CHAPTER 3 Drug calculations explained
• Select, understand and be able to accurately use a method of calculating drug doses and quantities to give with which you are comfortable
Introduction
The principles of how to do a drug calculation are the same whether for an adult or for a child and are considered in this chapter. Chapters 4, 5 and 6 include examples mainly for adults; Chapter 8 deals specifically with calculating in paediatrics. Ample opportunity is given to practise a range of examples which are likely to be met in the clinical area.
• This is not a clinical pharmacology textbook and therefore most examples do not have supporting background clinical information. Doses and routes reflect aspects of current practice but cannot cover all clinical situations that may be encountered. They should be considered as illustrative only. In practice, drug therapy is often complex involving several drugs in combination. Single drugs have been selected in order to demonstrate key principles of calculating doses.
• It is recognised that practice does vary in different centres and that a variety of prescription forms are in use. The key point is that the prescription (however generated) must contain all the relevant information and be valid according to local procedures and protocols
• Examples of labels given reflect current practice as far as this is possible within the limits of this textbook. Some abbreviations used on labels may differ from those used in the text. For example ‘ml’ is more commonly used than ‘mL’. Occasionally non-official abbreviations may be encountered on some labels
• In order to economise on the use of space, the prescription forms used in the examples that follow are necessarily abridged. However, all the information needed to complete the calculations is given. The full format of a prescription sheet is shown in Fig. 1.1
• Products mentioned in the examples may have several indications. An indication of the main pharmacological action(s) is given
• Labels will often be presented in various house styles. No particular significance should be attached to colour/design/art features, etc. In most cases, labels will give the proprietary and the approved name. All the examples have been prescribed using the approved name of the drug
Relating details on the label to the prescription
• Medicines are often available in different strengths in order to meet most clinical needs without the necessity for complex calculations. For example,
• Particular care is needed when working with units that are less than one milligram, such as micrograms and nanograms. Dosages given in micrograms and nanograms indicate that very little of the drug is required to provide a therapeutic effect
• When dealing with liquid medicines, remember to look on the label for the strength of the drug (e.g. 500 mg) and the volume in which it is contained (e.g. 5 mL)
• Whenever possible, try to avoid having to divide a tablet to get the required dose. In some cases, however, this cannot be avoided. Only attempt to split a tablet if it is scored. Never try to split a coated tablet as this will defeat the purpose of the coating and would introduce an inaccuracy. Where a tablet splitter (see p. 59) is available, use it. If there is difficulty, ask the pharmacist – he/she may be able to provide a more convenient formulation. Similarly, capsules should not be opened unless prescribed in this way
• The utmost care is required when dealing with doses involving thousands of units of activity (e.g. nystatin 100 000 units/mL). As always, carefully read the prescription and the label, paying particular attention to the number of zeros
Is a calculation necessary?
• Check that the units of strength on the prescription match the units of strength on the label. For example, do not accept a prescription where the dose is given as a decimal fraction of a milligram when the strength on the label is expressed in micrograms. A quantity that is less than 1 mg should be written in micrograms, e.g. 250 micrograms and not 0.25 mg. Similarly, a quantity less than 1 microgram should be expressed in nanograms.
If the strength of the available product exactly matches the prescribed dose, there is no need to make a calculation. The level of calculation (see p. 42) may therefore be classed as Level 0. The examples that follow are given to illustrate four situations where no calculation is required.
Where a calculation is required
Decide on a method for making the calculation with which you are comfortable (see pp. 43–50).
• It is often very helpful at this stage to consider whether the total quantity required by the patient will be more than or less than what is contained in a single unit dose of the medicine. For example:
Medicine label
• Where a calculation results in a quantity such as 1.78 mg or 2.333 mL, it will not be possible to administer these exact doses using standard clinical equipment. The decision to round up or down will be dependent on clinical factors such as the nature of the drug and the patient’s weight, age and clinical condition
• If in any doubt about a calculation, it is useful to validate the answer by repeating the calculation using another method
• Do not hesitate to ask a second nurse (or other competent colleague) to check your answer if you need reassurance
• Only proceed to give the medicine when you feel confident. Remember that, as with all your professional practice, you should be prepared to account for (i.e. explain, justify) your actions if required (NMC 2008b)
It is of the utmost importance that the principles of the calculation involved are fully understood and upheld at all times. The principles of the metric system and arithmetic are discussed in Chapters 1 and 2.
