Intended for healthcare professionals


Calculating drug doses

BMJ 1995; 310 doi: (Published 06 May 1995) Cite this as: BMJ 1995;310:1154
  1. Lee Baldwin
  1. Senior registrar in anaesthetics Nuffield Department of Anaesthetics, John Radcliffe Hospital, Oxford OX3 9DU

    Doctors need to be drilled to calculate doses accurately in emergencies

    About 70-80% of undergraduates entering medical training in Britain have passed mathematics at A level, and virtually all will have passed the subject at O level. Yet, as Rolfe and Harper show, among a representative sample of 150 doctors asked to perform simple calculations converting drug doses from a percentage or dilution to mass per volume the success rate was as low as 16% (p 1173).1 The results showed that senior doctors (consultants and senior registrars) were better at calculating the correct answer, and anaesthetists were notably better. While these data seem to suggest a hierarchy in numerical skill among doctors, in terms of both age and specialty, clearly there are other more likely reasons.

    The five questions used in the survey concerned tasks regularly performed by anaesthetists, most of whom would have known the answers even if unable to perform the calculation. It would have been interesting to see if the anaesthetists tested would have scored as highly if the questions had been more abstract—for example, how much drug is there in a one in 350000 solution? In any event, all anaesthetists are regularly required to convert drug doses in this way and—perhaps more important—have this skill reinforced with frequent practice. Nurses are another group who are regularly called on to calculate drug doses, and Adams and Duffield found that their accuracy in doing so improved with regular drills and testing.2 That study concluded, not surprisingly, that drills and experience in clinical practice have an important effect on nurses' ability to calculate drug doses accurately. Furthermore, after a relatively short break from this aspect of training nurses' skills deteriorated rapidly. The published study of Rolfe and Harper effectively reaches the same conclusions for doctors.1

    Having drug ampoules universally labelled in mg/ml would not necessarily eliminate confusion or the need to convert doses since many drugs—electrolytes, for example—are regularly considered and prescribed in terms of amount (millimoles), while biological substances such as insulin are usually measured in units of activity.

    What may we draw from this and a previous, similar study?3 Firstly, when drug doses are usually prescribed in terms of mg or mmol/kg body weight the ampoule containing these drugs should be labelled with the concentration of the contents in mg or mmol/ml. This labelling could be provided in addition to more traditional information about dilution or percentage content. One possible difficulty with dual labelling is ensuring legibility. Unlike the United States, Britain has no standard for the size of lettering in labelling on ampoules,4 so squeezing more information on to ampoules, particularly smaller ones, could increase illegibility. Adopting similar labelling standards to those in existence in the United States might prevent this potential problem.

    Secondly, drills for calculating doses of those drugs that make up the first line of treatment in resuscitation should be taught as rigorously and as dogmatically as other topics when advanced cardiac life support and advanced trauma life support are taught. Finally, if the situation is not an emergency and the doctor is unsure of the dose or contents of an ampoule the advice to “look it up first” or ask for advice holds especially true. With repeated reference to the British National Formulary the doctor will in effect be performing his or her own drill and with time will become adept at the calculations needed to convert drug doses.


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