The COVID-19 rapid serology test identifies early and late-stage antibodies against this virus that are present in the blood. Similar to other biochemical tests such as the influenza rapid test, these tests are the least expensive, but have varying sensitivities and specificities.
Rapid tests are weaker in antibody detection than immunofluorescence tests, having weaker positivity (i.e. the presence of antibodies, may register as negative). Also, very rarely false-positive lines may show up; those are usually faint lines. A doctor will evaluate positive results in all cases.
This is less reliable in detecting an ongoing infection at its early phase than the PCR test.