Kristian G. AndersenGeverifieerd account

Serology tests are great to measure people who have been exposed previously, but they typically cannot be used to accurately diagnose acute patients - that requires detecting the pathogen itself (in this case HCoV-19), and not the antibodies (IgM or IgG) produced against it. 2/n

Antibodies only come up much later in infection and typically too late to be useful for diagnosis. E.g., IgM likely takes 10+ days after infection to develop, while IgG takes longer still (likely at least 14+ days). 3/n

Typically when we talk about "rapid diagnostic tests" (RDTs) those tests detect the antigen derived from the pathogen, not the antibodies against it. RDTs are typically blood-based tests and can often be done doing a simple fingerstick. 4/n

The main issue trying to do this to diagnose COVID-19, is the fact that the virus typically can't be found in the blood, so these blood-based tests won't work for diagnosis. 5/n

RDTs could instead be made to work with sputum or nasal aspirates, but those fluids can be hard to handle compared to blood, which is typically quite 'clean'. That said, I'm quite sure we'll see those tests pop up, only it'll take time. 6/n

In summary: RDTs = measure pathogen antigens (by having antigen-specific IgGs on the test itself). Serology tests = measure antibodies (IgM and/or IgG) to pathogen antigens (by having pathogen-derived antigens (proteins) on the test itself). 7/n

Both of these tests are great and critical, but they serve different purposes - RDTs for diagnostics; Serology tests for understanding previous exposures (e.g., in populations, healthcare workers, etc.) - these can give us estimates of the real number of infected people. 8/n