Serum cystatin C concentration as a marker of acute renal dysfunction in critically ill patients

Introduction In critically ill patients sudden changes in glomerular filtration rate (GFR) are not instantly followed by parallel changes in serum creatinine. The aim of the present study was to analyze the utility of serum cystatin C as a marker of renal function in these patients. Methods Serum creatinine, serum cystatin C and 24-hour creatinine clearance (C-Cr) were determined in 50 critically ill patients (age 21-86 years; mean Acute Physiology and Chronic Health Evaluation II score 20 +/- 9). They did not have chronic renal failure but were at risk for developing renal dysfunction. Serum cystatin C was measured using particle enhanced immunonephelometry. Twenty-four-hour body surface adjusted C-Cr was used as a control because it is the 'gold standard' for determining GFR. Results Serum creatinine, serum cystatin C and C-Cr (mean +/- standard deviation [ range]) were 1.00 +/- 0.85 mg/dl (0.40-5.61 mg/dl), 1.19 +/- 0.79 mg/l (0.49-4.70 mg/l), and 92.74 +/- 52.74 ml/min per 1.73 m(2) (8.17-233.21 ml/min per 1.73 m(2)), respectively. Our data showed that serum cystatin C correlated better with GFR than did creatinine (1/cystatin C versus C-Cr: r = 0.832, P < 0.001; 1/creatinine versus C-Cr: r = 0.426, P = 0.002). Cystatin C was diagnostically superior to creatinine (area under the curve [AUC] for cystatin C 0.927, 95% confidence interval 86.1-99.4; AUC for creatinine 0.694, 95% confidence interval 54.1-84.6). Half of the patients had acute renal dysfunction. Only five (20%) of these 25 patients had elevated serum creatinine, whereas 76% had elevated serum cystatin C levels (P = 0.032). Conclusion Cystatin C is an accurate marker of subtle changes in GFR, and it may be superior to creatinine when assessing this parameter in clinical practice in critically ill patients.