Interpreting SARS-CoV-2 Diagnostic Tests: Common Questions and Answers

SARS-CoV-2 is the novel coronavirus that causes COVID-19. The spectrum of asymptomatic, presymptomatic, and symptomatic SARS-CoV-2 transmission presents challenges for evaluating SARS-CoV-2 test performance for diagnostic or screening purposes and for interpreting test results. Molecular and antigen tests can detect current SARS-CoV-2 infection and are used to diagnose COVID-19. Clinicians should consider a test's characteristics, test timing in relation to symptom onset, and the pretest probability of disease when interpreting results. Molecular and antigen SARS-CoV-2 tests both have high specificity. However, antigen tests generally have lower sensitivity and thus greater potential for false-negative results. Pretest probability of disease should be based on a patient's exposure to someone with a confirmed or probable case, signs or symptoms of COVID-19, local or population-specific COVID-19 prevalence, and presence of an alternative diagnosis. Using a leaf plot is an efficient way to visualize posttest probability of disease based on estimated pretest probability and the test's sensitivity and specificity. A negative molecular or antigen test result might not rule out SARS-CoV-2 infection when pretest probability is high, depending on the test's sensitivity. A symptom-based approach is preferred over a test-based approach for discontinuing isolation precautions for most patients with COVID-19 because prolonged shedding of viral RNA does not necessarily correlate with infectivity. Antibody tests might help identify past SARS-CoV-2 infection if performed two to four weeks after symptom onset; however, because of uncertainty about the extent and durability of postinfection or vaccine-induced immunity, they should not yet be used to infer immunity or guide discontinuation of personal protective measures.