The usual ELISA test for HIV tests the patients serum(blood) for antibodies to the HIV virus. Since the antibodies take some time to develop, there is a window phase in which the usual ELISA tests are negative in infected individuals. The p24 is a protein which forms the core of the HIV virus. It can be detected by various tests, including ELISA, spot EIA and western blot. This can be useful in detecting early infections, even in the window phase. Some recent ELISA kits are now incorporating the detection of p24 antigen along with the HIV antibody detection, thus increasing the sensitivity, specificity of testing and decreasing the window phase.