Both testare immuno assays, meaning they rely on antibodies.
In the HBsAg EIA, you are detecting viral antigen, indicating viremia
in the patient. This of course means that this person is actively
infected with HBV.
The anti-HIV PA test detects antibodies against HIV. A positive
result tells you you are infected, but doesn't rely on the presence of
circulating virus the way the HBsAg test does.
Jay Mone'
