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