['Acute kidney injury (AKI) has been associated with high morbidity and mortality rates among critically ill children. Cystatin C is a protease inhibitor, and studies have shown that it is a promising marker for the early diagnosis of AKI. Our goal in this study was to assess whether serum cystatin C could serve as an accurate marker for the diagnosis of AKI.', 'This prospective study was undertaken in the pediatric intensive care unit at King Abdulaziz University Hospital. Serum creatinine and serum cystatin C levels were both measured in patients on admission (0\xa0h) and at 6, 12, and 24\xa0h after admission. AKI was diagnosed according to the modified pRIFLE criteria. Receiver operating characteristic (ROC) curve analysis was performed to assess the utility of serum cystatin C for diagnosing AKI.', 'A total of 62 patients were enrolled in this study, and 32 were diagnosed with AKI according to the modified pRIFLE criteria (51.4\xa0%). The area under the ROC curve for serum cystatin indicated that it was a good marker for the diagnosis of AKI at 0, 6, 12 and 24\xa0h, with sensitivities of 78, 94, 94 and 83\xa0%, respectively. However, the specificities of serum cystatin C at 0, 6, 12, and 24\xa0h were 57, 57, 60 and 50\xa0%, respectively. The optimal cutoff value was 0.645\xa0mg/L. The area under the ROC for serum creatinine showed sensitivities of 50, 65.4, 69.2 and 57.7\xa0% and specificities of 67.7, 70, 60 and 70\xa0% at 0, 6, 12 and 24\xa0h, respectively. The optimal cutoff value for serum creatinine was 30\xa0μmol/l. Comparisons of ROC curves revealed that serum cystatin C was superior to serum creatinine for the diagnosis of AKI at 12\xa0h (p\u2009=\u20090.03), but no differences were detected at 0, 6 or 24\xa0h.'] Is serum cystatin a useful marker for the diagnosis of acute kidney injury in critically ill children : prospective cohort study?