Serologic Testing
Testing of IgM and IgG specific antibodies to SARS-CoV-2 is done readily by obtaining blood samples and does not require specialized PCR equipment. An initial study on the kinetics of antibody formation found that of 535 patients, the total seroconversion rate for total Ab, IgM, and IgG was 93.1%, 82.7%, and 64.7%, with median seroconversion time of 11, 12, and 14 days.28 The same authors reported that combining RT-PCR and antibody detection can improve the sensitivity for diagnosis of COVID-19.28
A patient with a positive serologic test, however, only means that they have been exposed to SARS-CoV-2. Whether the resulting antibodies confer immunity to the virus requires further study. As such, the presence of antibodies does not necessarily reflect a person’s potential to transmit the disease. There are reports showing that patients who developed COVID-19 and then resolved with negative serial PCR testing later again became positive on PCR assay.29 This could potentially represent a false positive result, or a viral carrier state despite a measured serologic conversion. As such, while serologic testing is important in the setting of confirming SARS-CoV-2 infection, this test does not indicate whether the patient is or is not actively infectious and shedding the virus. Until further research is done, the WHO does not currently recommend utilizing serologic testing to guide decision making.30