How soon can you test positive for a Sexually Transmitted Infection (STI) after exposure?

STI Testing Timeline After Sexual Exposure

Most bacterial STIs (chlamydia, gonorrhea, trichomoniasis) can be detected immediately after exposure, but HIV and syphilis require follow-up testing at 6 weeks and 3 months to definitively rule out infection. 1

Immediate Testing (Within 72 Hours to 2 Weeks)

Bacterial STIs can be detected early, though immediate testing primarily establishes a baseline:

• Chlamydia and gonorrhea: Test immediately using nucleic acid amplification tests (NAATs) on urine or genital specimens, as these can detect infection even within 72 hours of exposure 2, 1
• Trichomoniasis: Test immediately using vaginal NAAT for women 1
• Syphilis: Perform baseline serologic testing (RPR/VDRL and treponemal tests), though early infection may not be detectable 1
• HIV: Perform baseline laboratory-based antigen/antibody (Ag/Ab) test, recognizing this establishes pre-exposure status 2, 1
• Hepatitis B: Baseline serologic testing if not vaccinated 1

Critical caveat: Testing at anatomic sites of exposure is essential. For receptive anal sex, obtain rectal specimens for chlamydia and gonorrhea; for receptive oral sex, obtain pharyngeal specimens for gonorrhea (pharyngeal chlamydia testing is not generally recommended) 1

Follow-Up Testing at 1-2 Weeks

If initial tests were negative and presumptive treatment was not given, repeat STI testing can be performed 1-2 weeks after exposure 2

This timing is particularly relevant for bacterial STIs where the initial exposure may not have resulted in detectable infection immediately 2

Essential Follow-Up at 6 Weeks

HIV testing at 4-6 weeks post-exposure is critical, as the window period for laboratory-based Ag/Ab tests means early infection may be missed on initial testing 2

Definitive Testing at 3 Months (12 Weeks)

This is the most important follow-up timepoint to definitively rule out infection:

• HIV: Both laboratory-based Ag/Ab test AND diagnostic nucleic acid test (NAT) at 12 weeks after exposure (or 8 weeks after completing post-exposure prophylaxis if given) 2, 1
• Syphilis: Repeat serologic testing at 6-12 weeks if initial test was negative, as early syphilis may not be detectable initially 1
• Reinfection screening: If initial chlamydia or gonorrhea tests were positive and treated, retest at 3 months due to high reinfection risk 1

The rationale: The 3-month window accounts for the longest window periods of HIV and syphilis, while the 1-2 week timeframe is sufficient for most bacterial STIs 1

Special Considerations for High-Risk Individuals

More frequent screening (every 3-6 months) is recommended for individuals with ongoing risk factors including multiple partners, anonymous partners, substance use during sex, or history of STIs 1

Post-Exposure Prophylaxis Window

If considering post-exposure prophylaxis, this must be initiated within 72 hours of exposure 2. After this window closes, the focus shifts entirely to screening for acquired infections 1

For men who have sex with men, doxycycline post-exposure prophylaxis (200 mg within 72 hours of condomless sex) may be considered as part of comprehensive STI care 2

Testing Algorithm Summary

1. Immediate (≤72 hours): Baseline testing for all STIs; initiate PEP if indicated
2. 1-2 weeks: Repeat bacterial STI testing if initial tests negative and no presumptive treatment given
3. 4-6 weeks: HIV testing (can be deferred if PEP started within 24 hours, adherent to full course, and not considering PrEP)
4. 12 weeks: Definitive HIV (Ag/Ab + NAT) and syphilis testing; reinfection screening if previously positive

The one-month mark alone is insufficient to rule out HIV and syphilis, making the 3-month follow-up non-negotiable for complete risk assessment 1