Definition of Oliguria

Oliguria is defined as a urine output of <0.5 ml/kg per hour for at least 6 hours. 1

Standard Definitions of Oliguria

Oliguria has specific definitions that vary slightly depending on the clinical context:

General definition: Urine output <0.5 ml/kg/hour for at least 6 hours 1
Traditional definition: <400 ml/day total urine output (equivalent to 0.24 ml/kg/h in a 70-kg patient) 1
Pediatric definition: <0.5 ml/kg per hour for 8 hours 1
Anuria definition: <0.3 ml/kg per hour for 24 hours or 0 ml/kg per hour for 12 hours 1
Sepsis management definition: ≤0.5 mL/kg/h for at least 2 hours despite adequate fluid resuscitation 1

Oliguria serves as an important marker in several clinical contexts:

Acute Kidney Injury (AKI): Oliguria is incorporated into both RIFLE and AKIN classification systems for AKI diagnosis and staging 1
Early warning sign: Oliguria often precedes rises in serum creatinine, making it an early predictor of AKI 2
Mortality predictor: Oliguric patients without changes in serum creatinine have higher ICU mortality (8.8%) compared to non-oliguric patients without AKI (1.3%) 2
Duration matters: Oliguria lasting >12 hours or occurring in 3 or more episodes is associated with increased mortality 2

Oliguria can be categorized based on its persistence:

Transient oliguria: Resolves within 48 hours and has better outcomes 3
Prolonged oliguria: Resolves after 48 hours 3
Permanent oliguria: Persists throughout ICU stay or recurs at the end of stay (associated with worse outcomes) 3

Several important caveats exist when using oliguria as a diagnostic criterion:

Poor calibration: Studies show poor correlation between urine output criteria and serum creatinine criteria for AKI 1
Physiological response: Oliguria may represent an appropriate response to volume depletion rather than kidney injury 1
Weight-based limitations: The weight-based definition is problematic in obese patients due to the nonlinear relationship between body weight and urine output 1
Pharmacological manipulation: Diuretic administration can change oliguria classification without changing kidney function 1
Measurement method matters: Average versus persistent measurement approaches yield different diagnostic and prognostic implications (73% vs 54.3% incidence) 4

When oliguria is detected, a structured approach is recommended:

Immediate assessment: Evaluate volume status and implement judicious protocol-driven volume resuscitation 5
Medication review: Discontinue potentially nephrotoxic medications 5
Hemodynamic optimization: Ensure mean arterial pressure ≥60 mmHg; consider vasopressors if fluid resuscitation is inadequate 5
Further evaluation: Obtain serum creatinine to assess for AKI; consider the pattern and duration of oliguria 5
Cause identification: Assess for pre-renal, intrinsic renal, and post-renal causes 5, 6

Overdiagnosis: Not all oliguria represents kidney injury; transient oliguria may have relatively benign outcomes 3
Surrogate endpoint: Avoid using oliguria alone as a surrogate endpoint for clinical decisions 5
Delayed recognition: Failure to monitor urine output can lead to delayed recognition of AKI 2
Inconsistent definition: Using different definitions (average vs. persistent) leads to different diagnostic and prognostic implications 4