How are drug doses calculated per kilogram (kg) for pediatric patients?

Pediatric Drug Dosing: Weight-Based Calculation Methods

Pediatric drug doses are calculated using milligrams per kilogram (mg/kg) of body weight, with the patient's weight measured in kilograms serving as the foundation for safe and effective dosing. 1

Core Dosing Principles

Weight-based dosing (mg/kg) is the standard method for pediatric medication calculation, NOT simple linear scaling from adult doses. 2, 3 The traditional approach of proportionally reducing adult doses based on weight alone leads to underdosing in infants and children (whose elimination is more efficient per kg than adults) and overdosing in neonates (whose drug elimination pathways are immature). 3

Critical Weight Documentation Requirements

• Always document patient weight in kilograms on all prescriptions and medication orders. 4 This practice reduces pediatric dosing errors by approximately 4% overall, with more substantial reductions for specific medications like epinephrine (22% error reduction) and fentanyl (21% error reduction). 5

• For infants specifically, documenting weight in kilograms reduces dosing errors from 53% to 33%. 5 This age group benefits most from precise weight documentation.

Practical Dosing Examples from Guidelines

Tuberculosis Medications (First-Line Agents)

Isoniazid dosing: 1

• Children: 10-15 mg/kg daily (maximum 300 mg)
• Intermittent dosing: 20-30 mg/kg twice weekly (maximum 900 mg)

Rifampin dosing: 1

• Children: 10-20 mg/kg daily (maximum 600 mg)
• Intermittent dosing: 10-20 mg/kg twice weekly (maximum 600 mg)

Pyrazinamide dosing: 1

• Children: 15-30 mg/kg daily (maximum 2.0 g)
• Intermittent dosing: 50 mg/kg twice weekly (maximum 4 g)

Ethambutol dosing: 1

• Children: 15-20 mg/kg daily (maximum 1.0 g)
• Intermittent dosing: 50 mg/kg twice weekly (maximum 4 g)

Important caveat: Doses are based on ideal body weight, and children weighing more than 40 kg should be dosed as adults. 1

Emergency Medications

Epinephrine for cardiopulmonary resuscitation: 1

• IV/IO: 0.01 mg/kg of 1:10,000 solution (maximum 1 mg), repeated every 3-5 minutes
• Endotracheal: 0.1 mg/kg of 1:1000 solution (maximum 10 mg)

Epinephrine for anaphylaxis: 1

• IM/SC: 0.01 mg/kg of 1:1000 solution (maximum 0.3-0.5 mg), repeated every 5-20 minutes

Epinephrine for croup: 1

• Nebulized: 0.5 mL/kg of 1:1000 solution (maximum 5 mL = 5 mg)

Diphenhydramine for dystonic reactions: 1

• IV/IM: 1-2 mg/kg (maximum initial dose 50 mg)

Bronchodilator Therapy

Salbutamol (albuterol) for acute asthma: 6

• Nebulized: 0.15 mg/kg (minimum 2.5 mg) every 20 minutes for 3 doses, then 0.15-0.3 mg/kg every 1-4 hours as needed
• Weight-based shortcut: 2.5 mg for children <20 kg; 5 mg for children >20 kg
• MDI with spacer: 4-8 puffs (90 mcg/puff) every 20 minutes for 3 doses, then every 1-4 hours as needed

Levalbuterol dosing is half the racemic albuterol dose (e.g., 1.25 mg instead of 2.5 mg). 6

Antibiotics for Common Infections

Amoxicillin for otitis media: 1

• 40 mg/kg/day divided twice daily for 5 days

Ceftriaxone for severe infections: 1

• 50 mg/kg once daily (can increase to 50 mg/kg twice daily for severe infections)

Gentamicin for sepsis: 1

• Infants: 5-7.5 mg/kg once daily
• Older children: 7.5 mg/kg once daily

Ampicillin for sepsis/meningitis: 1

• 50 mg/kg every 6 hours (can increase to 50 mg/kg every 4 hours for severe infections)

Age-Based Dosing Considerations

Children ≥2 years are physiologically mature and differ from adults only in size. 3 Standard mg/kg dosing applies reliably in this age group.

Neonates and infants <2 years have immature drug elimination pathways. 3 These patients require:

• More conservative dosing approaches
• Longer dosing intervals for renally eliminated drugs
• Close monitoring for drug accumulation

Adult dosing typically begins at age 15 years. 1

Alternative Dosing Methods (When Weight Unknown)

For children up to 30 kg: Calculate dose as (weight × 2)% of adult dose 2

For children over 30 kg: Calculate dose as (weight + 30)% of adult dose 2

This method follows body surface area curves more closely than simple mg/kg calculations and reduces major prescribing errors. 2

Maximum Dose Considerations

Always apply maximum dose limits even when weight-based calculations exceed them. For example:

• Isoniazid: 300 mg maximum daily dose regardless of weight 1
• Rifampin: 600 mg maximum daily dose regardless of weight 1
• Epinephrine for anaphylaxis: 0.5 mg maximum single dose 1

Common Pitfalls to Avoid

Never use pounds for dosing calculations. 4, 5 Converting from pounds to kilograms introduces calculation errors. Always obtain and document weight in kilograms.

Do not assume simple linear scaling from adult doses. 2, 3 This results in systematic dosing errors across pediatric age groups.

Avoid using outdated weight measurements. 4 Obtain current weight for each prescription, as children's weights change rapidly during growth.

Do not skip maximum dose verification. 1 Weight-based calculations must always be capped at recommended maximum doses to prevent toxicity.

For neonates, recognize that standard pediatric mg/kg doses often require reduction or interval extension. 3 Immature elimination pathways necessitate dose adjustments beyond simple weight-based calculations.