McKenna's Pharmacology, 2e

5

Mathematics and dosage calculations

Learning objectives Upon completion of this chapter, you should be able to: 1. Perform basic mathematical calculations. 2. Calculate the correct dose of a drug when given examples of drug orders and available forms of the drugs ordered. 3. Discuss why children require different dosages of drugs to adults. 4. Explain the calculations used to determine a safe paediatric dose of a drug.

Test your current knowledge of mathematics and dosage calculations with a PrepU Practice Quiz!

Glossary of key terms decimal: number that contains a decimal point denominator: bottom number in a fraction fraction: part of a whole number improper fraction: fraction where the numerator is larger than the denominator metric system: the most widely used system of measure, based on the decimal system; all units in the system are determined as multiples of 10 nomogram: tool for calculating body surface area numerator: top number in a fraction proper fraction: fraction where the numerator is lower than the denominator whole number: a complete number

T o determine the correct dose of a particular drug for an individual, it is necessary to consider the person’s gender, weight, age and physical condition, as well as the other drugs that the person is taking. Frequently, the dose that is needed for a person is not the dose that is available, and it is necessary to convert the dose form available into the prescribed dose. Doing the necessary mathematical calculations to determine what should be given is the responsibility of the prescriber who orders the drug, the pharmacist who dispenses the drug and the nurse or midwife who administers the drug. This allows the necessary checks on the dose being given before the person actually receives the drug. Another check to

help prevent medication errors is that, in many institu- tions, drugs arrive at the ward area in unit-dose form, pre-packaged for each individual person. The nurse or midwife who will administer the drug may come to rely on this pre-packaged system, forgoing any recalculation or rechecking of the dose to match the written order. Unfortunately, mistakes still happen, and the nurse or midwife, as the person who is administering the drug, is legally and professionally responsible for any error that might occur. Practising nurses and midwives must know how to convert drug dosing orders into appropri- ate doses of available forms of a drug to ensure that the right person is getting the right dose of a drug.