Which baseline laboratory tests should be obtained after a potential exposure (e.g., needle‑stick or unprotected sexual contact) to assess human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), syphilis, and other sexually transmitted infections (STIs), and to evaluate suitability for post‑exposure prophylaxis?

Baseline Laboratory Testing After Potential HIV/HBV/HCV/STI Exposure

At the initial post-exposure visit, obtain an HIV antigen/antibody combination test (preferably rapid or laboratory-based), baseline HBV serology (HBsAg, anti-HBs, anti-HBc), HCV antibody, syphilis serology (RPR/VDRL), and if the exposed person had long-acting injectable PrEP in the past 12 months, add an HIV nucleic acid test (NAT). 1

Immediate Testing (Within 72 Hours of Exposure)

HIV Testing

• Perform both rapid (point-of-care) AND laboratory-based HIV Ag/Ab combination test at baseline 1
• Add diagnostic HIV NAT if the exposed person received long-acting injectable PrEP within the past 12 months - this is critical because residual drug levels can delay antibody development 1
• The dual testing approach (rapid + lab-based) provides immediate results while confirming with more sensitive laboratory methods

Hepatitis B Testing

• Test the exposed person for:
  • HBsAg (to detect existing chronic infection)
  • Anti-HBs antibody levels (to determine immunity status - responders have >10 mIU/mL)
  • Anti-HBc (to identify past exposure) 2
• Critical decision point: If anti-HBs >10 mIU/mL from prior vaccination, the person is protected and needs no further HBV prophylaxis 2
• If isolated anti-HBc positive, test for anti-HBc IgM and HBV-DNA to distinguish true infection from false positive 2

Hepatitis C Testing

• Baseline HCV antibody test 2, 3
• Baseline ALT (alanine aminotransferase) if considering early treatment strategy 2

Syphilis and Other STIs

• Baseline syphilis serology (RPR or VDRL) - particularly important for sexual exposures 4, 5
• Consider testing for other bacterial STIs based on exposure type (gonorrhea, chlamydia)

Source Patient Testing (If Available and Consent Obtained)

Test the source for HIV, HBsAg, HCV antibody, and syphilis to guide prophylaxis decisions 2, 6. This must occur within the first 24-72 hours to inform treatment decisions.

Follow-Up Testing Timeline

HIV Follow-Up

• 4-6 weeks post-exposure: Laboratory-based HIV Ag/Ab test PLUS diagnostic HIV NAT 1
  • Exception: May defer this if nPEP started within 24 hours AND no doses were missed
• 12 weeks post-exposure (final): Laboratory-based HIV Ag/Ab combination immunoassay AND diagnostic HIV NAT 1
• Extended to 12 months if source is HIV/HCV co-infected 2

HCV Follow-Up

• Standard approach: HCV antibody at 6 months post-exposure 2
• If early treatment strategy chosen: Monthly ALT for 4 months, then qualitative HCV-RNA when ALT elevation detected 2
• Alternative early detection: HCV RNA at 4-6 weeks can provide earlier diagnosis 3

HBV Follow-Up

• If HBIG and/or vaccine given, recheck anti-HBs at appropriate intervals based on vaccination status 2

Syphilis Follow-Up

• Repeat at 6 weeks, 3 months, and 6 months if initial exposure risk was high 4

Common Pitfalls to Avoid

1. Missing the 72-hour window: Post-exposure prophylaxis effectiveness drops significantly after this timeframe - initiate testing and treatment immediately 1

2. Forgetting HIV NAT for long-acting PrEP users: Standard antibody tests may be falsely negative due to residual drug suppression 1

3. Assuming HBV vaccination equals immunity: Always verify anti-HBs levels >10 mIU/mL; approximately 5-15% of vaccinated individuals are non-responders 2

4. Stopping nPEP prematurely: Complete the full 28-day course unless the source is definitively confirmed HIV-negative 1

5. Inadequate follow-up: The 12-week final HIV test is essential - earlier negative tests do not rule out seroconversion 1

6. Overlooking syphilis in sexual exposures: Neurosyphilis rates can reach 7.8% in HIV-positive individuals with sexual transmission, making baseline and follow-up testing critical 7