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Test for HIV / AIDS - p24 antigen in blood
Last reviewed: 06.07.2025
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Antigen p24 is normally absent in serum.
The p24 antigen is an HIV nucleotide wall protein that is an essential structural component of the retroviral particle and is estimated to be present in 2,000–4,000 molecules in each virion. P24 antigen testing is sensitive and specific for diagnosing HIV infection in children, window-phase infection, predicting CD4+ T-cell decline and clinical progression in early and late infection, and is suitable for monitoring antiretroviral therapy in both adults and children.
The stage of primary manifestations after HIV infection is a consequence of the onset of the replicative process. The p24 antigen appears within 2 weeks after HIV infection as a result of the initial outbreak of viral replication, which is associated with a high level of viremia, during which a person is highly infectious and can be detected by ELISA in the period from 2 to 8 weeks. After 2 months from the onset of infection, the p24 antigen disappears from the blood. Later in the clinical course of HIV infection, a second increase in the content of the p24 protein in the blood is noted. It occurs during the period of AIDS formation.
Existing ELISA test systems for detection of p24 antigen are used for early detection of HIV in blood donors and children, prognosis of the disease course and monitoring of therapy. The ELISA method has high analytical sensitivity, which allows detection of HIV-1 p24 antigen in blood serum at concentrations of 5-10 pg/ml and less than 0.5 ng/ml HIV-2, and specificity. At the same time, it should be noted that the content of p24 antigen in the blood is subject to individual variations, which means that only 20-30% of patients can be identified using this study in the early period after infection.
Antibodies to the p24 antigen of the IgM and IgG classes appear in the blood starting from the 2nd week, reach a peak within 2-4 weeks and remain at this level for different periods of time - IgM antibodies for several months, disappearing within a year after infection, and IgG antibodies can persist for years.
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