At-home COVID-19 tests are typically rapid antigen tests, which differ fundamentally from lab-based polymerase chain reaction (PCR) tests. While home tests offer speed and convenience, providing a result in about 15 minutes, their reliability depends heavily on the circumstances of use. Key factors include the timing of the infection and the amount of virus present in the person being tested. To understand what a result means, one must consider the specific metrics used to evaluate any diagnostic tool.
Understanding the Metrics of Test Performance
The performance of any medical test is measured using sensitivity and specificity. Sensitivity is the measure of a test’s ability to correctly identify a person who actually has the infection, meaning it correctly detects true positive cases. For COVID-19 rapid antigen tests, sensitivity often falls around 70% to 75% when compared to highly sensitive PCR tests. This range indicates that for every 100 people who have the virus, the rapid test may miss between 25 and 30 of them, resulting in what is known as a false negative.
Specificity measures the test’s ability to correctly identify a person who does not have the infection, correctly identifying true negative cases. The specificity of home antigen tests is consistently high, typically ranging between 98% and 99%. Because of this high specificity, a positive result from a home test is generally considered highly reliable, as a false positive is a rare occurrence. The lower sensitivity, however, is the primary reason for the common recommendation to retest after a negative result, as the test may have missed an early infection.
Real-World Factors Affecting Test Reliability
The theoretical accuracy metrics are complicated by real-world variables, making the user’s execution and timing critical to the result. The most important factor influencing a home test’s reliability is the viral load, which is the amount of SARS-CoV-2 virus particles present in the nasal sample. Antigen tests detect a protein on the surface of the virus and require a relatively high concentration of this protein to trigger a positive result.
If the viral load is low—such as in the very first or last days of an infection—the test may not be sensitive enough to detect it. Timing of testing directly correlates with viral load, as the amount of virus typically peaks around the time symptoms begin or within the first week of illness. A test taken too early after exposure, before the viral concentration has built up, is much more likely to result in a false negative.
The accuracy is also heavily influenced by the sample collection technique used at home. Improper swabbing, such as not inserting the swab far enough or not rotating it for the specified time, can lead to insufficient collection of the viral sample. If the sample collected does not contain enough viral antigen, a negative result will be displayed even if the person is actively infected. Following the instructions precisely, including the required waiting period before reading the result, minimizes user error.
Navigating Positive and Negative Results
Interpreting the result requires considering a person’s symptoms and exposure history due to the known limitations of rapid antigen tests. If a home test result is positive, it should be trusted because of the test’s high specificity, meaning it is highly likely that the person has COVID-19. The appropriate next steps include isolating immediately to prevent transmission and contacting a healthcare provider for guidance on treatment or follow-up care.
A negative result in an individual experiencing COVID-19 symptoms, such as a sore throat or fever, should be treated with caution. Since the test’s sensitivity is lower, this result is a potential false negative, especially if the person is early in their illness. Public health guidance recommends serial testing in this scenario, meaning testing again with a new test 48 hours after the first negative result.
For individuals who are asymptomatic but have had a known exposure, a single negative test does not rule out infection. The viral load may be too low to detect while the virus is incubating. The recommendation is to test immediately after exposure, and if negative, repeat the test two more times, each separated by 48 hours, for a total of three tests over five days. If symptoms develop at any point, a person should immediately isolate and consider seeking a lab-based PCR test for confirmation.