In this episode, I’ll discuss an alternative definition of oliguria.
The commonly used definition of oliguria is a urine output of 0.5 mL/kg/hr over a 6 hr period. However some small studies have
suggested that this definition of oliguria is too liberal.
To bring clarity to this clinical issue, a group of authors published a study in the journal Critical Care designed to determine the optimal threshold to define oliguria in critically-ill patients.
The study was a retrospective cohort that analyzed over 15,000 patients. The data was split into a training set and a validation set. In the training set the authors developed multiple models that examined urine output over 3, 6, 12, and 24 hours and compared this to 90 day mortality. These models were then applied to the validation set of data for evaluation of their performance.
The authors found that the relationship between minimum urine output and 90 day mortality was nonlinear and there was a clear inflection point where mortality increased below 0.2 mL/kg/hr for the 3 and 6 hr windows and 0.3 mL/kg/hr for the 12 and 24 hr windows.
When the authors considered a threshold of < 0.2 mL/kg/hr over 6 hr for oliguria, the proportion of patients with an episode of oliguria decreased substantially to just under 25%. Unlike the traditional definition of oliguria, this threshold of 0.2 mL/kg/hr over 6 hr identified a population with a higher predicted 90-day mortality, making this new definition clinically useful. The authors concluded:
The widely used cut-off for oliguria of 0.5 ml/kg/h over 6 h may be too conservative. A cut-off of 0.2 ml/kg/h over 3 or 6 h is supported by the data and should be considered in further definitions of oliguria.
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