The impact of acute kidney injury on the urea-to-creatinine ratio - A single-center retrospective analysis.

BACKGROUND: The urea-to-creatinine ratio (UCR) is used as a biomarker of protein catabolism in critical illness. However, the impact of acute kidney injury (AKI) severity on UCR dynamics remains poorly understood. METHODS: This retrospective study analyzed 1567 consecutive ICU (intensive care unit) patients at two different ICUs at the university hospital of Innsbruck. Patients were categorized by AKI stage (KDIGO criteria). Temporal trajectories of UCR, urea generation rate and creatinine clearance were modeled using generalized additive models. Risk factors for an elevated UCR (>75 mg/dl per mg/dl) were identified via multivariate logistic regression. RESULTS: Peak urea levels occurred consistently around days 6-7 across all groups. In contrast, creatinine peaked earlier (day 2 in stage 0; days 4-5 in stages I-III). The relationship between evolving AKI and UCR was time-dependent and non-monotonic. While AKI was associated with a significantly lower UCR during the early ICU course, this was followed by a reversal later during the ICU stay. Urea generation rate remained fairly comparable between groups. Multivariate analysis revealed that higher urea generation rate, protein intake, SAPS III, corticosteroid administration and older age independently predicted elevated UCR on day 4. However, the presence of AKI and male sex significantly reduced the risk of an elevated UCR. CONCLUSIONS: UCR dynamics are strongly influenced by asynchronous trajectories of creatinine and urea plasma levels across different stages of acute kidney injury. Evolving AKI was associated with a lower UCR during the early ICU course, limiting its reliability as a standalone marker of catabolism.