Definitions of Oliguria and Anuria
Oliguria is defined as urine output <0.5 ml/kg per hour sustained for at least 6 hours in adults, while anuria is defined as urine output <0.3 ml/kg per hour for 24 hours or 0 ml/kg per hour for 12 hours. 1
Standard Oliguria Definitions
The threshold of <0.5 ml/kg per hour represents the most widely accepted definition across major classification systems (RIFLE, AKIN, KDIGO), though the required duration varies by context 1:
- General adult definition: <0.5 ml/kg per hour for at least 6 hours 1
- Pediatric definition: <0.5 ml/kg per hour for 8 hours 2
- Sepsis management: ≤0.5 ml/kg per hour for at least 2 hours despite adequate fluid resuscitation 1
- Traditional threshold: <400 ml/day total urine output (equivalent to 0.24 ml/kg/h in a 70-kg patient) 1
Anuria Definition
Anuria is significantly more severe than oliguria and is defined by either 2, 1:
- Urine output <0.3 ml/kg per hour for 24 hours, or
- 0 ml/kg per hour for 12 hours (complete absence of urine production)
AKI Staging by Urine Output
The duration of oliguria determines AKI severity 1:
- Stage 1 AKI: <0.5 ml/kg per hour for 6-12 hours
- Stage 2 AKI: <0.5 ml/kg per hour for ≥12 hours
- Stage 3 AKI: <0.3 ml/kg per hour for ≥24 hours or anuria for ≥12 hours
Special Population Considerations
Pediatric patients require age-specific definitions 2:
- Oliguria: <0.5 ml/kg per hour for 8 hours
- Anuria: <0.3 ml/kg per hour for 24 hours or 0 ml/kg per hour for 12 hours
Neonates have higher baseline requirements, needing >1.0 ml/kg per hour to be considered normal 1.
Obese patients present a challenge because the weight-based definition becomes problematic due to the nonlinear relationship between body weight and expected urine output; consider using adjusted body weight for calculations 1.
Clinical Significance and Caveats
Oliguria may represent an appropriate physiologic response to volume depletion rather than true kidney injury 1. The key distinction is whether oliguria persists despite adequate fluid resuscitation 1.
Duration matters more than absolute presence: Transient oliguria (resolving within 48 hours) has significantly lower mortality than prolonged or permanent oliguria 3. Research shows that 28.9% of oliguric ICU patients had transient oliguria with mortality rates actually lower than non-oliguric patients 3.
Diuretic administration invalidates urine output assessment because it artificially increases output without improving kidney function 1. Similarly, in cirrhotic patients with ascites, urine output criteria become unreliable due to avid sodium retention despite potentially normal GFR 1.
The assessment method matters: Using an average urine output below threshold identifies 73% of patients as oliguric, while requiring persistent values below threshold identifies only 54.3% 4. The average method has higher sensitivity (85% vs 70.3% for mortality prediction) but lower specificity (29.8% vs 49.4%) 4.