COVID-19 - SARS-CoV-2

Indications for Testing

The CDC and the Infectious Diseases Society of America (IDSA) offer recommendations for SARS-CoV-2 laboratory testing. For detailed information about these recommendations, see the ­­­­­­­­­­­­­CDC’s Overview of Testing for SARS-CoV-2  and the IDSA Guidelines on the Diagnosis of COVID-19: Molecular Diagnostic Testing. 

Laboratory Testing

Molecular Diagnostic Testing

Molecular assays (eg, nucleic acid amplification tests [NAATs]) are used to detect SARS-CoV-2 in respiratory specimens and are considered the gold standard for COVID-19 diagnosis.   A negative result indicates that SARS-CoV-2 RNA was not present in the specimen above the limit of detection. However, a negative result does not exclude the possibility of COVID-19 and should not be used as the sole basis for treatment or patient management. The possibility of a false-negative result should be considered if the patient’s recent exposures or clinical presentation suggests that COVID-19 is likely. Retesting may be advisable in symptomatic individuals with an intermediate or high clinical suspicion of COVID-19 and should be considered based on clinical judgment in combination with the recommendation of public health authorities. 

Antigen Testing

Antigen testing, performed on nasal or throat swab specimens, can be used for rapid detection of a SARS-CoV-2 infection in a point-of-care setting. Under optimal conditions (ie, during early, symptomatic infection when viral load is highest), studies suggest the sensitivity and specificity of antigen testing are similar to those of molecular testing. False-positive results may still occur when there is a low pretest probability of SARS-CoV-2 infection, whereas false-negative results are possible in individuals with low viral loads (eg, late in the course of infection).  Clinicians should consider the performance characteristics of antigen tests when interpreting results.

Serologic Testing

Serologic testing, also known as antibody testing, is used to detect antibodies against SARS-CoV-2 in serum or plasma. Following infection, immunoglobulin M (IgM) and IgG antibodies develop almost simultaneously and take 7-21 days to reach detectable levels. Most infected individuals will test positive for SARS-CoV-2 antibodies 3 weeks after symptom onset and may continue to test positive for IgG antibodies for months afterward.   Because of this natural delay in antibody development, serologic testing is not recommended for initial diagnosis of COVID-19.  However, antibody testing may be used to aid in diagnosing multisystem inflammatory syndrome in children (MIS-C) and adults (MIS-A); refer to the CDC’s case definitions for MIS-C  and MIS-A  for more information.

Notably, vaccination leads to the production of antibodies against specific viral protein targets; therefore, vaccination status should be considered when interpreting the results of antibody testing. Antibody testing is not recommended to assess immunity following vaccination or to inform patient vaccination.  For additional information, including the role of specific antibodies in results interpretation, refer to the CDC’s Interim Guidelines for COVID-19 Antibody Testing.