What it measures
Antibodies to SARS-CoV-2, the virus that causes Covid-19. Antibodies to the spike protein come from vaccination or infection, while antibodies to the nucleocapsid protein point to a past infection. Some tests give an antibody level rather than a yes or no.
Why people test it
- Checking for antibodies after vaccination or a past infection.
- Checking whether a past illness may have been Covid-19.
- Measured in people with a weakened immune system, to see whether vaccination produced antibodies.
Typical range
The result is reported as positive or negative, and quantitative tests also give an antibody level. The scale and cut-off depend on the test kit, so results from different labs are not directly comparable. The report shows the lab's own cut-off.
Finger-prick or vein?
A finger-prick sample is usually fine for an antibody test like this, and some providers offer a home kit. Others take blood from a vein at a clinic.
An antibody test does not show whether someone has Covid-19 now. That needs a swab test.