Creatinine and Cystatin C: A Measure of Renal Function in Men With Testosterone-Induced Muscle Hypertrophy.

Creatinine (Cr) is often used as a standalone gold standard marker of kidney function. Cystatin C (Cys C) is a less physiologically labile marker of renal function, particularly in certain subgroups. Herein, we analyze trends in cystatin C as compared to creatinine in men on testosterone replacement therapy with varying body mass indices and percent body fat (PBF). This retrospective analysis observes 227 men with testosterone-induced muscle hypertrophy who visited a men's health tertiary care clinic. All participants were characterized as competitive or recreational athletes. In patients with a normal body mass index (BMI), there was no clinically significant correlation between Cr and Cys C. Slight correlation was seen with overweight (R² = .27) patients (p < .0001) and obese (R² = .29) patients (p < .0001). Patients with PBF of 0%-10% (n = 22) exhibited minimal (R² = .23) positive correlation between Cys C and Cr (p = .03). Positive correlation between Cys C and Cr in patients with PBF of 10%-20% was clinically negligible (R² = .17, n = 87), modest (R² = .49) in patients with PBF of 20%-30% (n = 42), and evident (R² = 1.00) in patients >30% (n = 3) (p < .0001, respectively). Cystatin C measurements display less variance compared with creatinine at differing BMI distinctions. At the upper limit of BMI or PBF in our patient population, cystatin C exhibits minimal to moderate variability compared with creatinine.