What are the baseline labs required for starting Pre-Exposure Prophylaxis (PrEP)?

Baseline Laboratory Testing for Starting PrEP

Before initiating PrEP, perform HIV testing with a combined HIV antibody/antigen test (or HIV RNA if acute infection suspected), and PrEP can be started the same day if negative without waiting for additional safety labs. 1

Essential Baseline Tests

HIV Testing (Most Critical)

• Combined HIV antibody and antigen test (4th generation) is the primary screening test 1
• If performed within 7 days of the visit and negative with no symptoms of acute HIV, PrEP can be initiated same-day 1
• If test unavailable, perform rapid point-of-care HIV test at the visit, but also send laboratory-based antibody/antigen test 1
• Add HIV RNA testing if clinical suspicion of acute HIV infection exists—withhold PrEP until results available 1

Renal Function Assessment

• Serum creatinine with calculated creatinine clearance (CrCl) should be obtained at baseline 1
• However, do not delay PrEP initiation while waiting for creatinine results 1
• Patients with CrCl 30-60 mL/min who are MSM may preferentially receive tenofovir alafenamide/emtricitabine if they have osteopenia/osteoporosis risk 1

Additional Baseline Testing (Obtain but Don't Delay)

• Hepatitis B surface antigen 1
• STI screening: gonorrhea and chlamydia nucleic acid amplification testing (genital and extragenital sites based on exposure), plus syphilis serology 1
• Hepatitis C antibody 1
• Pregnancy test (if applicable) 1

Critical Clinical Approach

The key principle is that PrEP should NOT be delayed waiting for any test results except HIV testing. 1 The primary goal of baseline testing is preventing PrEP administration in persons with undiagnosed HIV infection 1.

Same-Day PrEP Initiation Algorithm

1. Perform rapid HIV test at visit (if combined antibody/antigen test not available within 7 days) 1
2. If rapid test negative and no symptoms of acute HIV: start PrEP immediately 1
3. Send laboratory-based combined antibody/antigen test simultaneously 1
4. Order creatinine, hepatitis B surface antigen, STI panel, and hepatitis C antibody—but prescribe PrEP without waiting 1
5. Follow up quickly on results to ensure safety 1

When to Withhold PrEP

• Any symptoms suggestive of acute HIV infection (fever, rash, lymphadenopathy, pharyngitis)—obtain HIV RNA and withhold PrEP pending results 1
• Positive or indeterminate HIV screening test 1

Important Caveats

Renal Monitoring Considerations

• Older patients (≥50 years) and those with baseline CrCl <90 mL/min have substantially higher risk of renal impairment on tenofovir disoproxil fumarate 2
• Baseline renal dysfunction increases risk 28-fold for developing renal impairment during PrEP 2
• Consider tenofovir alafenamide formulation for MSM with pre-existing bone or renal concerns 1, 3

Acute HIV Infection Risk

• PrEP initiated during undiagnosed acute HIV infection can lead to drug resistance (typically M184V/I mutation) 1, 4
• Primary HIV infection in PrEP users presents with lower viral loads and fewer symptoms, complicating diagnosis 4
• HIV RNA testing is particularly important if recent ARV exposure or symptoms present 1, 4

Recent Exposure Scenario

• If high-risk exposure occurred within 72 hours: start 3-drug PEP regimen first, then transition to 2-drug PrEP after 28 days 1
• Perform combined antibody/antigen test plus HIV RNA at PEP conclusion before transitioning to PrEP 1

Prescribe no more than 30 days initially, then 90-day supplies thereafter, with follow-up visit at 1 month for repeat HIV testing. 1