Dose calculation and medication error--why are we still weakened by strengths?

In 1995 Rolfe and Harper showed that there is substantial confusion about the different means by which the concentration of drugs in solution are expressed [1]. They conducted a survey of 150 doctors in a UK University teaching hospital, and demonstrated that barely half could correctly identify the mass of drug in a solution when its concentration was expressed as a ratio. There were also problems with percentages: less than half could convert the concentration of lidocaine from a percentage to mass concentration, and only one-third knew how many millimoles of sodium bicarbonate are in 100 mL of an 8.4% solution. Less than a third could work out the mass of epinephrine in 10 mL of the mixture of 0.25% bupivacaine with 1 : 200 000 epinephrine used in infiltration anaesthesia. Anaesthesiologists performed substantially better than physicians and surgeons. The authors recommended that the expression of drug concentration should be standardized to mass concentration for all solutions (e.g. milligrams per millilitre). It seems a compelling argument as it involves little or no cost and should only reduce the likelihood of dosing errors. They pointed out that other researchers had been making similar calls since the early 1980s to no avail [2,3]. The correspondence generated by this study debated whether it was desirable, sensible or possible to make these changes. Some argued that not all drugs were suited to having their concentration expressed as mass concentration, for example vaccines [4]. A senior executive in the medicines sourcing department of the National Health Service (NHS) Executive who had also previously called for a universal labelling standard of mass concentration [5] then reported that wider discussion of the proposed changes had identified potential hazards, although these were not specified [4]. Another correspondent highlighted the importance of education and experience, suggesting that students should be drilled rigorously in calculating doses of important emergency drugs [6]. Still, little has changed. In the UK, solutions of drugs are presented in packaging with concentrations expressed in a variety of ways. Epinephrine and norepinephrine are still presented as 1 : 1000 or 1 : 10 000 solutions. Local anaesthetics are still typically presented as percentage solutions, with the notable recent exception of levobupivicaine (ChirocaineTM; Abbot Laboratories, Queenborough, UK). Having previously shown that medical students are just as confused by ampoule labels [7], Wheeler and colleagues recently conducted a novel survey of doctors in the UK using the Internet [8]. They posed six multiple-choice questions concerning the mass of drug contained in solutions of epinephrine, lidocaine and atropine. Additionally, three common clinical scenarios were presented and participants were asked to calculate the correct volume of each drug to give. Almost 3000 doctors took part. Only 85% and 65% correctly identified the mass of drug in the epinephrine and lidocaine solutions, respectively. However, 93% identified the correct concentration of atropine. More would have administered the correct volume of epinephrine and lidocaine in clinical scenarios (89% and 81%, respectively) but only 65% identified the correct volume of atropine. Again there were clear differences between the specialties; anaesthesiologists performed amongst the best. The authors argued that these findings further strengthen the case for standardized labelling of drug solutions as mass concentration, and highlighted an unexpected finding. Less than two-thirds of participants had correctly calculated the volume of atropine to be given in the clinical scenario, even though its concentration is expressed as milligrams per millilitre and the vast majority had calculated its concentration correctly. Atropine may be less familiar than epinephrine or lidocaine, but the clinical scenario – a patient with a symptomatic bradycardia – was not overly esoteric. It did, however, involve a conversion from micrograms to milligrams so it seems likely that the problem was Correspondence to: Daniel W. Wheeler, University of Cambridge, Division of Anaesthesia, Box 93, Addenbrooke’s Hospital, Cambridge CB2 2QQ, UK. E-mail: dww21@cam.ac. uk; Tel: 44 1223 217 889; Fax: 44 1223 217 887