How to dose medications for a 5-year-old girl with a BSA (Body Surface Area) of 18.85 sqm according to BNFC (British National Formulary for Children)?

Body Surface Area Calculation for a 5-Year-Old Girl

The reported body surface area (BSA) of 18.85 square meters for a 5-year-old girl is extremely inaccurate and physiologically impossible. This value should be recalculated using the appropriate formula recommended by the British National Formulary for Children (BNFC).

Correct BSA Calculation Methods

• The BNFC recommends using the Gehan and George equation for calculating BSA: BSA (m²) = 0.0235 × (height [cm])^0.42246 × (weight [kg])^0.51456 1
• For a typical 5-year-old girl, the BSA would typically be around 0.7-0.8 m² (not 18.85 m²) 1, 2
• The Mosteller formula is another commonly used and simple method: BSA (m²) = √[(height [cm] × weight [kg])/3600] 2

Importance of Accurate BSA Calculation

• Accurate BSA calculation is critical for pediatric medication dosing, especially for drugs with narrow therapeutic indices 1
• Using an incorrect BSA value of 18.85 m² would result in potentially fatal overdoses of medications 3, 4
• Age-based dosing guidelines can lead to inappropriate dosing, particularly in underweight or overweight children, making accurate BSA calculation even more important 4

BSA-Based Medication Dosing in Children

• For medications dosed according to BSA:
  • Methotrexate for juvenile idiopathic arthritis: 10-15 mg/m² BSA per week (not exceeding 15 mg/m² BSA) 5
  • Fluid requirements for children with primary hyperoxaluria: 2-3 L/m² BSA 5
  • Peritoneal dialysis exchange volumes: 1,100 mL/m² BSA for pediatric patients 5

Common Pitfalls in BSA Calculation

• Transcription errors (such as 18.85 instead of 0.885) can lead to dangerous medication dosing 3, 6
• Using adult-based formulas without appropriate pediatric adjustment 6
• Not accounting for the child's actual height and weight, which can vary significantly at the same age 4
• Relying solely on age-based dosing rather than BSA when indicated 4, 6

Recommendation for Clinical Practice

• Always verify that the calculated BSA is physiologically plausible for the child's age before prescribing medications 1, 3
• For a 5-year-old girl, recalculate the BSA using the child's actual height and weight measurements 1, 2
• When using BSA for medication dosing, double-check calculations, especially when the result seems unusually high or low 3, 4
• Consider using electronic calculators or validated apps specifically designed for pediatric BSA calculation to minimize errors 1, 2