may be more beneficial from a public health standpoint. For instance, performing both PCR tests and rapid antigen tests on 6000 patients at the Baltimore Convention Center Field Hospital during a 10-day period in 2021, Siddiqui and colleagues demonstrated that rapid antigen tests prove to have sufficient accuracy, specificity, and reliability for detecting SARS-CoV-2 in a high-volume setting. Accordingly, the researchers argued that while PCR testing may be superior from a clinical standpoint, rapid antigen tests may be better for public health as they are easier to distribute, apply, and process enabling the cost-effective screening of large populations in a short time.
References
- Crozier, A., Rajan, S., Buchan, I., & McKee, M. (2021). Put to the test: Use of rapid testing technologies for COVID-19. BMJ, n208. https://doi.org/10.1136/bmj.n208
- Smith, R. L., Gibson, L. L., Martinez, P. P., Ke, R., Mirza, A., Conte, M., Gallagher, N., Conte, A., Wang, L., Fredrickson, R., Edmonson, D. C., Baughman, M. E., Chiu, K. K., Choi, H., Jensen, T. W., Scardina, K. R., Bradley, S., Gloss, S. L., Reinhart, C., . . . Brooke, C. B. (2021). Longitudinal assessment of diagnostic test performance over the course of acute SARS-CoV-2 infection. The Journal of Infectious Diseases, 224(6), 976–982. https://doi.org/10.1093/infdis/jiab337
