Weight-based prescribing and dose checking

Paediatric prescribing combines more variables than adult prescribing: weight, age, gestation, body surface area, maturation, indication, route, organ function, formulation and a dose ceiling. RCPCH identifies weight-based errors and tenfold overdose as major preventable harm and recognises BNFC as the authoritative UK basis for paediatric dosing. Electronic systems can reproduce an incorrect weight or default just as efficiently as a correct one, so clinical verification remains necessary.

Start with identity and anthropometry. Use an accurate weight measured in kilograms during the current encounter or a clinically recent stable value. Record the date and whether oedema, dehydration or critical illness changes it. In an emergency without a weight, use the organisation's approved length-based tape or estimation method and replace it with a measured value as soon as possible. Never enter pounds into a kilogram field or calculate from an undocumented caregiver estimate when weighing is feasible.

Select the exact BNFC monograph entry. The same medicine may have different doses for infection, seizure, pain or resuscitation and different intervals for oral, intravenous or intramuscular routes. Check the age band, gestational and postnatal age for neonates, maximum single and daily doses and whether use is unlicensed. A licensed adult preparation may be used off label appropriately in children; explain material uncertainty and document specialist rationale when it affects the decision.

Distinguish expressions. If a medicine is prescribed as 10 mg/kg/dose and the child weighs 18 kg, each dose is 180 mg before applying the maximum. If it is 10 mg/kg/day in two divided doses, the daily amount is 180 mg and each dose 90 mg. Write the units at every step. For body-surface-area dosing, use an approved calculator with verified height and weight and then apply the protocol maximum and dose-rounding rule.

Apply ceilings after calculation. A 60 kg adolescent receiving a medicine at 15 mg/kg would calculate to 900 mg, but a 500 mg maximum single dose means 500 mg, not 900 mg. Check the maximum total over 24 hours across regular, as-required and combination products. Calendar-day totals can miss doses given late the previous evening; inspect the actual rolling administration record.

Convert to volume as a separate controlled step. Read the label in mg per mL, not just a familiar product name. A 225 mg dose from a 10 mg/mL product is 22.5 mL; confusing mg and mL can cause a tenfold error. Select an oral syringe that measures the volume accurately and avoid kitchen spoons. For intravenous products, check whether concentration refers to the ampoule, final dilution or infusion and use standardised concentrations when available.

Decimals require deliberate presentation. Write 0.5 mg rather than .5 mg and 5 mg rather than 5.0 mg. Prefer whole units that minimise decimals when the prescription system supports them, but never change units mentally during administration. Write micrograms in full for high-risk contexts and confirm that pumps, labels and charts display the same unit. Challenge a dose that looks implausible even when electronic decision support does not alert.

Obesity changes distribution and clearance differently by medicine. RCPCH and UKMI advise drug-specific choice among actual body weight, ideal body weight, adjusted body weight or a capped adult dose and recommend measuring height. In a genuine emergency, actual or estimated weight can prevent treatment delay, but the dose must then be reviewed. Do not cap every medicine automatically at an adult dose when a specialist paediatric protocol explicitly differs, and do not use actual weight universally for narrow-therapeutic-index drugs.

Renal and hepatic function modify both dose and interval. Estimate renal function with an age-appropriate method and recognise that serum creatinine can appear low in a child with little muscle. Review dehydration, urine output, dialysis or extracorporeal support. For hepatically metabolised medicines, assess synthetic function, bilirubin, interactions and disease-specific advice. Therapeutic drug monitoring is essential for selected antibiotics, anticonvulsants and immunosuppressants; define sampling time relative to dose.

Administration completes the calculation. Confirm child, medicine, dose, route, time, allergy and indication, and match the label with prescription. Check vascular access and compatibility for infusions and program pumps through a standard library when available. For a child unable to swallow, do not crush modified-release or enteric products without pharmacy advice. Flush feeding tubes appropriately and consider whether feeds alter absorption.

At discharge, reconcile the exact product strength because community supply may differ from inpatient stock. Demonstrate the oral syringe, ask the caregiver to measure a dose and give a written schedule in dose and volume. Discuss storage, missed doses, vomiting after a dose and duplicate ingredients. For shared-care or school administration, specify who monitors response and toxicity, who adjusts for weight and how an expired or changed prescription is replaced.