Quest only offers SARS-CoV-2 IgM (spike) in a panel that includes a separate IgG (nucleocapsid) determination.4,5 The results from this qualitative test for SARS-CoV-2 IgM can be positive (reactive) or negative (non-reactive).5,17
A positive (reactive) result indicates that antibodies to SARS-CoV-2 were detected and the individual has potentially been infected with SARS-CoV-2.1,3,5,17 SARS-CoV-2 IgM is generally detectable in blood several days after initial infection. Detection of IgM may indicate a recent infection.1,5,12,13,17 A spike SARS-CoV-2 IgM test may detect an immune response in individuals who have received a COVID-19 vaccination (and who have not been infected naturally), but the clinical significance is unknown. The clinical significance is unknown, in part, because the performance characteristics of this test in these individuals has not been established.
It is unknown how long SARS-CoV-2 IgM remains detectable, post-infection.1,3,5,17
Whether SARS-CoV-2 antibodies confer immunity to infection also remains unknown.1,3,5,17 Incorrect assumptions of immunity may lead to premature discontinuation of physical distancing requirements and could increase the risk of infection for individuals, their household members, and the public.1
A negative test result means that SARS-CoV-2 IgM was not present in the specimen at levels above the limit of detection.1,5,17 However, patients tested early after infection may not have detectable antibodies despite active infection.1,5,17 A negative result should not be used to rule out infection. Testing of patient specimens for the presence of virus material (eg, RNA or antigen) should be performed if acute infection is suspected.1,3,5,11
A recent publication supports the use of simultaneous IgG/IgM measurements (regardless of method) to significantly improve assay sensitivity.9,10 Regardless of the test result, individuals should continue to follow CDC guidelines to reduce the risk of infection, including social distancing and wearing masks.1