The urine output definition of acute kidney injury is too liberal

Abstract

IntroductionThe urine output criterion of 0.5 ml/kg/hour for 6 hours for acute kidney injury (AKI) has not been prospectively validated. Urine output criteria for AKI (AKIUO) as predictors of in-hospital mortality or dialysis need were compared.MethodsAll admissions to a general ICU were prospectively screened for 12 months and hourly urine output analysed in collection intervals between 1 and 12 hours. Prediction of the composite of mortality or dialysis by urine output was analysed in increments of 0.1 ml/kg/hour from 0.1 to 1 ml/kg/hour and the optimal threshold for each collection interval determined. AKICr was defined as an increase in plasma creatinine ≥26.5 μmol/l within 48 hours or ≥50% from baseline.ResultsOf 725 admissions, 72% had either AKICr or AKIUO or both. AKIUO (33.7%) alone was more frequent than AKICr (11.0%) alone (P <0.0001). A 6-hour urine output collection threshold of 0.3 ml/kg/hour was associated with a stepped increase in in-hospital mortality or dialysis (from 10% above to 30% less than 0.3 ml/kg/hour). Hazard ratios for in-hospital mortality and 1-year mortality were 2.25 (1.40 to 3.61) and 2.15 (1.47 to 3.15) respectively after adjustment for age, body weight, severity of illness, fluid balance, and vasopressor use. In contrast, after adjustment AKIUO was not associated with in-hospital mortality or 1-year mortality. The optimal urine output threshold was linearly related to duration of urine collection (r2 = 0.93).ConclusionsA 6-hour urine output threshold of 0.3 ml/kg/hour best associated with mortality and dialysis, and was independently predictive of both hospital mortality and 1-year mortality. This suggests that the current AKI urine output definition is too liberally defined. Shorter urine collection intervals may be used to define AKI using lower urine output thresholds.

Derivation of Urine Output Thresholds That Identify a Very High Risk of AKI in Patients with Septic Shock

“…In their analysis, a 6-hour urine output threshold of 0.3 ml/kg per hour yielded the optimal sensitivity and specificity for death or need for dialysis (61% and 77%, respectively). Our 3-hour urine output threshold of ,0.5 ml/kg per hour in a septic shock population had similar predictive power for stage II+ AKI, need for dialysis, and 28-day mortality (31).…”

Classifying AKI by Urine Output versus Serum Creatinine Level

“…They also appear to contrast with prior work by Ralib and colleagues, who found that the oliguria threshold of 0.5 ml/kg per hour was not predictive of survival, whereas 0.3 ml/kg per hour was. 5 These authors did not examine the effects of UO and SC together and their sample size was only 725 patients, limiting their statistical power. Other investigators have found UO to be a sensitive and early marker for AKI and to be associated with adverse outcomes in critically ill patients.…”

Oliguria and Biomarkers of Acute Kidney Injury: Star Struck Lovers or Strangers in the Night?

“…In septic shock, a duration of at least 5 h, however, appears to be a good predictor of AKI [15]. Finally, Md Ralib et al [16] found a correlation between the volume of urinary output and duration of oliguria which suggested that a lower threshold of 0.3 ml/kg/h should be used, if the 6-hour observation interval is used as a minimum for the definition of stage of AKI. Also in the study from Koyner et al [17] patients with progressing AKI were mostly oliguric.…”