How are pediatric doses calculated for a child based on their age, weight, and medical history?

Pediatric Dose Calculation Methods

Pediatric doses are calculated primarily using weight-based dosing (mg/kg), with the critical threshold being 40 kg—below which weight-based calculations apply, and at or above which adult dosing becomes standard. 1, 2, 3, 4

Primary Calculation Framework

Weight-Based Dosing (Standard Method)

• For children weighing less than 40 kg: Calculate dose as mg/kg of body weight, using ideal body weight rather than actual weight 1
• For children weighing 40 kg or more: Use adult dosing protocols 1, 2, 3, 4
• Age consideration: Adult dosing typically begins at 15 years of age, though weight remains the primary determinant 1

Body Surface Area (BSA) Dosing

• BSA-based calculations are used for specific medications, particularly chemotherapeutic agents and certain antibiotics like caspofungin 1
• After 6 months of age: BSA becomes a reliable marker for drugs requiring this calculation method 5
• Hydrophilic drugs with low volume of distribution in adults should be normalized to BSA in children under 2 years 5

Age-Specific Dosing Considerations

Neonates (0-28 days)

• Dosing must account for postnatal age AND weight, as drug elimination pathways are immature 1, 6
• Extreme caution required for hepatically metabolized drugs until 2 months of age 5
• Example dosing complexity: 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

Infants (1 month to 2 years)

• First dose based on volume of distribution, subsequent doses determined by clearance 5
• Age 2-6 months: General weight-based guidelines become applicable 5
• Renal function markers (serum creatinine, p-aminohippuric acid clearance) should guide dosing for renally excreted drugs 5

Children (2 years to adolescence)

• Children ≥2 years are essentially mature and differ from adults primarily in size, not physiology 6
• Weight-based dosing remains standard until 40 kg threshold is reached 1, 2, 3, 4
• Simplified calculation rule: For children up to 30 kg, dose = (weight × 2)% of adult dose; over 30 kg, dose = (weight + 30)% of adult dose 7

Critical Dosing Adjustments

Renal Impairment

• Dose reduction required based on creatinine clearance, with specific adjustments varying by medication 2
• First 2 years of life: Renal excretion rate must be determined by markers of renal function before dosing renally cleared drugs 5

Hepatic Metabolism Considerations

• Drugs metabolized by CYP2D6 and UGT should be normalized to bodyweight even after 6 months of age 5
• Maturation of hepatic enzymes is complete by approximately 2 months for most pathways 5

Common Pitfalls to Avoid

• Never simply scale adult doses by weight alone—this results in underdosing in infants/children and overdosing in neonates 6
• Do not use adult dosing for children under 40 kg, even if they appear large for their age 1, 2
• Avoid assuming "small adult" dosing—neonates and infants have fundamentally different pharmacokinetics requiring individualized consideration 8, 6
• Weight estimation errors are high-risk in emergency settings where actual weight is unknown 1
• Dose calculation errors are the most common medication errors in pediatrics, particularly in emergency departments 1

Medical History Integration

Factors Requiring Dose Modification

• Organ function status: Renal and hepatic impairment necessitate specific dose reductions 2, 5
• Genetic polymorphisms: CYP2D6 poor metabolizers require different dosing strategies 5
• Disease states: Conditions like cystic fibrosis or febrile neutropenia may require higher doses (e.g., gentamicin 30 mg/kg/day divided every 8 hours) 1
• Concomitant medications: Drug interactions (e.g., protease inhibitors with rifampin) require dose adjustments 1

Developmental Considerations

• Visual acuity monitoring: Ethambutol should be used with caution in children <5 years who cannot report visual changes 1
• Bone/cartilage concerns: Fluoroquinolones require risk-benefit assessment in children due to growth plate effects 1
• Hyperbilirubinemia: Ceftriaxone is contraindicated in hyperbilirubinemic neonates 1