How do I calculate the appropriate medication dose for a pediatric patient based on age, weight, and height?

Pediatric Medication Dosing: A Weight-Based Algorithmic Approach

For children weighing less than 40 kg, calculate medication doses using mg/kg of body weight; for children weighing 40 kg or more, use standard adult dosing protocols. 1

Primary Dosing Framework

Weight-Based Calculation (Children <40 kg)

• Use ideal body weight rather than actual weight when calculating mg/kg doses to avoid overdosing in obese children or underdosing in malnourished children 1
• Multiply the child's weight in kilograms by the prescribed mg/kg dose to determine the total dose per administration 1
• Always verify that the calculated dose does not exceed the maximum adult dose for that medication, regardless of the child's weight 1

Transition to Adult Dosing (≥40 kg)

• Children weighing exactly 40 kg or more should receive standard adult doses rather than weight-based calculations 2, 1, 3
• Age 15 years typically marks the transition to adult dosing, though weight remains the primary determinant 1

Age-Specific Considerations

Neonates and Young Infants (<3 months)

• Dosing in neonates must account for both postnatal age and weight because drug elimination pathways are immature 1
• **Example: Gentamicin in premature neonates <1000g requires 3.5 mg/kg every 24 hours**, while term neonates >2000g and >7 days old require 2.5 mg/kg every 8 hours 1
• For oseltamivir in infants 0 to <3 months, use 3 mg/kg/dose twice daily for treatment (though chemoprophylaxis is not recommended unless the situation is judged critical) 2

Infants 3-12 Months

• Oseltamivir dosing for infants 3 to <12 months is 3 mg/kg/dose twice daily for treatment and 3 mg/kg/dose once daily for chemoprophylaxis 2
• Weight-based dosing is required and preferred over age-based dosing whenever possible 2

Children ≥12 Months

• For children >12 months, use weight-band dosing for oseltamivir:
  • ≤15 kg: 30 mg twice daily (treatment) or once daily (prophylaxis)

  • 15-23 kg: 45 mg twice daily or once daily

  • 23-40 kg: 60 mg twice daily or once daily

  • 40 kg: 75 mg twice daily or once daily 2

Critical Dosing Adjustments

Renal Impairment

• Dose reduction is required based on creatinine clearance, with specific adjustments varying by medication 1
• For oseltamivir in patients with creatinine clearance 10-30 mL/min: 75 mg once daily for 5 days (treatment) or 30 mg once daily for 10 days after exposure (prophylaxis) 2

Disease-Specific Modifications

• Certain disease states require higher doses: for example, cystic fibrosis or febrile neutropenia may require gentamicin 30 mg/kg/day divided every 8 hours 1
• Organ function status (renal and hepatic impairment) necessitates specific dose reductions 1

Drug Interactions

• Concomitant medications may require dose adjustments: for example, protease inhibitors with rifampin require careful dosing modifications 1

Body Surface Area (BSA) Considerations

When to Use BSA

• Hydrophilic drugs with a low volume of distribution in adults should be normalized to BSA in children <2 years 4
• After 6 months of age, BSA is a good marker for drug dosing for most medications 4
• BSA-based dosing is particularly important for chemotherapy agents 5, 6

BSA Calculation Alternatives

• When height is unavailable, BSA can be estimated from weight alone with acceptable accuracy (deviation <10%) 5
• For children up to 30 kg, a simplified rule is: dose = (weight × 2)% of adult dose; over 30 kg: dose = (weight + 30)% of adult dose 7

Common Pitfalls to Avoid

Calculation Errors

• Dose calculation errors are the most common medication errors in pediatrics, particularly in emergency departments 1
• Never use adult dosing for children under 40 kg, even if they appear large for their age 1
• Weight estimation errors are high-risk in emergency settings where actual weight is unknown 1

Age-Related Mistakes

• Do not assume all children of the same age require the same dose—weight is the primary determinant 1
• Drugs metabolized by CYP2D6 and UGT should be normalized to bodyweight even after 6 months of age, not BSA 4

Organ Maturation Issues

• Drugs primarily metabolized by the liver should be administered with extreme care until age 2 months, with dosing based on therapeutic drug monitoring 4
• For drugs significantly excreted by the kidney, determine renal function using serum creatinine in the first 2 years of life before calculating doses 4

Practical Dosing Algorithm

1. Obtain accurate weight in kilograms (and height if BSA calculation is needed)
2. Determine if the child weighs <40 kg or ≥40 kg
3. For <40 kg: Calculate dose as mg/kg × weight
4. For ≥40 kg: Use standard adult dosing
5. Verify the calculated dose does not exceed maximum adult dose
6. Adjust for renal/hepatic impairment if present
7. Consider disease-specific modifications
8. Check for drug interactions requiring dose adjustment

Special Considerations for Specific Age Groups

• Neonates (<1 month): Base dosing on postnatal age, weight, and organ maturation 1, 4
• Infants (1-24 months): Use weight-based dosing with careful attention to renal and hepatic maturation 4
• Children (2-12 years): Weight-based or BSA-based dosing depending on drug characteristics 4
• Adolescents (≥40 kg): Transition to adult dosing 1, 3