Definitions of pediatric acute kidney injury (AKI) use changes of serum creatinine. There is a paucity of well-designed studies in infants because of creatinine age-dependency. The emerging role of cystatin C as a superior marker of renal dysfunction led to a carefully conducted study on AKI in infants by Zappitelli et al. This Commentary calls for the development of age-independent serum creatinine and estimated glomerular filtration rate z scores. © 2011 International Society of Nephrology.