Excretion

Renal function in neonates is less than adults, matures by 1 year of age and is then better than adults before declining. The dosing of renally excreted drugs reflects this, for example the recommended frequency of dosing of gentamicin increases with age from 2.5mg/kg every 24hrs in a neonate less than 29 weeks postmenstrual age, up to 2.5mg/kg every 8 hrs in a 1 month year old (Paediatric Formulary Committee, 2011)

Sources of Information on dosing

You should obtain doses of medicines from a paediatric dosage handbook. In the UK, although the adult BNF contains some paediatric doses, the recommended gold standard source is the BNF for children (BNF-C) (Paediatric Formulary Committee 2011).

How do you calculate doses in paediatrics?

There are three main methods that you will come across when calculating doses for children

Body weight

This is the most common method to calculate doses. In the BNF-C this is stated as mg/kg. It is good practice to document bodyweight on prescriptions to allow doses to be double checked by nursing or pharmacy staff. Generally the calculated dose should not exceed the maximum recommended dose for an adult. For obese children you may need to use ideal body weight. Doses are usually expressed in terms of a single dose together with the recommended frequency (e.g. 1mg/kg twice daily). Occasionally doses are quoted as total daily dose (e.g. 3mg/kg daily in 3 divided doses). You should check this carefully, as confusing the total daily dose with the single dose could result in an overdose.

Body surface area

Many physiological parameters relevant to drug handling correspond better with body surface area. This can be calculated from weight and height using nomograms, but this can be difficult to obtain in acutely unwell children. For this reason relatively few drugs are dosed in this way e.g. cytotoxics. A table of body surface area according to body weight is available at the back of the BNF-C

Dosing by age

Some drugs with a wide therapeutic range are quoted as a single dose for an age range (e.g. for amoxicillin 1 month-1year 62.5mg three times daily, 1-5years 125mg three times daily). This simplifies dosing regimens, however you should take care if children are underweight for their age as this could result in an overdose. Tables of mean weight by age are available at the back of the BNF-C. Whatever method is used to calculate the dose, make sure that the dose can be practically measured. The standard oral syringe is supplied when liquid medicines are prescribed in doses other than multiples of 5ml, and is marked in 0.5ml divisions.

Calculating drug doses 1

A 54 Kg child of 12 years requires prophylactic trimethoprim 2mg/Kg for recurrent urinary tract infections. Use the BNF-C to calculate an appropriate dose