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Antibody Testing - Are We There Yet?
Short Answer…..No.
The most used and most reliable test to date is the RT-PCR test (swabs of upper resp tract), these target various RNA genes. Positivity of these tests can be as early as day 1 and decline by week 3, depending on severity of illness. Specificity of RT-PCR is almost 100% because of the genome target design for SARS-Cov-2. But there are many false negatives due to prevalence of viral RNA and swab technique.
Lately, there has been a large ask for antibody testing for SARS-CoV-2.
Many of us want to know if the mild symptoms we felt were COVID-19 or were we an asymptomatic carrier? If we had the infection does it convey protection to ourselves and to spreading it to our families?
COVID-19 infection can indirectly be measured by looking at our immune response. Total antibodies are the most sensitive and earliest marker found (given higher concentration). IgM and IgG ELISA have found symptoms as soon as 4 days from symptom onset, higher levels occur after 3 weeks of illness.
IgM, largest, first to come up.
IgG, takes little longer to show up, but could represent immunity (unsure duration and protection).
Many antibody tests have been developed and marketed but are being found of variable quality. Many manufacturers however do not reveal nature of antigens used and only indicate the presence or absence of SARS-CoV-2 antibodies. We are unsure still if these antibodies are neutralizing antibodies and convey immunity.
Of the antibody tests based on sensitivity ELISA antibody testing seem to be the best followed by CLIA, LFIA, and FIA, Lateral Flow.
However from a study by Crook et al. if prevalence is low, then there is the potential for positive test.