# 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?

## TL;DR

This paper argues that serum creatinine and urine output should be unified into a single AKI severity measure based on creatinine excretion rate.

## Contribution

The paper proposes a novel framework unifying serum creatinine and urine output as expressions of urine creatinine excretion.

## Key findings

- Serum creatinine and urine output are physiologically linked and should not be treated as independent AKI markers.
- Urine creatinine excretion rate may serve as a more accurate and unified AKI severity criterion.
- Current AKI evaluation practices may be improved by considering creatinine excretion dynamics.

## Abstract

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.