The 4th-generation HIV test detects both HIV-1/2 antibodies and the HIV p24 antigen. p24 is a viral capsid protein that appears in blood early in infection (typically ~2–4 weeks after exposure), before antibodies form. Detecting p24 plus antibodies shortens the diagnostic window and enables earlier identification of acute HIV infection compared with antibody-only tests.

The p24 antigen is an HIV capsid protein that appears in blood early after infection. A combined p24 antigen with HIV‑1/2 antibody test (fourth‑generation) detects both p24 and antibodies to HIV‑1 and HIV‑2, shortening the diagnostic window and allowing earlier detection (often within 2–6 weeks post‑exposure). A reactive result requires confirmatory testing and healthcare follow‑up.

The p24 antigen usually becomes detectable about 2–3 weeks after HIV exposure, peaks around weeks 3–4, and then declines as the body produces antibodies. It commonly becomes undetectable by about 4–6 weeks post-infection, though timing varies between individuals. Fourth‑generation antigen/antibody tests use p24 to shorten the window period; nucleic acid tests can detect infection even earlier.

The HIV antigen–antibody test (fourth-generation) detects HIV p24 antigen and antibodies to HIV-1/2 in blood, allowing earlier detection than antibody-only tests. It can identify infection about 2–6 weeks after exposure; antibodies may take several weeks longer. A reactive result needs confirmatory testing. It’s commonly used for screening in clinics and labs because it shortens the diagnostic window and improves early detection.

Fourth-generation HIV Ag/Ab tests are highly accurate: sensitivity and specificity exceed 99% once the window period has passed. They detect p24 antigen plus antibodies, shortening the window to about 2–4 weeks (most infections are detectable by 4 weeks). Very early infections (first 1–2 weeks) can be missed, and positive results require confirmatory testing.

The four major HIV antigens are gp120 and gp41 (envelope glycoproteins) and the Gag proteins p24 (capsid) and p17 (matrix). gp120 binds the CD4 receptor, gp41 mediates membrane fusion, p24 forms the viral capsid and is a key early diagnostic marker, and p17 forms the matrix beneath the viral envelope and aids viral assembly.