The usual dose of a medicine is the amount that produces the desired therapeutic responses in majority of patients. The medicine’s usual dosage range is the range of dosage determined to be safe and effective in normal adults population group. For medicines administered to a certain category of patient (e.g. Children), a usual paediatric dose needs to be determined on the basis of specific patient parameters. Generally children require smaller doses than adults.
Calculation of children’s dose is commonly performed by the pharmacist. When a pharmacist compounds and dispenses a prescription he is held responsible for the doses of medicines in the prescription. Many rules exist for calculation of doses for children based on patient’s age, weight and surface area. Many reference text books recommend that medicine doses for children be calculated according to body surface area (BSA). This is due to the fact that many physiological properties such as plasma volume, glomerular filtration, and requirements of body fluids, electrolytes and calories have been shown to be proportional to the body surface area. The drawbacks of all the rules including one that is based on BSA are not simple and accurate. The BSA Method is more cumbersome in clinical practice.
Salisbury Rule: The dose calculated by the Salisbury rule (Lack JA, Stuart-Taylor ME. Calculation of drug dosage and body surface area of children. Br J Anaesth 1997;78:601-5) is closer to the BSA-based value.
The Rule is as follows:
Less than 30 Kg: weight x 2; More than 30 Kg: weight +30 percentage of adult dose of a medicine
This rule allows a dosage calculation that is comparatively “accurate” for clinical use and not having complicated mathematics. Recently the use of Salisbury rule as a criterion for doses gained wide acceptance due to its simplicity in calculation and more reliable. It is remembered as: “less than 30 Kg, double the body weight; more than 30 kg add 30 to the body weight” percentage of adult dose of a medicine.