The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus responsible for the global illness known as COVID-19. Understanding the presence and spread of this virus relies heavily on specialized laboratory tests. An “assay” in simple terms is a laboratory procedure designed to measure or detect a specific substance. These tests have become fundamental tools for monitoring the virus and guiding public health responses.
What Are SARS-CoV-2 Assays?
SARS-CoV-2 assays identify either the virus itself or the immune system’s response to it. This distinction helps determine if someone has an active infection or past exposure. Tests that look for components of the virus, such as its genetic material or proteins, indicate a current infection. Conversely, tests that detect antibodies produced by the body suggest a prior infection or vaccination. The choice of test depends on what information is needed.
Detecting Active Infection
Detecting an active SARS-CoV-2 infection primarily involves two types of assays: molecular tests and antigen tests. Molecular tests, often referred to as Nucleic Acid Amplification Tests (NAATs) like RT-PCR, detect the virus’s genetic material, specifically its RNA. RT-PCR works by converting viral RNA into complementary DNA (cDNA) and then amplifying specific target sequences, making even small amounts of virus detectable.
Sample collection for molecular tests commonly involves a nasal or nasopharyngeal swab. Saliva can also be used, offering a less invasive method. These tests are considered highly sensitive and specific, meaning they are very good at identifying infected individuals and correctly ruling out infection in those who are not infected.
Antigen tests, on the other hand, detect specific viral proteins, or antigens. These tests typically use a nasal swab and provide quicker results, often within minutes, making them suitable for rapid screening. Antigen tests are generally less sensitive than molecular tests, meaning they may miss some active infections, particularly if the viral load is low.
Despite their lower sensitivity compared to molecular tests, antigen tests maintain a high specificity. This means a positive antigen test result is highly accurate in indicating a current infection. However, a negative antigen test may need to be repeated or confirmed with a molecular test, especially if symptoms are present, due to potential false negatives.
Identifying Past Exposure
Antibody tests, also known as serology tests, identify if an individual had a past SARS-CoV-2 infection or vaccination. These tests do not diagnose a current infection; instead, they look for specific proteins called antibodies produced by the immune system in response to the virus. Antibodies are found in blood samples, typically collected via a blood draw or a finger prick.
The immune system produces different types of antibodies over time, such as IgM and IgG. IgM antibodies usually appear earlier in infection and diminish more rapidly. IgG antibodies typically develop a few days after IgM and can persist for weeks or months, indicating a more established immune response.
Detecting anti-nucleocapsid antibodies generally indicates a past SARS-CoV-2 infection, while anti-spike protein antibodies may result from either infection or vaccination. Antibody tests are useful for understanding viral spread and identifying individuals with an immune response, even if asymptomatic. However, the presence of antibodies does not currently guarantee future immunity to reinfection.
Understanding Your Test Results
Interpreting SARS-CoV-2 test results requires considering the type of test performed and the individual’s symptoms and exposure history. A positive result from a molecular or antigen test indicates that the virus’s components were detected, suggesting an active infection. For antibody tests, a positive result means antibodies to SARS-CoV-2 were found, indicating a past infection or vaccination.
A negative result from a viral test (molecular or antigen) means the virus was not detected. It does not rule out infection, as false negatives can occur. Similarly, a negative antibody test suggests no detectable antibodies, but it could mean the test was too soon after exposure or that the body did not produce a detectable response.
An inconclusive or invalid result means the test could not provide a clear answer. In such cases, retesting may be necessary for a definitive outcome. Test results should be evaluated in the broader clinical context, including symptoms and potential exposures, to guide appropriate actions.