Baseline Laboratory Tests Before Starting Tenofovir Disoproxil Fumarate
Before initiating tenofovir disoproxil fumarate (TDF), you must obtain: hepatitis B virus (HBV) testing, HIV testing, serum creatinine with estimated creatinine clearance, urine glucose, urine protein, and serum phosphorus in patients with chronic kidney disease. 1
Required Testing Based on FDA Label Requirements
The FDA label is explicit about mandatory baseline testing 1:
Essential Tests for All Patients:
- HBV testing - Required before or when initiating TDF to identify chronic HBV infection, as discontinuation can cause severe acute exacerbations of hepatitis B
- HIV testing - HIV antibody testing should be offered to all HBV-infected patients; conversely, all HIV patients should be tested for HBV before starting TDF
- Serum creatinine and estimated creatinine clearance - TDF is principally eliminated by the kidney and can cause renal impairment including acute renal failure and Fanconi syndrome
- Urine glucose - To detect baseline glucosuria and monitor for proximal renal tubulopathy
- Urine protein - To assess baseline proteinuria and renal tubular function
Additional Test for Patients with Chronic Kidney Disease:
- Serum phosphorus - Required in patients with pre-existing chronic kidney disease to monitor for hypophosphatemia associated with Fanconi syndrome
Clinical Context and Rationale
The renal monitoring is critical because TDF undergoes active tubular secretion and can cause dose-dependent nephrotoxicity 2. Patients with creatinine clearance below 50 mL/min require dose-interval adjustments (every 48 hours for CrCl 30-49 mL/min, every 72-96 hours for CrCl 10-29 mL/min) 1.
The HBV/HIV co-testing requirement addresses two key safety concerns: (1) preventing severe hepatitis B flares upon TDF discontinuation in unrecognized HBV patients, and (2) avoiding HIV resistance development if TDF is used as monotherapy in undiagnosed HIV/HBV co-infected patients 1.
Important Caveats
Avoid initiating TDF if the patient is currently using or recently used nephrotoxic agents (e.g., high-dose or multiple NSAIDs), as this significantly increases acute renal failure risk 1. Recent case reports document HIV patients stable on TDF who developed acute renal failure requiring hospitalization and renal replacement therapy after starting NSAIDs.
No baseline hematologic testing (complete blood count) or routine liver function tests are mandated by the FDA label for TDF initiation, though guidelines note these have limited utility for contemporary first-line regimens 3.
Monitoring Schedule After Initiation
Once baseline labs are obtained and TDF is started, continue assessing serum creatinine, estimated creatinine clearance, urine glucose, and urine protein "on a clinically appropriate schedule" throughout treatment 1. The 2025 guideline evidence suggests that frequent early monitoring (at 4-8 weeks) provides minimal clinical benefit for contemporary regimens 3.