Clinical use of clinician-initiated cystatin C testing in assessing renal function in a subset of patients taking tenofovir disoproxil/emtricitabine as HIV pre-exposure prophylaxis: a retrospective chart review

Objective Clinical guidelines recommend monitoring of serum creatinine concentrations in users of tenofovir disoproxil/emtricitabine for HIV pre-exposure prophylaxis (PrEP) due to its potential nephrotoxicity. Cystatin C might aid clinicians in determining whether creatinine elevations reflect true kidney injury. We reviewed the use of Cystatin C in our practice. Methods Clinic records from Gay Men’s Health Service were reviewed until March 2024, and those from St. James’s Hospital were reviewed until October 2024, specifically for patients who had a cystatin C test requested due to concerns regarding their renal function. Descriptive statistics were used to analyse the data. Results Seventy PrEP users, with a median age of 39 years (IQR 34–45), were included. Data on body mass index (BMI) and blood pressure were available in 57/70 (81.4%) and 41/70 (58.6%), respectively; 37/57 (64.9%) had a BMI≥ 25 kg/m 2 and 14/41 (34.1%) had hypertension. Protein supplements, creatine and anabolic agents were used by 43/70 (61.4%), 29/70 (41.4%) and 11/70 (15.7%), respectively. Recreational drugs were used by 17/70 (24.3%). The median cystatin C was 0.87 mg/L (IQR 0.81–0.99); 21/70 (30%) had a result above the upper limit of normality. At this time, the median creatinine was 106.5 µmol/L (97–114). An estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m 2 according to Modification of Diet in Renal Disease, Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) (2021), CKD-EPI creatinine-cystatin (2021) and CKD-EPI cystatin C (2012) was found in 11/70 (15.7%), 1/70 (1.4%), 0/70 (0%) and 1/70 (1.4%), respectively. Median eGFR was 67 (61–74), 78 (70–86), 90.5 (82–96), 102.5 (84–111) mL/min/1.73m 2 , respectively. Urinary protein-to-creatinine ratio was <15 mg/mmol in 65/70 (92.9%), and nobody had significant proteinuria of >50 mg/mmol. Conclusion In this review, the addition of cystatin C to calculate eGFR lessened concerns regarding renal dysfunction, facilitating safe ongoing PrEP prescribing and potentially avoiding unnecessary referral to nephrology services.