Normal Urine Output
Normal urine output in adults and children is ≥0.5 mL/kg/hour, while neonates require higher thresholds of >1.0 mL/kg/hour. 1
Standard Definitions by Age Group
Adults and Children
- Minimum acceptable output: ≥0.5 mL/kg/hour 1, 2
- For a 70 kg adult, this translates to at least 35 mL/hour or approximately 840 mL/day 2
- Patients with normal renal function not receiving diuretics should produce at least 0.8-1 L per day 2
Neonates and Infants
- Premature infants may have urine output frequently exceeding 5 mL/kg/hour due to renal immaturity 3
- Term neonates require higher baseline output than older children due to high water turnover and immature concentrating ability 3
- Insensible water loss is significantly higher: 0.8-0.9 mL/kg/hour in premature infants versus 0.5 mL/kg/hour in term neonates 3
Pediatric Considerations
- Oliguria in children: <0.5 mL/kg/hour for 8 hours 3, 1
- Anuria in children: <0.3 mL/kg/hour for 24 hours or 0 mL/kg/hour for 12 hours 3, 1
Clinical Context and Thresholds
When Urine Output Becomes Concerning
The threshold of 0.5 mL/kg/hour serves as the dividing line between normal kidney function and potential acute kidney injury (AKI), though this represents a liberal definition that may overdiagnose AKI 4. Research suggests that 0.3 mL/kg/hour over 6 hours may be a more specific threshold for clinically significant kidney injury associated with mortality and dialysis need 4.
AKI Staging by Urine Output (KDIGO Criteria)
- Stage 1 AKI: <0.5 mL/kg/hour for 6-12 hours 3, 1
- Stage 2 AKI: <0.5 mL/kg/hour for ≥12 hours 3, 1
- Stage 3 AKI: <0.3 mL/kg/hour for ≥24 hours or anuria for ≥12 hours 3, 1
Critical Action Thresholds
- Severe oliguria requiring immediate intervention: <4 mL/kg over 8 hours 1, 2
- This represents an absolute indication to suspend nephrotoxic therapies 1, 2
- Sepsis-related oliguria: ≤0.5 mL/kg/hour for at least 2 hours despite adequate fluid resuscitation 1
Important Clinical Caveats
When Urine Output Is Unreliable
Cirrhotic patients with ascites present a unique challenge where urine output criteria become problematic 3. These patients are frequently oliguric with avid sodium retention yet may maintain relatively normal glomerular filtration rate 3, 2. Diuretic treatment can artificially increase urine output without improving kidney function 3, 2.
Obesity Considerations
The weight-based definition becomes problematic in obese patients due to the nonlinear relationship between body weight and expected urine output 1, 5. Consider using adjusted body weight for calculations in this population 1.
Diuretic Effects
Diuretic administration can change oliguria classification without changing actual kidney function 1, 5. Do not use urine output as the sole criterion in patients receiving diuretics, as thresholds become less reliable for predicting renal recovery 2.
Physiologic Oliguria vs. Pathologic Oliguria
Oliguria may represent an appropriate physiologic response to volume depletion rather than true kidney injury 1, 5. This distinction is critical—oliguria in the setting of hypovolemia requires fluid resuscitation, not diuretic therapy or immediate concern for intrinsic renal damage 5.
Practical Monitoring Approach
Calculation Method
Use the formula: mL/kg/hour = total urine volume (mL) ÷ body weight (kg) ÷ hours of collection 2
Collection Intervals
While hourly monitoring is ideal, 6-hour collection intervals are practical and clinically valid for detecting significant changes in kidney function 6. This matches nursing shift patterns and provides adequate sensitivity for AKI detection 6.
Surgical Context
Recent evidence suggests that in patients undergoing major abdominal surgery without significant kidney injury risk factors, a minimum target of 0.2 mL/kg/hour is noninferior to 0.5 mL/kg/hour and results in substantial fluid sparing 7. However, this lower target should only be applied in carefully selected low-risk surgical patients 7.