The most common HIV tests used are based on the detection of three biological markers of HIV infection (see Figure1):
 - 

Viral RNA (genetic material), earliest to appear: detected by individual or pooled nucleic acid amplification testing (NAAT)
 - p24 antigen (a viral core protein), next to appear: detected by 4th generation enzyme immunoassay testing (EIA)
 - HIV antibody, last to appear: detected by 3rd or 4th generation EIA, point-of-care (PIC) or rapid HIV testing, and Western Blot


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