It is possible to have a COVID-19 infection even after receiving a negative test result. This common phenomenon stems from the inherent limitations and mechanics of available diagnostic tests. Understanding why a negative result might not reflect the true status of an infection is important for making informed decisions about personal safety. The chance of missing an active infection relates directly to the type of test used and the timing of the test relative to the virus’s progression.
Defining the False Negative
A false negative occurs when a diagnostic test indicates a person is not infected, but they are actively carrying the virus. This concept relies on two primary metrics of test performance: sensitivity and specificity. Sensitivity is the test’s ability to correctly identify people who have the disease, producing fewer false negatives. Specificity is the ability to correctly identify people who do not have the disease, resulting in fewer false positives.
At-home rapid tests, known as antigen tests, are highly specific, meaning a positive result is reliable. However, these tests have lower sensitivity, with reported rates often falling around 69% to 73% in symptomatic individuals, and even lower in asymptomatic cases. This lower sensitivity is a trade-off for rapid results and ease of use, making the antigen test the primary source of false negative results compared to laboratory-based tests.
How Test Timing and Viral Load Influence Results
The biggest reason for a false negative is testing too early, when the viral load is too low for detection. Viral load is the concentration of virus particles in the body, which must reach a certain threshold for a diagnostic test to pick it up. The average incubation period for COVID-19 is about five to six days, but people can become infectious two to three days before symptoms appear.
Antigen tests require a high concentration of viral protein to display a positive line, making them most effective during the peak infectious period, which typically coincides with the first few days of symptoms. If a person tests on the day they develop symptoms, the probability of a false negative result on a highly sensitive test like a PCR is estimated to be around 38%. The chance of a false negative is even higher for rapid antigen tests when the viral concentration is low.
Laboratory-based molecular tests, such as Polymerase Chain Reaction (PCR) tests, are significantly more sensitive than rapid antigen tests because they amplify the virus’s genetic material. This higher sensitivity allows PCR tests to detect the virus earlier and for a longer period, even when viral loads are low. For a rapid antigen test to be positive, the viral load must be high enough to be easily transmissible. Therefore, a negative result from a rapid test, especially soon after exposure or symptom onset, does not definitively rule out an infection.
Next Steps After a Negative Test Result
If you have symptoms consistent with COVID-19 but receive a negative result on an at-home antigen test, perform a repeat test. The Food and Drug Administration (FDA) advises retesting 48 hours after the first negative result for symptomatic individuals. If the second test is also negative, you likely do not have COVID-19, but a third test may be warranted if symptoms persist.
For individuals exposed to the virus but showing no symptoms, the recommendation is to wait at least five full days after exposure before testing. If the initial test is negative, you should retest 48 hours later, and then again 48 hours after the second test, for a total of three tests over five days. This staggered schedule increases the chance of catching the infection by waiting for the viral load to increase to detectable levels.
While waiting for retest results, or if symptoms are present despite a negative result, wear a high-quality mask and limit contact with others to prevent potential spread. If you are concerned about ongoing symptoms or high-risk exposure, seek a laboratory-based PCR test. PCR tests provide a more definitive result due to their superior sensitivity.