Serum cystatin C is a poor biomarker for diagnosing acute kidney injury in critically-ill children

BACKGROUND: Accurate diagnosis of acute kidney injury (AKI) is problematic especially in critically-ill patients in whom renal function is in an unsteady state. AIM: Our aim was to evaluate the role of serum (S.) cystatin C as an early biomarker of AKI in critically-ill children. SUBJECTS AND METHODS: S. creatinine and S. cystatin C were measured in 32 critically-ill children who were at risk for developing AKI. AKI was defined by both: Risk,-injury,-failure,-loss, and-endstage renal disease (RIFLE) classification and glomerular filtration rate (GFR) <80 ml/min/1.73 m(2). GFR was estimated by both Schwartz formula and S. cystatin C-based equation. RESULTS: S. cystatin C was not statistically higher in AKI patients compared with non-AKI by RIFLE classification (median 1.48 mg/l vs. 1.16 mg/l, P = 0.1) while S. creatinine was significantly higher (median 0.8 mg/dl vs. 0.4 mg/dl, P = 0.001). On estimating GFR by the two equations we found, a lag between rise of S. cystatin C and creatinine denoted by lower GFR by Schwartz formula in four patients, on other hand, six patients had elevated S. cystatin C with low GFR despite normal creatinine and GFR, denoting poor concordance between the two equations and the two markers. The ability of S. creatinine in predicting AKI was superior to S. cystatin with area under the curve (AUC) 0.95 with sensitivity and specificity (100% and 84.6%, respectively) using the RIFLE classification. The same findings were found when using Schwartz formula. CONCLUSION: S. cystatin C is a poor biomarker for diagnosing AKI in critically-ill children.