What is the Body Surface Area (BSA) for a 46.95 kg boy according to the British National Formulary for Children (BNFC)?

BSA Calculation for a 46.95 kg Boy

For a 46.95 kg boy, the BSA is approximately 1.42 m² using the weight-based formula recommended by BNFC: BSA (m²) = √[(weight in kg × height in cm) / 3600], or when height is unavailable, the simplified weight-only formula: BSA (m²) = (weight in kg)^0.5378 × 0.024265.

BNFC-Recommended Approach

The British National Formulary for Children specifically recommends calculating drug dosages based on body surface area for many medications, particularly those with narrow therapeutic indices 1. However, the practical challenge is that conventional BSA calculations require both weight and height measurements 2.

Weight-Only BSA Estimation

When height is not available (as in this question), the BNFC approach supports using weight-alone formulas that have been validated in pediatric populations 1, 2:

• For a 46.95 kg child, using the Meeh-type equation: BSA = 0.024265 × (weight)^0.5378

  • This yields: BSA = 0.024265 × (46.95)^0.5378 ≈ 1.42 m² 3

• This Meeh-type formula has been shown to predict Mosteller equation results with 99.2% of values falling within 0.9-1.1 ratio across the pediatric age range 3

Alternative Calculation if Height is Known

If height were available, the Mosteller formula (the gold standard) would be used 4:

• BSA (m²) = √[(weight in kg × height in cm) / 3600] 4
• For example, if this boy were 150 cm tall: BSA = √[(46.95 × 150) / 3600] ≈ 1.44 m² 4

Clinical Context and Dosing Implications

For drug dosing purposes in a child weighing 46.95 kg (over 30 kg), the BNFC-referenced rule suggests using (weight + 30)% of adult dose = 76.95% of adult dose 1. This weight-based approach actually follows the BSA curve more closely than conventional mg/kg dosing and reduces prescription errors 1.

Important Caveats

• BSA-based dosing should be used cautiously in obese children or those with cachexia, as weight-only formulas may be less accurate in these populations 2
• The precision of BSA estimation decreases in children under 10 kg, where dose modifications are necessary 2
• No single scaling method (weight, BSA, or weight^0.75) performs optimally across all pediatric age ranges, with BSA methods tending to overestimate doses by up to 2.86-fold in infants under 1 year 5

Practical Application

For this 46.95 kg boy:

• Estimated BSA: 1.42 m² (using weight-only Meeh formula) 3
• This falls within the typical range for a child of this weight
• For chemotherapy or other BSA-dosed medications, this value provides sufficient accuracy (within 10% deviation) for clinical use 2