Receiving both a negative and a positive COVID-19 test result on the same day can be confusing, but this scenario is common and scientifically explainable. This discrepancy highlights the different ways each test works and where the person is in the infection timeline. A conflict in results is often linked to the rapidly changing amount of the virus in the body, known as the viral load, and the distinct sensitivity thresholds of the two primary test types. Understanding the mechanics of these tests explains why a single day can yield two different outcomes.
Understanding the Different Types of COVID-19 Tests
The two main types of tests are the molecular test, typically a reverse transcription-polymerase chain reaction (RT-PCR), and the rapid test, which is an antigen test. These methods detect different parts of the virus and operate with vastly different levels of sensitivity. The RT-PCR test is considered the standard for diagnosis due to its high sensitivity.
Molecular tests like RT-PCR detect the virus’s genetic material, or RNA, using a process that amplifies tiny amounts of the material. This allows the RT-PCR test to return a positive result even when the amount of virus present is very low. Due to this high sensitivity, a PCR test can remain positive for weeks after a person is no longer infectious, detecting residual genetic fragments.
Antigen tests, or rapid tests, detect specific proteins found on the surface of the SARS-CoV-2 virus. They are faster and can be performed at home, but they are significantly less sensitive than PCR tests. A rapid test requires a relatively high concentration of the virus to register a positive result. This difference in detection limits creates the potential for conflicting results.
Why Results Conflict: The Role of Viral Load and Timing
The primary reason for a same-day conflict is the dynamic nature of the viral load, which changes rapidly during the initial phase of infection. After exposure, the virus multiplies in the body, and the viral load rises from undetectable to its peak. This peak often occurs around the fourth or fifth day after symptoms begin. During this early phase, a person may be on the cusp of the rapid test’s detection limit.
Consider a scenario where a person is early in their infection, perhaps on the day symptoms first appear. The viral load may be too low to cross the threshold required for the less sensitive antigen test, resulting in a negative rapid test. However, the highly sensitive RT-PCR test can easily detect the small but growing amount of viral RNA, leading to a positive result. Studies have shown that the sensitivity of rapid antigen tests can be as low as 30% to 60% on the first day of symptoms.
As the viral load increases over the next few days, the rapid test’s sensitivity improves dramatically, often reaching 80% to over 90% by the fourth day of symptoms. The timing of the test relative to the infection’s onset is the most important factor in the discrepancy. Poor sample collection technique is another factor that can cause a false negative on either test. If the nasal swab does not collect enough material, even a PCR test may miss the infection.
A false positive from an antigen test, although rare, can occur due to user error or a problem with the testing kit itself. Because the RT-PCR test is specific, a positive PCR result is highly reliable and generally overrides a negative rapid test. The conflicting results indicate that while a transmissible amount of the virus may not be present yet, the infection has already begun.
Interpreting Conflicting Results and Next Steps
When faced with a negative rapid test and a positive RT-PCR test on the same day, the general public health rule of thumb is to assume you are positive. The high reliability of a positive molecular test means it should take precedence over a negative rapid test, especially if you are experiencing symptoms. The positive result confirms the presence of the virus’s genetic material in your body.
If you receive a positive result from any test, you should immediately follow current public health guidelines for isolation to prevent further spread. These guidelines typically involve isolating for a period of time, regardless of the negative rapid test result. Isolation helps ensure that you are not transmitting the virus while your viral load is building up to its peak.
Public health advice often recommends re-testing if your initial rapid test was negative but you have a high suspicion of infection or a positive PCR result. If your initial rapid test was negative, repeating the antigen test 24 to 48 hours later is advised to confirm if your viral load has increased enough to be detected by the rapid test. Consulting a healthcare provider is also a prudent step if symptoms worsen or if you have underlying conditions that place you at a higher risk for severe illness.