Final answer:
A positive EIA test, often applied in drug screening or disease diagnosis such as HIV screening, is indicated by a color change on the test membrane.
Step-by-step explanation:
A positive result for an enzyme immunoassay (EIA) test, which can be used to test for the presence of illicit drugs or viral antigens in urine (UA), is typically indicated by a color change on the membrane filter. In the context of clinical focus and resolution such as HIV screening, indirect ELISA tests use this principle.
When a patient's serum or plasma is applied to the membrane embedded with antibodies specific to the drug or viral antigen, any antibodies present will bind to the antigen. After adding an antibody conjugate and washing off the excess, a substrate is then added to stimulate a color-producing, enzyme-mediated reaction, where the presence of color indicates a positive test result.