Pediatric Dose (Clark Rule)

Pediatric dose by Clark's rule

Clark's rule takes the standard adult dose and scales it down by the child's weight: child_dose = adult_dose · (child_weight_kg / 70), with 70 kg standing in as the reference adult. It came after the older Young's rule, which scaled by age (age/(age+12)) and turned out to be unreliable, since two kids the same age can weigh very differently. When you're dealing with cytotoxic drugs or anything with a narrow therapeutic index, BSA scaling does a better job: child_dose = adult_dose · (child_BSA / 1.73). Example: a 500 mg adult dose given to a 20 kg child works out to 500 · (20/70) ≈ 143 mg.

Clinical context

Think of Clark's rule as a fallback. It comes out in emergency departments, urgent care, and at the prescription pad when the label doesn't list a specific pediatric dose. It works for plenty of antibiotics, analgesics and antihistamines. What it can't handle are neonates, whose pharmacokinetics (protein binding, renal clearance, hepatic enzymes) are different enough to need their own regimens, and drugs that don't scale linearly with weight. The drug label or pediatric formulary (BNF for Children, Lexicomp Pediatric, AAP Red Book) is your first stop; reach for Clark's rule only when no pediatric dose is published.

FAQ

Clark or Young? Go with Clark, the weight-based one. Young's rule keys off age and is treated as obsolete now, since two children of the same age can be 2× apart in weight.

When use BSA scaling instead? Chemotherapy, immunosuppressants, and any drug whose toxicity tracks metabolic rate. Past the age of 1, BSA lines up with organ function better than weight does.

Why not use Clark's rule in newborns? Their renal and hepatic clearance is still immature, protein binding is altered, and they carry more water per kg. Scale linearly by weight and you'll overdose or underdose them in ways that are hard to predict. Stick with neonatal dosing tables.

What if the calculated dose exceeds the adult dose? Cap it at the adult dose. Weight scaling can push an obese adolescent past the adult ceiling, but for most drugs that adult maximum is still where the safety line sits.