Calculation of Doses of Drugs in Solution

Introduction: Our hypothesis was that clinical medical students find the different means of expressing the concentration of drugs in solution confusing. We are concerned that lack of formal teaching on this topic may make students liable to make drug dosing errors after they have qualified. Administering the wrong volume of a drug may have serious consequences for patient safety.

Study design and participant group: Web-based electronic multiple-choice examination of clinical medical students.

Methods: We asked clinical medical students at our university three multiple-choice questions concerning the concentration of lidocaine (lignocaine) and epinephrine (adrenaline) in solution and the maximal recommended dose of lidocaine. The incorrect options were wrong by factors of between 4 and 1000.

Results: One hundred and sixty-eight clinical students out of 350 contacted responded to an invitation to participate (response rate 48%). Twenty-seven percent answered every question incorrectly and 10% answered all three correctly. The mean score for all students was only 1.24 out of 3 (standard error 0.96). However, final-year students performed significantly better (p = 0.016), implying that some knowledge had been acquired informally. Their higher mean score resulted from correctly identifying the amount of epinephrine (p = 0.005) and lidocaine (p = 0.018) more frequently. Only 27% knew the maximal recommended dose of lidocaine, with no difference between years (p = 0.724).

Conclusions: A substantial majority of medical students are unable to calculate the mass of a drug in solution correctly. There is evidence that some students are picking up this skill during the course, because final-year students performed significantly better than first-year students. Modern medical student pharmacology teaching is highly sophisticated, encompassing genomics, molecular and cell biology. The ability to calculate drug doses safely appears to have been overlooked. Students should be familiar with these concepts, so as to avoid dose errors and associated morbidity, mortality and cost when they begin prescribing. To simplify calculations, drug packaging should express the concentration of drugs in solution solely as mass per unit volume, e.g. milligrams per millilitre.