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