Viral replication increases exponentially shortly after HIV infection. This can be detected using nucleic acid testing (NAT); however, NAT is not considered first-line testing, as it is not cost- or time-effective. Instead, HIV screening tests currently consist of serological tests, which detect antibodies against HIV. The time between infection and the production of antibody is known as the "eclipse period." During the eclipse period, IgG and IgM antibodies against HIV will not be present and thus, a patient with HIV virus may screen as negative. Such a patient would be considered seronegative yet still viremic; that is, they have virus present but they don't yet have antibodies present.
To help address this false-negative patient scenario, newer screening tests have been developed, which not only detect antibodies to HIV but also detect the HIV virus itself. These are known as HIV Ag/Ab assays since they combine tests for HIV antigen and antibodies.