Which baseline laboratory tests are needed before initiating pre‑exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate/emtricitabine in a male patient who has sex with men?

Baseline Laboratory Testing for PrEP Initiation in MSM

Before initiating PrEP in a male patient who has sex with men, you must obtain: combined HIV antibody/antigen testing, serum creatinine, hepatitis B surface antigen, hepatitis C antibody, hepatitis A antibody, and nucleic acid amplification testing (NAAT) for gonorrhea and chlamydia at all exposure sites (genital, rectal, and pharyngeal). 1

Essential Pre-Initiation Labs

The 2020 International Antiviral Society-USA Panel guidelines provide the most comprehensive and recent framework for PrEP initiation testing 1:

HIV Testing (Critical - Do Not Skip)

• Combined HIV antibody and antigen test (4th generation test) is mandatory 1
• Add HIV RNA testing if there is any clinical suspicion of acute HIV infection or if the patient reports unprotected sex with an HIV-positive partner in the preceding month 1
• Do not initiate PrEP until HIV RNA results confirm HIV-negative status if acute infection is suspected 1

Critical Pitfall: Starting PrEP in someone with undiagnosed acute HIV infection can lead to drug resistance, as the two-drug regimen (TDF/FTC) is insufficient for HIV treatment. This is particularly concerning during the "window period" when antibody tests may still be negative but the person is infected and highly infectious 2, 3.

Renal Function Assessment

• Serum creatinine level with calculated creatinine clearance (use Cockcroft-Gault formula) 1, 4
• Confirm creatinine clearance ≥60 mL/min before prescribing TDF-based PrEP 4
• This is essential because tenofovir can cause renal toxicity, including acute renal failure and Fanconi syndrome 5

Hepatitis Screening

• Hepatitis B surface antigen (HBsAg) 1
  • If positive, TDF/FTC can serve dual purpose as both PrEP and hepatitis B treatment 4
  • Warning: Abrupt discontinuation of TDF/FTC in HBV-infected patients can cause severe acute hepatitis exacerbation 5
• Hepatitis C IgG antibody (if not previously known to be positive) 1
  • If known positive, confirm hepatitis C RNA if not recently documented
• Hepatitis A IgG antibody - specifically recommended for MSM 1
  • Vaccinate if not immune

STI Screening (Site-Specific)

• Gonorrhea and chlamydia NAAT testing at all potential exposure sites 1:
  • Genital (urethral or cervical)
  • Rectal
  • Pharyngeal
• This multi-site testing is crucial for MSM given the high prevalence of extragenital infections that are often asymptomatic

Additional Considerations

Rapid PrEP Start Approach

The 2024 IAS-USA guidelines support rapid initiation 6:

• If HIV test results from within the past 7 days are negative, you can initiate PrEP while awaiting other safety assessments 6
• If no recent test available, conduct testing and initiate once negative, assuming reliable follow-up communication 6

What You Don't Need at Baseline

• Pregnancy testing (not applicable to this patient)
• Syphilis serology (this is recommended quarterly during PrEP, not necessarily at baseline, though reasonable to obtain) 1
• Bone mineral density testing (only if specific risk factors present) 5

Common Pitfalls to Avoid

1. Using only point-of-care HIV antibody tests: While these facilitate rapid initiation, guidelines recommend confirming with laboratory-based antigen/antibody testing 1, 7. Recent data shows only 35.3% of PrEP initiators received proper laboratory-based HIV testing 7.

2. Missing extragenital STI screening: Many MSM have asymptomatic rectal or pharyngeal infections that urethral testing alone will miss 1.

3. Forgetting hepatitis A screening: This is specifically recommended for MSM and often overlooked 1.

4. Inadequate HIV RNA testing when indicated: If there's any suspicion of recent exposure or acute HIV symptoms, HIV RNA must be obtained before starting PrEP 1, 2.

5. Not calculating creatinine clearance: Simply checking serum creatinine without calculating clearance is insufficient 4.