Using numbers and handling data
Using numbers and handling data

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Using numbers and handling data

2.4 Sources of errors

The following is a list of common problems that can lead to medication errors. They fall into three broad categories according to where they occur in the sequence from a drug being prescribed to it being administered to a patient. As you can see, the same types of mistake can occur in each category. Those errors that involve maths are highlighted in italics:

Prescription errors

  • Wrong drug prescribed (contraindicated, or allergy, or interferes with existing drug therapy).

  • Prescription illegible/misread (number, decimal point, units of measurement) or unsigned.

  • Wrong dosage form selected (tablets vs. liquid).

  • Wrong administration route selected (oral, intravenous, etc.).

  • Wrong prescribed drug concentration, or drug quantity/volume, or drug rate of administration.

  • Verbal (orally communicated) orders.

Dispensing errors

  • Drug not available.

  • Drug preparation error (wrong drug selected or misreading/miscalculation of drug concentration, or drug quantity/volume).

  • Equipment failure/malfunction.

  • Labelling error (e.g. wrongly labelled syringe).

Administration/omission errors

  • Misreading/miscalculating drug quantity/volume or drug rate of administration, or dose of radiation, or route of administration.

  • Wrong drug administration technique/equipment.

  • Equipment failure/malfunction.

  • Drug chart not kept up to date/needs re-writing.

  • Drug not given at correct time, or correct frequency, or not given at all.

  • Inadequate patient ID or drug given to wrong patient.

In addition, the following factors can all contribute to medication errors:

  • excessive workload;

  • lapses in individual performance;

  • inadequate training;

  • inadequate communication.

(Adapted from ASHP, 1993)

Box 4 How can you help to minimise errors?

Take a pro-active role in minimising errors by keeping yourself up to date with the latest alerts and safe medical practices.

The Medicines and Healthcare Products Regulatory Agency website ( is run by the UK Government (website accessed 13 March 2014). It has up-to-date safety information and alerts about problems with medicines, especially problems concerned with the labelling and packaging of medicines that might lead to drug errors.

The Institute for Safe Medication Practices (in the USA) also has a good website ( that you can browse for more information (website accessed 13 March 2014). Of particular interest are the newsletters that highlight potential risks to be aware of and good practice to adopt. You may wish to look at some of the articles and extracts that are available from the current issue and past issues of the newsletter, or search for a particular topic of interest (note: for the purposes of this course it is not necessary to subscribe to the newsletters). At present (2007), this website only covers America and Spain, so whilst there's much useful and practical advice here, please be aware that procedural differences may occur in the UK.

The four audio tracks linked below contain information that reinforces what you have just learned here. Listen to them now.

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As you have just seen from the errors list above, it is not always safe to assume that a prescription has been filled out correctly. The most common numerical mistakes are differences by a factor of 10, 100 or 1000, either because of a mistake with a calculation or because the wrong unit of measurement has been written down. For instance, as you have seen, when handwritten, the symbols for 'milli' (m) and 'micro' (μ) can often be confused, but are different by a factor of 1000, which is why 'mcg' is routinely used in hospitals to indicate micrograms.

With experience, you will gain a common-sense knowledge of when particular calculations are correct or not, and by developing a reflective sense of your working practices you will quickly become better and more confident with these routine calculations. In particular, you should get used to the recommended dose ranges of the drugs or reagents you routinely deal with, and check the dose range against your calculations for all new drugs that you come across.


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