Serum cystatin C level for better assessment of glomerular filtration rate in cystic fibrosis patients treated by amikacin

SUMMARY Background and objective: Monitoring of renal function in cystic fibrosis (CF) patients is essential. The dosage regimen of amikacin is regularly modified according to the patient’s glomerular filtration rate (GFR). The aim of the study was to evaluate the use of cystatin C (CyC) for monitoring amikacin therapy along with other markers of renal tubular and glomerular function, and damage [N-acetyl-b-D glucosaminidase (NAG), creatinine level and creatinine clearance]. Methods: We compared the GFR, estimated from the serum concentrations of creatinine (Cockcroft‐Gault formula) and CyC (Grubb’s formula). Seventy-one patients (mean age 12 years; range 4‐28 years) with CF were treated by intermittent intravenous infusion of amikacin. Tubular nephrotoxicity was investigated by measurement of urine NAG ⁄urine creatinine ratio (U-NAG ⁄ U-creatinine). Concentrations of all markers were measured before starting amikacin therapy and at days 3, 5, 7, 10 and 12. Fluorescence polarization analysis, turbidimetry, enzymatic phototometric creatinine deaminase method and fluorimetry were used for determination of serum amikacin, serum CyC, creatinine and urine NAG activity. Receiver operating characteristic (ROC) analysis was performed to assess the influence of GFR estimated from serum creatinine and serum CyC for the prediction of amikacin clearance during aminoglycoside therapy. Results: Significant differences in the rate of U-NAG ⁄U-creatinine were noted before and after treatment with amikacin (P