Rapid diagnostic tests for COVID-19, or rapid antigen tests (RATs), are an accessible tool for quick screening. Designed for use at home or point-of-care facilities, they provide results in minutes, serving as an initial screening method to determine if an individual might be actively infected with the SARS-CoV-2 virus.
These tests provide a fast, qualitative result—a simple positive or negative—to guide decisions about isolation. They are a form of lateral flow assay, a technology used for years in tests for conditions like strep throat and influenza. This established technology was adapted to identify the SARS-CoV-2 virus, making testing broadly available to the public.
The Science Behind Rapid Antigen Tests
A rapid antigen test functions by detecting specific proteins, called antigens, from the SARS-CoV-2 virus. The most common target is the nucleocapsid (N) protein, which is abundant in the virus and can be detected early in an infection. The test works like a lock-and-key system where the test strip contains immobilized antibodies (the locks) designed to bind only to the SARS-CoV-2 N-protein antigens (the keys).
When a nasal sample is mixed with a buffer solution, any virus present is broken apart, releasing these proteins. This liquid is applied to the test strip and moves up via capillary action. It passes over mobile antibodies attached to colored particles. If viral antigens are present, they bind to these mobile, colored antibodies, forming a complex that continues to travel up the strip.
This process differs from a polymerase chain reaction (PCR) test, which detects the virus’s genetic material (RNA). PCR tests are more sensitive because they amplify the viral RNA, making millions of copies to detect even small amounts of the virus. Antigen tests do not amplify their target; they detect the proteins present in the sample, which is why they are faster but less sensitive than PCR tests.
Understanding Test Accuracy
The reliability of a rapid antigen test is described by two main factors: sensitivity and specificity. Sensitivity is the test’s ability to correctly identify individuals who have the virus (a true positive). Specificity is its ability to correctly identify those who do not have the virus (a true negative). The accuracy of these tests is heavily dependent on the viral load, which is the amount of virus present in a person’s system.
Tests are most accurate when the viral load is at its peak, which typically occurs a few days after infection and often coincides with the onset of symptoms. For this reason, rapid antigen tests are most reliable when used within the first week of symptoms. If a test is taken too early, the viral load may be too low to detect, potentially resulting in a false negative.
Beyond viral load, accuracy is also influenced by sample collection. An inadequate nasal swab that fails to collect enough material can lead to an inaccurate result. Following the kit’s instructions for both timing the test and collecting the sample gives the best chance of an accurate reading.
Proper Test Administration and Result Interpretation
To correctly perform the test, first wash your hands thoroughly and lay out all components on a clean, flat surface. Collect a sample using the sterile nasal swab, typically by inserting it into both nostrils as directed by the manufacturer’s instructions. The swab is then submerged and stirred in the tube of liquid buffer solution, which helps release the viral proteins from the sample.
After mixing, discard the swab and secure the dropper cap onto the tube. Apply the specified number of drops to the sample well on the test cassette. The results will be ready to read within the time frame indicated by the brand, often 15 to 30 minutes. Reading the result outside this window may lead to an incorrect interpretation.
Interpreting the result is a visual process. A single line next to the ‘C’ (Control) indicates a negative result. Two lines, one at ‘C’ and one at ‘T’ (Test), indicate a positive result, and any visible test line, no matter how faint, counts. An invalid result occurs if the control line does not appear, meaning the test failed and a new one should be taken.
Actionable Steps After Testing
A positive test result requires immediate action to prevent transmission. Follow the most current public health guidelines for isolation and inform close contacts so they can monitor for symptoms and test themselves. If your symptoms are severe or you have underlying health conditions that put you at higher risk, consult a healthcare provider.
A negative result does not provide a definitive all-clear, particularly if you have symptoms or a recent known exposure. This may be due to a viral load that is too low for the test to detect. In such cases, continue taking precautions like wearing a mask and practicing social distancing. Retesting within 24 to 48 hours is often recommended to confirm the result as the viral load may increase to a detectable level.