The main aim of this study whether serum cystatin C can detect early changes in development of acute kidney injury compared to serum creatinine in critically ill patients and to study the confounding factors that normally can interfere in the analysis of serum creatinine. Serum cystatin C showed a faster increase in critically ill patients who were in the early stages of acute kidney injury compared to serum creatinine. Serum cystatin C showed superior sensitivity in adolescents, adults and in aged peoples compared to serum creatinine in detecting acute kidney injury. Cystatin showed better sensitivity in classifying patients having acute kidney injury into Risk, Injury and Failure categories compared to creatinine. | Serum cystatin C compared to serum creatinine as an early marker of renal failure