Laboratory Testing Prior to Starting PrEP
Before initiating PrEP, you must obtain a combined HIV antibody/antigen test (4th generation) and confirm it is negative—this is the only test that should delay PrEP initiation. 1, 2, 3
Mandatory HIV Testing (Gates PrEP Initiation)
Perform a combined HIV antibody and antigen test (4th generation) and do not start PrEP until results confirm HIV-negative status. 1, 2, 3
If the test was performed within the past 7 days and is negative with no symptoms of acute HIV, initiate PrEP immediately on the same day. 2, 3
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, 2, 3
This is critical because initiating PrEP during undiagnosed acute HIV infection can lead to drug resistance, particularly M184V/I mutations, which compromises future treatment options. 2, 4
PrEP users with acute HIV present with lower viral load peaks and fewer symptoms, making diagnosis more challenging and prolonging seroconversion stages. 4
Baseline Tests (Do Not Delay PrEP While Awaiting These Results)
Renal Function:
Obtain serum creatinine with calculated creatinine clearance (CrCl) at baseline, but do not delay PrEP initiation while waiting for results. 1, 2, 3
TDF-based PrEP is contraindicated if CrCl <60 mL/min. 2
Hepatitis Screening:
Test hepatitis B surface antigen (HBsAg) because stopping TDF in HBV-positive patients can cause severe hepatitis flares. 1, 2
Test hepatitis C IgG antibody if not known to be previously positive. 1, 2
For MSM and people who inject drugs, test hepatitis A IgG antibody if immunity status is unknown. 1, 2
STI Screening:
Perform nucleic acid amplification testing (NAAT) for Neisseria gonorrhoeae and Chlamydia trachomatis from all exposure sites (genital, rectal, pharyngeal). 1, 2
Obtain syphilis serology. 2
Pregnancy Testing:
Critical Clinical Approach
The primary goal is preventing PrEP administration in individuals with undiagnosed HIV infection—only HIV testing should gate PrEP initiation. 2, 3 All other baseline tests are important for monitoring and safety but should not delay starting PrEP in someone at high risk for HIV acquisition. 2, 3
Common Pitfalls to Avoid
Do not delay PrEP initiation waiting for creatinine, hepatitis, or STI results—only HIV testing should delay initiation. 2, 3
Do not skip HIV RNA testing when acute HIV is suspected clinically—standard antibody/antigen testing may be negative during the window period, and starting PrEP during acute infection leads to resistance. 1, 2, 4
If high-risk exposure occurred within 72 hours, start a 3-drug PEP regimen first, then transition to 2-drug PrEP after 28 days with repeat HIV testing. 3
For patients with pre-existing bone or renal concerns, consider tenofovir alafenamide (TAF) formulation instead of TDF. 3