Paediatric Body Surface Area Dosing

Body surface area dosing scales the dose to surface area in m2, which tracks metabolic rate, cardiac output and glomerular filtration better than weight for some drugs. It is used for cytotoxic drugs, some hormones and immunosuppressants, and for normalising kidney function.

Calculation. Body surface area in m2 equals the square root of (height in cm multiplied by weight in kg, divided by 3600). A child of 20 kg and 110 cm: 110 x 20 = 2200; divided by 3600 = 0.61; square root = 0.78 m2.

Weight based alternative. In infants under 10 kg or under 1 year, surface area formulae are less reliable and many drugs are dosed by weight (often in mg/kg) or by an age-banded table.

Examples where m2 dosing is usual: - Hydrocortisone replacement in adrenal insufficiency: 8 to 10 mg/m2/day orally in 3 divided doses (higher, up to 15 mg/m2/day, in congenital adrenal hyperplasia in early childhood), with the largest dose in the morning. - Hydrocortisone for stress cover or adrenal crisis: 50 to 100 mg/m2 intravenously as an initial dose, then 50 to 100 mg/m2/day in divided doses.

When surface area is unknown, an age-banded dose is used: 25 mg under 3 years, 50 mg for 3 to 12 years, 100 mg over 12 years. - Cytotoxic chemotherapy: dose in mg/m2 set by the treating regimen, calculated by the oncology team.

Children under 10 kg or 1 year are often dosed in mg/kg with a fixed reduction, set by the specialist. - Methotrexate, cyclophosphamide, vincristine and similar drugs have regimen-specific doses with maximum caps (for example vincristine capped at 2 mg).

Kidney function. Report glomerular filtration as mL/min/1.73 m2. Convert a measured value to the child’s own surface area when dosing renally cleared drugs: multiply by the child’s m2 and divide by 1.73.

Obesity. Use actual height and weight for chemotherapy unless the specialist regimen states otherwise. Capping at 2 m2 is not usual in children; adult caps apply only where the regimen sets them.

Organ dysfunction. Reduce cytotoxic doses for hepatic or renal impairment as the regimen directs, and delay a cycle for unresolved toxicity.

Special groups. Pregnant adolescents need specialist review before any cytotoxic or immunosuppressant is started. Where oncology services are distant, a specialist-written dose sheet should accompany the child to the local facility.

Monitoring. Remeasure height and weight before each cycle or at least every 3 months for chronic therapy, since surface area changes with growth. Monitor blood counts, liver enzymes and creatinine as the regimen requires, and for hydrocortisone track growth velocity, blood pressure and signs of over- or under-replacement.

Referral and escalation. Any surface area calculated dose should be checked by a second prescriber or pharmacist. Cytotoxic drugs are prescribed only within a specialist unit or on a specialist-written regimen.

Stopping. Do not stop hydrocortisone abruptly; double or triple the dose for 48 hours during fever above 38.5 C, and give the intravenous dose if vomiting.