Doctors' confusion over ratios and percentages in drug solutions: the case for standard labelling

Abstract

SUMMARYThe different ways of expressing concentrations of drugs in solution, as ratios or percentages or mass per unit volume, are a potential cause of confusion that may contribute to dose errors. To assess doctors' understanding of what they signify, all active subscribers to doctors.net.uk, an online community exclusively for UK doctors, were invited to complete a brief web-based multiple-choice questionnaire that explored their familiarity with solutions of adrenaline (expressed as a ratio), lidocaine (expressed as a percentage) and atropine (expressed in mg per mL), and their ability to calculate the correct volume to administer in clinical scenarios relevant to all specialties. 2974 (24.6%) replied. The mean score achieved was 4.80 out of 6 (SD 1.38). Only 85.2% and 65.8% correctly identified the mass of drug in the adrenaline and lidocaine solutions, respectively, whilst 93.1% identified the correct concentration of atropine. More would have administered the correct volume of adrenaline and lidocaine in clinical scenarios (89.4% and 81.0%, respectively) but only 65.5% identified the correct volume of atropine.

Factors influencing doctors’ ability to calculate drug doses correctly

“…There had been 2975 participants in the original study (1); the response rate was 24.6%. The study population did not differ significantly from the medical workforce on the basis of specialty (p = 0.29), age (p > 0.999) or seniority (p > 0.999).…”

“…http://www.Doctors.net.uk is an internet service and content provider based in the UK and aimed exclusively at doctors. Subscribers had been invited to participate in an online survey of drug administration skills in 2003 (1). The invitation included information that answers would be recorded, that anonymity would be maintained, and data of interest published; however, written consent was not obtained.…”

“… The six questions about drug administration from the multiple choice questionnaire (correct answers marked). Adapted from the work of Wheeler and colleagues (1) …”

“…Doctors have great difficulty calculating the doses of drugs in solution correctly, and expressing the concentrations of drugs in solution as ratios or percentages causes particular problems (1). Only 85.2% of doctors responding to an online study correctly identified that 1 in 1000 adrenaline (epinephrine) equates to 1 mg/ml, and just 65.8% knew that 10 ml of 1% lidocaine contains 100 mg of drug.…”

“…Doctors’ difficulties with drug administration imply that they also require additional education or reinforcement of their current knowledge. We subjected the original data collected in the online survey of 2975 UK doctors (1) to further analysis so as to identify whether there are any groups of doctors to whom extra teaching should be directed. We examined the influence of each doctor's length of medical practice, their grade and the medical school at which they trained upon their overall performance.…”