What are the recommended screening labs for individuals considering Pre-Exposure Prophylaxis (PrEP)?

Screening Labs for PrEP

Before initiating PrEP, you must obtain HIV testing (preferably a laboratory-based antigen/antibody test plus HIV RNA), serum creatinine with calculated creatinine clearance, hepatitis B surface antigen, STI screening at all relevant anatomic sites, hepatitis C antibody, and pregnancy test if applicable—but only HIV testing results should delay PrEP initiation. 1, 2

Essential Baseline Laboratory Tests

HIV Testing (Must Complete Before Starting PrEP)

• Laboratory-based combination antigen/antibody test (4th generation) is mandatory to confirm HIV-seronegative status before initiating PrEP 1, 2
• HIV RNA testing should be added if acute HIV infection is suspected based on symptoms (fever, rash, lymphadenopathy, pharyngitis) or if the patient has recently taken antiretrovirals or received cabotegravir injection within the past year 1, 3
• If HIV test results from within the past 7 days are negative, you can initiate PrEP immediately while awaiting other diagnostic results 1, 2
• A rapid HIV antibody test alone is insufficient—you need laboratory-based testing for adequate sensitivity 1, 2

Renal Function Assessment (Do Not Delay PrEP)

• Serum creatinine with calculated creatinine clearance is recommended before initiation, but PrEP should not be delayed while waiting for these results 1, 2
• This is particularly important because tenofovir-based regimens can affect renal function 1

Hepatitis Screening (Do Not Delay PrEP)

• Hepatitis B surface antigen (HBsAg) testing is recommended at baseline but should not impede PrEP initiation 1, 2
• Hepatitis C antibody testing should be performed at baseline 1, 2

STI Screening (Do Not Delay PrEP)

• Comprehensive STI screening at all anatomic sites of exposure (pharyngeal, urogenital, rectal) for gonorrhea and chlamydia is recommended 1
• Syphilis serology should be obtained at baseline 1
• Testing should be based on sexual history and anatomic sites of potential exposure 1

Pregnancy Testing (Do Not Delay PrEP)

• Pregnancy test should be performed at baseline for individuals who can become pregnant, but PrEP initiation should not be delayed while waiting for results 2

Critical Clinical Approach

The overriding principle is that only HIV testing results should delay PrEP initiation—all other tests can be pending. 1, 2 The primary goal is preventing PrEP administration to someone with undiagnosed HIV infection, which could lead to drug resistance 2

Rapid Start Protocol

• If no recent HIV test is available, conduct testing and initiate PrEP once results are negative 1
• If symptoms suggest acute HIV infection, withhold PrEP pending HIV RNA results 1, 2
• For substantial HIV exposure within 72 hours, start a 3-drug post-exposure prophylaxis (PEP) regimen first, then transition to PrEP after completion if HIV tests remain negative 1, 2

Follow-Up Laboratory Monitoring

HIV Testing During PrEP

• Every 3 months HIV testing is mandatory—PrEP prescription should not exceed 90 days without interval HIV testing 1
• A visit 30 days after PrEP start is recommended for follow-up HIV testing to assess for acute infection that may have been in the window period at initiation 1
• Use laboratory-based antigen/antibody testing for ongoing monitoring 1

STI Screening During PrEP

• Quarterly STI screening (every 3 months) at contact sites and blood syphilis testing is recommended 1
• This frequent screening is critical because STI prevalence remains high among PrEP users (6-10% for gonorrhea and chlamydia at follow-up visits) 4, 5

Renal Function Monitoring

• Creatinine measurement every 6 months at minimum 1
• More frequent monitoring (every 3 months) for patients over 50 years, taking hypertension or diabetes medications, or with baseline creatinine clearance <90 mL/min 1

Hepatitis C Monitoring

• Annual HCV serologic testing at minimum, with more frequent testing for elevated transaminases or high-risk individuals (people who inject drugs) 1

Common Pitfalls to Avoid

• Do not delay PrEP initiation waiting for non-HIV laboratory results—this is the most common error in clinical practice 1, 2
• Do not use rapid oral fluid HIV tests for PrEP screening, as they are less sensitive for acute infection than blood tests 1, 6
• Do not rely on antibody-only HIV tests at baseline, as they will miss acute infection 3, 6
• Do not skip anatomic site-specific STI testing—pharyngeal and rectal infections are frequently asymptomatic but highly prevalent 1
• Do not forget the 30-day follow-up visit after PrEP initiation, which can catch acute HIV infections that were in the window period at baseline 1