Laboratory Monitoring for Patients on PrEP
For HIV-negative adults starting daily oral tenofovir-based PrEP, perform combined HIV antibody/antigen testing, serum creatinine with estimated creatinine clearance, hepatitis B surface antigen, hepatitis C antibody, STI screening by NAAT at all exposure sites, and pregnancy testing (if applicable) at baseline, then repeat HIV testing at 1 month and quarterly thereafter, with renal function monitoring at the first quarterly visit and annually for stable patients. 1
Baseline Testing (Prior to PrEP Initiation)
Before prescribing any tenofovir-based PrEP regimen, obtain the following mandatory tests:
HIV Testing
- Combined HIV antibody and antigen test is required to exclude chronic HIV infection 1
- If acute HIV infection is clinically suspected (fever, rash, lymphadenopathy, or recent high-risk exposure), add HIV RNA testing and withhold PrEP until results confirm HIV-negative status 1
- PrEP can be initiated same-day if a combined antibody/antigen test performed within 7 days was negative and no symptoms of acute HIV are present 1
Renal Function Assessment
- Measure serum creatinine and calculate estimated creatinine clearance (Cockcroft-Gault) or eGFR (CKD-EPI) 1, 2
- TDF-based PrEP (Truvada) is contraindicated if creatinine clearance is <60 mL/min/1.73 m² 2, 3
- For MSM with creatinine clearance 30-60 mL/min or those with osteopenia/osteoporosis, prescribe TAF-based PrEP (Descovy) instead 1, 3
Hepatitis Screening
- Hepatitis B surface antigen (HBsAg) to identify chronic HBV infection 1, 3
- Hepatitis C IgG antibody (if not previously known positive; if positive, confirm HCV RNA) 1
- Hepatitis A IgG antibody for MSM and people who inject drugs (if immunity unknown) 1
STI Screening
- Genital and nongenital Neisseria gonorrhoeae and Chlamydia trachomatis testing by NAAT at all potential exposure sites (urogenital, rectal, pharyngeal) 1, 3
- For MSM, three-site screening (rectal, pharyngeal, urogenital) is mandatory 3
Pregnancy Testing
Follow-Up Monitoring During PrEP
At 1 Month After Initiation
Quarterly Monitoring (Every 3 Months)
- Combined HIV antibody and antigen test at every quarterly visit 1, 3
- Estimated creatinine clearance at the first quarterly visit (month 3), then annually thereafter if stable 1
- Three-site STI screening (gonorrhea and chlamydia by NAAT at urogenital, rectal, and pharyngeal sites) 1, 3
- Syphilis testing 1
- Pregnancy testing for individuals of childbearing potential 1, 4
Annual Monitoring (Every 12 Months)
- Combined HIV antibody and antigen test 1
- Estimated creatinine clearance 1
- Hepatitis C antibody test (every 3-6 months for people who inject drugs and MSM who use recreational drugs during sex if liver function tests are abnormal) 1
Intensified Renal Monitoring for High-Risk Patients
Certain patients require more frequent renal function monitoring every 1-3 months rather than the standard annual schedule: 2
- Baseline creatinine clearance 60-90 mL/min/1.73 m² 2
- Age ≥40 years (independent predictor of eGFR decline) 5
- Taking antihypertensive or diabetes medications 2
- Decompensated cirrhosis 2
- Poorly controlled hypertension 2
- Proteinuria 2
- Uncontrolled diabetes 2
- Active glomerulonephritis 2
- Concurrent nephrotoxic drugs 2
- Solid-organ transplantation 2
If creatinine clearance falls below 60 mL/min/1.73 m² or serum phosphate drops below 2 mg/dL during PrEP, increase monitoring frequency to every 1-3 months. 2
Special Considerations by PrEP Formulation
TDF-Based PrEP (Truvada)
- Mean creatinine increases by approximately 0.03 mg/dL (4.6%) and creatinine clearance decreases by 4.8 mL/min (3.0%) within the first 12 weeks, then stabilizes 5
- These changes are nonprogressive after 12 weeks but warrant baseline and ongoing monitoring 5
- Approximately 15.7% of patients develop worsening proteinuria by week 12 5
TAF-Based PrEP (Descovy)
- Measure creatinine clearance at the first quarterly visit (approximately 3 months) and then annually thereafter, reflecting TAF's improved renal safety profile compared to TDF 2
- Markers of glomerular filtration and proximal renal tubule dysfunction improve or remain stable over 144 weeks 6
- Bone mineral density increases or remains stable, making TAF preferable for patients with bone or renal morbidities 6
Critical Pitfalls to Avoid
Missed Acute HIV Infection
- Never initiate PrEP if acute HIV infection is suspected clinically, even if antibody/antigen testing is negative, because the window period can result in false-negative results 1
- Starting PrEP during undiagnosed acute HIV infection risks development of drug resistance 7
Inadequate Renal Monitoring
- Do not prescribe TDF-based PrEP to patients with creatinine clearance <60 mL/min 2, 3
- Patients with baseline eGFR <90 mL/min/1.73 m² have a 9.59-fold increased risk of developing new-onset eGFR <70 mL/min/1.73 m² 5
Hepatitis B Flare Risk
- If a patient with chronic HBV discontinues tenofovir-based PrEP, monitor closely for severe hepatitis flare or hepatic decompensation, particularly in those with cirrhosis 3, 4
Prescription Duration
- Limit initial PrEP prescriptions to 30 days, then 90-day supplies thereafter 1
- Never prescribe PrEP for >90 days without interval HIV testing 4
Restarting PrEP After Interruption
- If PrEP has been stopped for ≥7 consecutive days, repeat combined HIV antibody/antigen testing before restarting 1