Foundation Laboratory - Diagnostic Laboratory Services, California
Dec. 18, 2020
Antibody vs. Antigen Testing

When testing for viral or bacterial infections, there are two general categories of tests: antigen and antibody.  For patients, understanding the difference can be a little tricky.  Antigen testing is used primarily for identifying an active infection.  Antigens are usually found on the outside of cells and they can be attached to specific antibodies or receptors.  When a pathogen invades the body, these antigens are what normally triggers the body’s immune response.  So, in regard to testing, when you hear an antigen test you know that that test is looking for the actual bacterial or viral cells within your body.

 

There are a few categories of antibodies, however, only two are generally used for viral or bacterial infection testing: immunoglobulin M (IgM) and immunoglobulin G (IgG).  Antibodies are created by the body at different rates in response to pathogen antigens.  When the body first becomes infected, IgM is created and released.  Once IgM levels begin to climb, the body begins creating and releasing IgG.  IgG testing is often requested after vaccinations, like a Hepatitis B vaccination, to confirm immunity.

 

In general, a negative antigen test and a negative IgM test with a positive IgG test indicates a past exposure to the infection or a successful vaccination usually indicating immunity.  A positive antigen test with negative antibody tests indicates a very recently acquired current infection.  A positive antigen and a positive IgM test with negative IgG also indicates a current infection, but also suggests that the infection was acquired long enough ago for the body to react but not too long ago since the IgG is negative.  Finally, a positive antigen, IgM, and IgG test indicates a current infection that has been going on for at least a week since the body has had enough time to start producing IgG.