What is the window period for HIV testing?

Fourth-generation Ag/Ab combo tests are reliable at 4 weeks and conclusive at 45 days. Rapid antibody-only tests may require up to 90 days. Nucleic acid tests (NAT/PCR) detect infection as early as 10–14 days post-exposure. If initial testing after known exposure is negative, retesting at 45 days and 90 days is recommended.

Can hepatitis B vaccination cause a positive test?

Only for anti-HBs (surface antibody), which should be positive after vaccination. Vaccination does not cause positive HBsAg (surface antigen), anti-HBc (core antibody), or HBV DNA. A positive HBsAg always indicates infection regardless of vaccination history.

What does a reactive RPR with non-reactive TPHA mean?

This is almost always a biological false positive. RPR can be falsely reactive in pregnancy, lupus, recent vaccination, and other conditions. A non-reactive TPHA confirms there is no syphilis. No treatment is needed, but the underlying cause of the false-positive should be investigated.

Should I get TORCH screening outside of pregnancy?

Generally no. TORCH screening is most valuable before or during pregnancy because these infections can cause birth defects. Outside pregnancy, individual tests are ordered based on specific clinical indications rather than as a panel.

Infection Screening: STI, Hepatitis, HIV, Syphilis, TORCH

Infection screening is a critical component of preventive healthcare. Many serious infections — HIV, hepatitis B and C, chlamydia, syphilis — remain asymptomatic for months or years while silently progressing or spreading. This guide explains every major infection screening test, what each marker means, and how to interpret your results according to current CDC and WHO guidelines.

Disclaimer: This information is for educational purposes only and does not replace professional medical advice. Always consult a qualified healthcare provider for diagnosis and treatment.

Why Infection Screening Matters

The CDC, USPSTF, and WHO recommend routine screening because early detection enables effective treatment and prevents transmission:

HIV: All adults aged 15–65 at least once; more frequently for high-risk groups
Hepatitis C: All adults aged 18–79 at least once
Hepatitis B: All adults at least once (USPSTF 2020)
Chlamydia/gonorrhea: Sexually active women under 25 annually
Syphilis: All pregnant women; high-risk groups regularly

HIV Testing: The Modern Three-Step Algorithm

Step 1: Fourth-Generation Screening (Ag/Ab Combo)

Detects both HIV p24 antigen and HIV-1/2 antibodies simultaneously. Window period: approximately 18–45 days.

Non-reactive: No HIV detected (if tested after the window period)
Reactive: Requires confirmatory testing — a reactive screen is not a diagnosis

Step 2: HIV-1/HIV-2 Differentiation Assay

Confirms the presence of antibodies and distinguishes between HIV types.

Step 3: HIV-1 RNA NAT (if discordant)

Detects acute infection before antibodies develop. Can identify HIV as early as 10–14 days post-exposure.

Once diagnosed, viral load (goal: undetectable, below 50 copies/mL) and CD4 count (normal: 500–1,500 cells/mm3; below 200 defines AIDS) guide treatment monitoring.

Hepatitis B: The Complex Marker Panel

Hepatitis B uses the most nuanced set of serologic markers in infection screening.

Key Markers

Marker	Significance
HBsAg (surface antigen)	Active infection (acute or chronic)
Anti-HBs (surface antibody)	Immunity from vaccination or resolved infection
Anti-HBc total (core antibody)	Ever exposed — past or present
Anti-HBc IgM	Recent/acute infection
HBeAg (e-antigen)	High viral replication, high infectivity
HBV DNA	Quantifies active viral replication

Common Patterns

HBsAg	Anti-HBs	Anti-HBc	Interpretation
Neg	Pos (>10 mIU/mL)	Neg	Vaccinated, immune
Neg	Pos	Pos	Natural immunity — past resolved infection
Pos	Neg	Pos (IgM+)	Acute hepatitis B
Pos	Neg	Pos	Chronic hepatitis B (HBsAg positive >6 months)
Neg	Neg	Neg	Susceptible — never infected, never vaccinated

Hepatitis C: Two-Step Algorithm

Step 1: Anti-HCV Antibody

Non-reactive: No HCV exposure (unless tested within the 4–10 week window period)
Reactive: Exposure detected — does not distinguish active from resolved infection

Step 2: HCV RNA (PCR)

Detected: Active hepatitis C. Current direct-acting antivirals (DAAs) achieve cure rates above 95% in 8–12 weeks.
Not detected: Past resolved infection (spontaneous clearance or prior treatment). No treatment needed, but re-infection is possible.

Syphilis: RPR/TPHA Two-Test Algorithm

Syphilis testing combines non-treponemal and treponemal tests.

Traditional Algorithm

RPR or VDRL (non-treponemal) — screens for disease activity. Reports reactive/non-reactive with titer.
If reactive: FTA-ABS or TP-PA (treponemal) — confirms true syphilis.

Reverse Algorithm (Quest, LabCorp)

Treponemal immunoassay (EIA/CIA) — automated screening
If reactive: RPR with titer — quantifies activity

Interpreting Results

RPR/VDRL	Treponemal	Interpretation
Non-reactive	Non-reactive	No syphilis
Reactive	Reactive	Syphilis (stage determined clinically); titer guides treatment
Reactive	Non-reactive	Biological false positive (pregnancy, lupus, other)
Non-reactive	Reactive	Past treated syphilis or very early/late infection

RPR titers track treatment: a fourfold decline (e.g., 1:32 to 1:8) within 6–12 months indicates success. A fourfold rise suggests reinfection.

TORCH Panel: Pregnancy Infection Screening

TORCH screens for infections crossing the placenta:

T — Toxoplasmosis (Toxoplasma gondii)
O — Other (syphilis, varicella, parvovirus B19)
R — Rubella
C — Cytomegalovirus (CMV)
H — Herpes simplex virus (HSV)

IgG vs. IgM Interpretation

IgG	IgM	Meaning
Neg	Neg	No prior exposure — susceptible
Pos	Neg	Past infection or vaccination — immune, low fetal risk
Neg	Pos	Possible acute infection — urgent further testing
Pos	Pos	Recent or reactivated infection — IgG avidity testing needed

IgG avidity distinguishes timing: high avidity (>60%) means infection occurred more than 3–4 months ago (reassuring); low avidity (<30%) suggests recent infection (concerning in pregnancy).

For comprehensive preconception planning including TORCH, see preconception blood tests for women and preconception tests for men.

STI Panel: Chlamydia, Gonorrhea, and More

Chlamydia and Gonorrhea (NAAT)

Nucleic acid amplification testing from urine or swabs (cervical, urethral, rectal, pharyngeal) is the gold standard. Sensitivity >95%, specificity >99%. Test all relevant anatomical sites based on sexual history.

Herpes Simplex Virus (HSV) Serology

HSV-1 IgG positive: Past exposure to HSV-1 (oral herpes; can also cause genital infection)
HSV-2 IgG positive: Past exposure to HSV-2 (primarily genital herpes)
IgM: Not recommended for routine screening due to high false-positive rates

For active lesions, PCR swab testing is preferred over serology.

Trichomonas and Mycoplasma

NAAT testing is available for both Trichomonas vaginalis and Mycoplasma genitalium. Resistance-guided therapy is recommended for M. genitalium due to high macrolide resistance rates.

Preparing for Infection Screening

For the most accurate results:

No fasting required for most infection screening tests (antibody and antigen tests are unaffected by food intake)
Inform your doctor about recent vaccinations — hepatitis B vaccination causes a positive anti-HBs (this is expected), and some vaccines can transiently affect non-treponemal syphilis tests
Disclose all medications — antiviral drugs, immunosuppressants, and recent antibiotic courses can affect test sensitivity
Timing matters after exposure — each pathogen has a different window period. Testing too early may produce false-negative results. Your doctor will advise on optimal timing based on your exposure history.

When to Get Tested

Routine checkup: HIV, hepatitis B/C per CDC guidelines
New sexual partner: Full STI panel before unprotected contact
Pregnancy: First prenatal visit (HIV, HBV, syphilis, rubella immunity, chlamydia/gonorrhea)
After exposure: Chlamydia/gonorrhea at 2 weeks; syphilis and HIV at 4–6 weeks; all confirmatory at 3 months

Related Tests

Infection screening is most informative alongside:

Complete blood count (CBC) — lymphocyte and eosinophil changes indicate viral and parasitic infections
Blood chemistry panel — liver enzymes (ALT, AST) are critical for hepatitis assessment
CRP blood test — elevated CRP alongside positive markers helps assess disease activity
HPV test — often performed alongside STI screening during cervical cancer evaluation
Immunogram — immune function assessment relevant for HIV and recurrent infections

Get Your Results Interpreted

If you have infection screening results — STI panels, hepatitis markers, HIV tests, or TORCH panels — and want a clear explanation of every finding, upload your results at Evallume for an instant AI-powered interpretation that accounts for your risk factors and clinical context.

This article is for informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Consult a healthcare professional for any medical concerns.

Infection Screening Interpretation: STI Panel, Hepatitis, HIV, Syphilis RPR/TPHA, TORCH