Structural, physiological or clinical outcomes to define urine output threshold in acute kidney injury

Abstract

To the editor:

We read with interest the comments by Zhu and Li [1] regarding our article on evaluating various urine output (UO) thresholds and timeframes for defining acute kidney injury (AKI) [2]. The authors rightly emphasize that the core principle of AKI diagnosis is to reflect impaired excretion of metabolic waste. However, the concept of AKI must evolve in parallel with advancements in our understanding and the development of interventions to alter its natural progression and clinical impact.

From a physiological perspective, the traditional definition of oliguria (< 400 mL/24 h) is based on the assumption that a healthy adult excretes 600–800 mOsm of metabolic waste daily. At maximal urine concentration (1200 mOsm/L), at least 400 mL of urine is required for adequate waste excretion [3]. However, critically ill patients likely produce less metabolic waste and rarely achieve maximal urine concentration, making this physiological definition impractical in such contexts.

In our study, we also compared proposed UO thresholds to KDIGO standards for predicting progression to AKI stages 2/3, as measured by serum creatinine—a marker of metabolic waste. Our proposed criteria not only predicted hospital mortality but also outperformed KDIGO standards in predicting serum creatinine increases, a secondary outcome [2].

An accurate AKI definition is also essential to enable early interventions that can modify its course. Currently, these interventions are mostly preventive and include avoiding nephrotoxins, controlling hyperglycemia, monitoring serum creatinine and UO closely, and optimizing fluid balance and hemodynamics [4]. However, no studies have identified specific UO thresholds for initiating such measures. Future research should evaluate whether interventions targeting sensitive renal biomarkers—detectable before significant impairment in waste excretion—can effectively reduce AKI-related morbidity and mortality. If proven effective, we agree these biomarkers should be incorporated into AKI definitions.

As Zhu and Li pointed out [1], AKI stage 1—especially when based solely on UO—often does not correlate with increased mortality. Furthermore, there is limited evidence supporting the 0.5 mL/kg/h UO threshold as a definitive AKI marker for outcomes such as mortality, prolonged hospitalization, need for renal replacement therapy, or long-term morbidity. Additionally, no studies have demonstrated the efficacy of preventive or therapeutic interventions at this early stage.

Until higher UO thresholds are validated to guide AKI therapy or identify conditions associated with morbidity and mortality, overdiagnosing AKI poses risks of unnecessary costs and potential harm. This underscores the need for further research to define the optimal timing for AKI diagnosis, aiming to balance the benefits of early intervention with the financial, psychological, and clinical implications of the diagnosis.

Regarding timeframes, we computed a 3-h moving average for each hour, based on the mean UO within the preceding 3 h. Similarly, we calculated 6-, 12, and 24 h means using sliding windows.

No datasets were generated or analysed during the current study.

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A.B.L. was supported by Fundação Edson Queiroz and Conselho Nacional de Desenvolvimento Científico e Tecnológico (Grant No. 306377/2022-5).

Authors and Affiliations

1. Medical Sciences Postgraduate Program, Universidade de Fortaleza- UNIFOR, Fortaleza, Ceará, Brazil

   Guido Dias Machado & Alexandre Braga Libório

2. Medical Program, Universidade de Fortaleza- UNIFOR, Fortaleza, Ceará, Brazil

   Leticia Libório Santos

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1. Guido Dias MachadoView author publications

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2. Leticia Libório SantosView author publications

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A.B.L. wrote the letter. All authors have reviewed and approved the letter.

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Correspondence to Alexandre Braga Libório.

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The authors declare no competing interests.

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Machado, G.D., Santos, L.L. & Libório, A.B. Structural, physiological or clinical outcomes to define urine output threshold in acute kidney injury. Crit Care 28, 410 (2024). https://doi.org/10.1186/s13054-024-05195-5

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• Received: 25 November 2024

• Accepted: 27 November 2024

• Published: 18 December 2024

• DOI: https://doi.org/10.1186/s13054-024-05195-5

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