Defining urine output criterion for acute kidney injury in critically ill patients

BACKGROUND: The widespread use of RIFLE and AKIN classification systems for acute kidney injury (AKI) diagnosis and staging has established the association between AKI severity and adverse outcomes. However, as a result of the difficulties in measuring and recording the urine output every hour, a few prospective studies have validated the urine output criterion as stated in these classification systems. We assessed hourly urine output in ICU patients using an automated and accurate device to determine if changes in urine flow and volume could be a sensitive marker of AKI. Additionally, we assessed various definitions of oliguria to determine whether measurement of urine output using a fixed 6-h interval that matches nurses' shifts would be equivalent to the current standard for AKI diagnosis and staging. METHODS: Hourly urine output was recorded continuously using a digital monitor in a medical ICU. Serum creatinine measurements were done at least once per 24 h. We assessed changes in urine output by four different definitions of oliguria. Patients with no AKI by either criterion were compared with patients diagnosed exclusively by the urine output criterion, exclusively by serum creatinine criterion and by both criteria. RESULTS: Fifty-five percent of patients had an episode of oliguria during the ICU stay. There was no significant difference assessing urine output every hour or the total urine volume in a 6-h period for the detection of episodes of oliguria. Twenty-one patients (28%) were diagnosed as AKI using the serum creatinine criterion, whereas additional 24 (32%) were identified by the urine output criterion. CONCLUSIONS: Episodes of oliguria occur frequently in ICU patients and identify a higher percentage of AKI patients compared to serum creatinine criterion. Alterations in urine flow may be a sensitive marker of renal dysfunction and need to be validated in larger cohorts.