Kidney function assessment using cystatin C and serum creatinine in heart transplantation recipients: Implications for valganciclovir dosing

Background: Among heart transplantation (HT) recipients, the accuracy of serum creatinine (sCr)-based estimated glomerular filtration rate (eGFR) may be limited by fluctuations in muscle mass. Cystatin C (cysC) is less influenced by muscle mass, but its levels may increase with obesity and steroid use. Herein, we (1) longitudinally compared eGFRcysC and eGFRsCr among HT recipients; (2) investigated the association of body mass index (BMI), steroid use, and muscle mass with discrepancies between eGFRs; and (3) explored the implications of eGFRcysC use on valganciclovir (VGC) dosing. Methods: cysC and sCr were measured in 294 blood samples obtained from 80 subjects. Intraindividual differences between eGFRs (eGFRdiff_cysC-sCr) were calculated with negative values corresponding to eGFRsCr > eGFRcysC and positive values to eGFRcysC > eGFRsCr. In a patient subset (n = 21), pectoralis muscle measures were obtained. Results: Marked differences between eGFRcysC and eGFRsCr were observed, particularly early post-HT (1-week post-HT, median eGFRdiff_cysC-sCr −28 ml/min/1.73 m²). eGFRcysC demonstrated stability following a transient postoperative decline, while eGFRsCr decreased in the first year post-HT. Lower BMI and higher prednisone dose displayed a modest association with more negative eGFRdiff_cysC-sCr values. Pectoralis muscle measures indicative of greater muscle mass and better tissue quality exhibited a stronger association with more positive eGFRdiff_cysC-sCr values. The use of eGFRcysC would have led to VGC dose adjustment in 46% of samples, predominantly resulting in dose reduction. Conclusions: Among HT recipients, eGFRcysC and eGFRsCr markedly differ with implications for VGC dosing. The observed discrepancies may reflect changes in body composition and steroid use.