What is the standard fluid output in milliliters per kilogram (ml/kg) for children and adults?

Standard Fluid Output for Children and Adults

The standard fluid output for children and adults varies by age, with neonates having the highest output at 5 ml/kg/hour, decreasing to 0.5-1 ml/kg/hour in adults. 1

Age-Based Fluid Output Standards

Neonates and Infants

• Preterm infants, especially VLBW (very low birth weight) infants, may have urine output frequently exceeding 5 ml/kg/hour due to renal immaturity 1
• Term neonates typically have urine output around 0.5 ml/kg/hour 1
• Maximum urinary concentrations are limited to approximately 550 mOsm/L in preterm infants and 700 mOsm/L in term infants (compared to 1200 mOsm/L in adults), contributing to higher fluid output 1

Children

• Older children typically have urine output of approximately 0.5-1 ml/kg/hour 1
• Children with acute oliguria are defined as having urine output ≤0.5 ml/kg/hour for at least 2 hours despite adequate fluid resuscitation 1
• Water turnover is higher in children compared to adults due to higher metabolic rates and growth velocity 1

Adolescents and Adults

• Adolescents have urine output of approximately 0.3-0.5 ml/kg/hour 1
• Adults typically have urine output of 0.5-1 ml/kg/hour, with 0.5 ml/kg/hour considered the lower limit of normal 1
• In critically ill adults, urine output <0.5 ml/kg/hour for more than 6 hours may indicate acute kidney injury 1

Physiological Factors Affecting Fluid Output

Body Composition Differences

• Water contributes to approximately 90% of body weight in a 24-week-old fetus, 75% in term infants, and around 50% in adults 1
• The proportion of extracellular fluid decreases from infancy to adulthood, affecting fluid distribution and output 1
• Blood volume in neonates is 85-100 ml/kg body weight compared to 60-70 ml/kg in adolescents and adults 1

Renal Maturation

• Glomerular filtration rate increases significantly during the first week of life and continues to rise over the first two years 1
• The velocity of this increase is slower in premature infants, affecting their fluid handling capacity 1
• Immaturity of the distal nephron leads to reduced ability to concentrate urine in neonates and young infants 1

Clinical Applications and Monitoring

Fluid Management in Clinical Settings

• In fluid resuscitation for severe burns, urine output of 0.5-1 ml/kg/hour is commonly targeted in adults with thermal burns 1
• For children with severe burns, fluid resuscitation should be adjusted based on clinical and hemodynamic parameters, with urine output being a key parameter 1
• In sepsis management, acute oliguria is defined as urine output ≤0.5 ml/kg/hour for at least 2 hours despite adequate fluid resuscitation 1

Special Considerations

• In premature infants, a lower plasma oncotic pressure and higher capillary wall permeability increase the risk of edema, requiring careful fluid management 1
• In hemodialysis patients, ultrafiltration rates of 10-18 ml/kg/hour in pediatric patients were not associated with increased mortality, unlike in adults where rates >13 ml/kg/hour may be harmful 2
• Elderly patients may require special attention to fluid balance due to age-related changes in body composition and renal function 3, 4

Common Pitfalls in Fluid Output Assessment

• Failing to adjust expected output based on age and weight can lead to misinterpretation of normal versus abnormal output 1
• Not accounting for insensible water losses, which are higher in neonates (0.8-0.9 ml/kg/hour) compared to older children (0.4 ml/kg/hour) and adolescents (0.3 ml/kg/hour) 1
• Overlooking that water turnover is related to lean body mass rather than fat mass, potentially affecting interpretation in patients with different body compositions 1, 5
• Disregarding that endogenous water production (from metabolism) may be particularly important in pediatric patients due to their high metabolic rates 1