What labs are required before starting a patient on Pre-Exposure Prophylaxis (PrEP) and what ongoing monitoring labs are recommended?

Laboratory Testing for PrEP Initiation and Monitoring

Baseline Testing Before Starting PrEP

Before initiating PrEP, you must obtain a combined HIV antibody/antigen test (4th generation) and serum creatinine with calculated creatinine clearance; however, PrEP should not be delayed while waiting for creatinine results—only HIV testing should gate initiation. 1, 2

Essential Pre-Initiation Labs

HIV Testing:

• Combined HIV antibody/antigen test (4th generation) is mandatory and must be negative before starting PrEP 1
• If the test was performed within the past 7 days and is negative with no symptoms of acute HIV, initiate PrEP immediately 1, 2
• Add HIV RNA testing if any clinical suspicion of acute HIV infection exists (fever, rash, lymphadenopathy, pharyngitis)—withhold PrEP until RNA results return negative 1
• This is critical because initiating PrEP during undiagnosed acute HIV can lead to drug resistance, particularly M184V/I mutations 1, 3

Renal Function:

• Serum creatinine with calculated creatinine clearance (CrCl) 1
• Do not delay PrEP initiation while waiting for these results 2
• Tenofovir disoproxil fumarate (TDF)-based PrEP is contraindicated if CrCl <60 mL/min 1

Hepatitis Screening:

• Hepatitis B surface antigen (HBsAg)—essential because stopping TDF in HBV-positive patients can cause hepatitis flares 1
• Hepatitis C antibody (if not previously positive) 1
• Hepatitis A IgG antibody for men who have sex with men (MSM) and people who inject drugs 1

STI Screening:

• Nucleic acid amplification testing (NAAT) for Neisseria gonorrhoeae and Chlamydia trachomatis from all exposure sites (genital, rectal, pharyngeal as appropriate) 1
• Syphilis serology 1

Pregnancy Testing:

• For individuals of childbearing potential 1

Important Caveat for Recent High-Risk Exposure

If the patient reports unprotected high-risk exposure within the past 72 hours, start a 3-drug post-exposure prophylaxis (PEP) regimen immediately, then transition to 2-drug PrEP after 28 days 1, 2. Perform both combined antibody/antigen test and HIV RNA at PEP conclusion before transitioning to PrEP 1.

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Ongoing Monitoring While on PrEP

At 1 Month After Initiation

• Combined HIV antibody/antigen test 1
• This catches any breakthrough infections early, particularly if PrEP was inadvertently started during the window period 1

Quarterly (Every 3 Months)

HIV and STI Testing:

• Combined HIV antibody/antigen test at every visit 1
• NAAT for gonorrhea and chlamydia from all exposure sites 1
• Syphilis serology 1
• Pregnancy testing for applicable individuals 1

Renal Monitoring:

• Calculate creatinine clearance at the first quarterly visit, then annually thereafter 1
• Exception: Monitor every 3-6 months for patients at increased risk of kidney injury: age >50 years, baseline CrCl <90 mL/min, diabetes, hypertension 1

Annually

• Creatinine clearance (if not being monitored more frequently) 1
• Hepatitis C antibody 1
• Exception: Test hepatitis C every 3-6 months for people who inject drugs or MSM who use recreational drugs during sex 1

Critical Monitoring Pitfalls to Avoid

If PrEP is stopped for ≥7 consecutive days:

• Repeat combined HIV antibody/antigen test before restarting 1
• This prevents restarting PrEP during an undiagnosed acute infection acquired during the gap 1

If substantial nonadherence with high-risk exposure occurs:

• Discontinue PrEP and initiate 28-day course of 3-drug PEP instead 1

If HIV infection is suspected while on PrEP:

• Order immediate HIV RNA and genotype testing 1
• Add a boosted protease inhibitor (ritonavir-boosted darunavir) and/or dolutegravir to the TDF/emtricitabine pending results 1
• This prevents development of resistance while awaiting confirmatory testing 1

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Algorithm Summary

Before Starting PrEP:

1. Combined HIV antibody/antigen test (add RNA if acute HIV suspected) → Must be negative
2. Serum creatinine/CrCl (but don't delay PrEP for results)
3. HBsAg, hepatitis C antibody
4. STI screening (gonorrhea/chlamydia NAAT, syphilis serology)
5. Pregnancy test if applicable

After Starting PrEP:

• 1 month: HIV antibody/antigen test
• Every 3 months: HIV test, STI screening, pregnancy test; CrCl at first visit then annually (or every 3-6 months if high-risk for renal injury)
• Annually: Hepatitis C antibody (more frequently if PWID or MSM using drugs during sex)

This structured approach ensures early detection of HIV acquisition (which can occur despite PrEP with poor adherence) and monitors for the primary adverse effects of TDF-based regimens (renal and gastrointestinal toxicity) 1, 4.