A patient presented with fever, cervical lymphadenopathy and night sweats four weeks after unprotected sexual contact. Next investigation

Ref: PubMed* This is a possible suspicion of HIV infection.* p24 antigen is a viral protein that makes up most of the viral core.* Serum concentrations of p24 antigen are high in the first few weeks after infection; tests sensitive to p24 antigen are therefore useful for diagnosing very early infection when antibody levels are still low.