the p24 antigen is the earliest marker for HIV infections. Acute HIV syndrome develops 3-6 weeks after the infection. The acute HIV syndrome represents the initial or primary response of an immunocompetent adult to the HIV infection. It is characterized by a high level of virus production, viremia and widespread seeding of the lymphoid tissue. During this phase, the virus can be rapidly isolated from the blood and there are high levels of HIV p.24 Ag in the serum. Thus, this phase of HIV infection is detected by demonstrating p24 Ag of the HIV virus. The HIV antibody-based test such as ELISA and western blot cannot be used for diagnosing this phase of infection because the immune response has not yet developed. Soon, however, a virus-specific immune response develops evidenced by seroconversion (development of antibody). The seroconversion usually occurs within 3-7 weeks of presumed exposure.  Once the antibody develops, the antibody-based tests ELISA and western blot are used in the diagnosis.