Complex sentence: Serostatus is a term denoting the presence or absence of particular antibodies in an individual's blood--prior to seroconversion, the blood test is "seronegative" for the antibody; after seroconversion, the blood test is "seropositive" for the antibody.
Split into simpler sentences: Serostatus is a term denoting the presence or absence of particular antibodies in an individual's blood. Prior to seroconversion, the blood test is "seronegative" for the antibody; after seroconversion, the blood test is "seropositive" for the antibody.