Fact Check: How Accurate Is a Rapid Antigen Test?
Rapid antigen tests provide quick and cheap results, however, their accuracy has recently come under fire. Depending on the brand and timing of the test, sensitivity can range from a low of 45% to a high of almost 97%. Despite the inconsistent performance, antigen tests remain an important tool for the quick detection of COVID-19. Keep reading to learn more about how antigen tests work, when they’re most accurate, and when to use them instead of a PCR.
How does a rapid antigen test work?
The rapid antigen test kit is a simple hand-held device. One end is an absorbent pad, where you place the swab specimen. On the other end is a window that displays the results. The test is triggered as positive when viral proteins are detected. It can be used as a home test; no laboratory equipment is needed. Within minutes, you receive an easy-to-understand test result.
Antigen vs RT PCR
A PCR test is more accurate than the rapid antigen test. Lab technicians isolate the virus’s genetic material and make many copies of it before analysis. This allows the PCR test to detect even tiny amounts of coronavirus, making it a more sensitive test. It can detect the coronavirus even when viral load is low. Rapid antigen tests do not amplify the virus by making copies. To trigger a positive test, you need a sufficient amount of the virus in your system.
How accurate are rapid antigen tests?
If you take a rapid antigen test too early, when viral load is low, you might receive a false negative. Research suggests that the overall sensitivity of rapid antigen tests is 85%. That means they detect around 85% of positive cases, while 15% are missed.
If antigen tests are less accurate, why are they used?
Let’s unpack those numbers. The 85% sensitivity refers to the testing of both asymptomatic and symptomatic people. If you have symptoms and a high viral load, antigen tests are still very good at catching COVID infection.
“The more virus you have in your nose, the more virus you’re breathing out into the air, and the more virus other people can then breathe in,” said Dr. Gigi Gronvall, a testing expert at John Hopkins University. “The tests are very accurate, and correlate very well with PCR when people are most infectious.”
Rapid antigen tests also have speed, convenience, and affordability on their side. Easy-to-read results are delivered in minutes, at the point of care. Instead of waiting days for test results, people who test positive can isolate immediately to protect others. Rapid antigen tests also cost a fraction of the price of the PCR test.
When and how should I use the rapid antigen test?
If you have symptoms, you can test immediately. If you’ve been exposed to coronavirus, but have no symptoms, wait 3-5 days before testing. Rapid antigen tests are best for:
- Symptomatic patients. Rapid testing can identify an active infection sooner, helping patients isolate before they spread the virus to others.
- Group settings. Rapid testing can detect infected people before they expose others at school, work, and events.
- Quick results. Elderly and immunocompromised people may need quick medical intervention to ward off severe illness.
- Affordable and convenient testing. Rapid antigen tests are cheaper than a PCR test and can be performed anywhere.
- Recovered patients. A PCR test may continue to detect the virus many weeks after COVID recovery. If you’ve recovered from COVID in the last 90 days, an antigen test is better for detecting active infection.
Repeat testing improves rapid test accuracy
Repeat testing can make up for the lower sensitivity of antigen tests. A recent study investigated the COVID surveillance program at the University of Illinois. Researchers found that serial testing of students every three days identified 98% of infections. This is almost equal to the accuracy of a PCR test result.
How to choose the right type of antigen test
- Covid test brands: Some COVID test brands perform better than others. For example, the Roche Rapid Antigen Test and Abbot Panbio have a sensitivity of 96.52%, and 98.1% respectively, outperforming the average rapid test accuracy. Before purchasing a test, you can check the manufacturer’s information for test sensitivity and specificity. Sensitivity refers to the test’s ability to accurately detect the presence of the virus. Specificity refers to how likely the test is to give a false positive result.
- Collection method: Many people worry whether the method of collection matters. There's no advantage in getting a nasopharyngeal swab over a nasal tip swab test, as they both collect the same fluids.
- Trustworthy vendor: The surge in Omicron cases has led to a corresponding flood of counterfeit tests in the Philippines. How can you be sure your antigen test is legitimate? The supply chain is an important factor. Look for a vendor who sources their tests directly from the manufacturer. Buyers should also be mindful of proper test handling and storage. Heat is a perennial problem in the Philippines and many antigen test kits do not work properly over 25° C. Find a vendor that guarantees temperature-controlled tests.
Do rapid antigen tests detect Omicron?
The reliability of rapid antigen testing has been hotly debated since the arrival of Omicron. Based on preliminary research, the US Food and Drug administration released an update on the tests’ accuracy. “Early data suggests that antigen tests do detect the Omicron variant but may have reduced sensitivity,” the agency said.
Reduced sensitivity means the tests will miss more positive cases. Just how many more is uncertain as research is ongoing. But that doesn’t mean the tests are ineffective. On CNN, Dr. Fauci offered reassurance, saying, "The tests are still worthwhile. Don't let anybody think that the FDA was saying that tests are no longer good. They say they're less sensitive now. They never were 100% sensitive."
Note that neither PCR or rapid antigen test kit can tell which variant you’re infected with. Genomic sequencing is needed to identify whether you have Omicron, Delta, or any other strain of COVID.
I tested negative. What should I do?
Rapid antigen tests offer a window in time. The best way to prevent a false negative result is to test serially. Even if you don’t have symptoms, health experts recommend taking a second test to confirm negative results. "If that test is negative, all that's telling you is: At this point in time, you don't have a ton of virus in you," said Dr. Abraar Karan, an infectious disease specialist at Stanford University.
If your test is negative but you are experiencing symptoms–assume you are infected. Self-isolate as you await the results of a confirmatory PCR test.
I tested positive. What should I do?
If you receive a positive result, stay home and isolate yourself. Most positive patients suffer mild illness and can recover at home with rest, hydration, and OTC medicines. You should not leave your home except to receive medical care. Alternatively, you can consult a doctor online and avoid exposing others. If you experience any of the following emergency symptoms, seek urgent care: trouble breathing, persistent pain or pressure in the chest, confusion, inability to stay awake, or discoloration of the skin or lips (blue, grey, or pale color).
After your quarantine period ends, you can use use the antigen test to ensure you're no longer infectious. The less sensitive antigen test is best for this purpose. This is because the RT-PCR can detect tiny pieces of coronavirus long after you've recovered. According to the CDC, you may test positive up to 90 days after recovery if using the PCR test.
More Covid Testing Options
Still not sure which test you need? Learn more in our in-depth guide to different COVID tests, including test accuracy, collection methods, and which tests are available in the Philippines.