Short answer
STI blood tests detect immune responses and viral markers for HIV, syphilis, hepatitis B (and hepatitis C in risk- and age-based screening), and herpes. Complementing, never replacing, urine and swab NAAT testing.
Blood covers systemic and latent infections; NAAT covers mucosal ones.
Complete panels draw both: blood plus site-matched swabs or urine in the same visit.
Reactive blood screens need confirmatory cascades. Single blood results rarely diagnose alone.
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See Tests & Prices →HIV testing. Generations and windows
- Fourth-generation antigen/antibody immunoassays detect infection typically within 2–6 weeks (p24 antigen closing the early window antibody-only tests leave open).
- Reactive results reflex automatically to HIV-1/HIV-2 antibody differentiation and, where indicated, nucleic-acid testing. Staged results over days, each narrowing.
- Self-tests use the same biology with longer windows (oral-fluid and fingerstick antibody tests missing early infection fourth-generation labs catch).
Baseline plus window-dated follow-ups structure every exposure workup; see Syphilis Testing: Blood Tests Explained for the parallel two-test system.
Syphilis testing. The two-test system
Treponemal tests confirm exposure ever (staying positive for life. History markers).
Non-treponemal RPR/VDRL titers track activity and treatment response (rising with active disease, falling fourfold with cure, rising again with reinfection).
Titers, not single positives, guide follow-up over months.
Screening runs traditional (RPR first) or reverse (treponemal first) algorithms by laboratory; clinicians interpret both identically.
Consistent laboratories across follow-ups keep trends comparable.
Hepatitis panels. Immunity versus infection staging
Hepatitis B panels (surface antigen, surface antibody, core antibody combinations) distinguish susceptible (vaccinate), immune (vaccinated or recovered), acutely infected, and chronically infected states. Four patterns from three markers, read together.
Hepatitis C antibody screening follows risk and age guidance with RNA confirmation of active infection (antibody alone never proves current infection).
STI visits are efficient hepatitis screening opportunities. The blood is already drawing.
Herpes serology. Strict limits, frequent misuse
Type-specific IgG detects HSV-1/HSV-2 antibodies after 12–16 weeks but cannot locate infection (most HSV-1 positives reflect oral cold sores), cannot date it, and false-positives meaningfully in low-risk testers. Hence no routine screening recommendation.
Sore swabs diagnose; blood tests confuse in most asymptomatic scenarios.
Unexpected positives need confirmatory pathways before disclosure or treatment decisions.
Windows and confirmation. Timing decides meaning
Blood-test windows run weeks to months (longer than NAAT’s days to weeks). Test timing relative to exposure decides what each result can exclude.
Unexpected reactives get confirmation before conclusions or disclosure (screening-reactive lines are questions, confirmatory lines are answers).
Dated retests close window-period gaps; paired acute-convalescent draws resolve ambiguous serologies.
When to seek care
For baseline panels when establishing care, after exposures past the relevant windows, during pregnancy per schedules, and to resolve any reactive result.