Unifying Serum Creatinine and Urine Output in a Single On-Time AKI Severity Criterion: Is It All About the Rate of Creatinine Being Excreted by the Kidneys?

Serum creatinine (sCr) and urine output (UO) have long been considered the cornerstones of acute kidney injury (AKI) severity criteria. Many articles were previously published discussing the prognostic relevance of fulfilling either one or both AKI criteria. However, sCr and UO must not be considered independent variables because they are physiologically linked despite having distinct chronologies as AKI markers. An increase in sCr is a late manifestation of decreased renal function and body creatinine accumulation and not an on-time surrogate for a decreasing glomerular filtration rate. On the other hand, oliguria is not a single entity, and its interpretation relies on urine's biochemical composition as well as its threshold pathological output value, which is somewhat controversial. In the present article, the current practice of evaluating sCr and UO separately is questioned and the idea that they can eventually be considered different expressions of the same variable of interest (the urine creatinine excretion) is highlighted.