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Mass or molar? Recommendations for reporting concentrations of therapeutic drugs

Units used to report any measurement are an integral component of the result. Errors in communication of units have caused catastrophic failures, for example the Mars Climate Orbiter spacecraft crash in 1999.1 One area of laboratory medicine where different units are in common use is therapeutic drug concentrations, where results for the same drug may be reported in either mass units (eg, mg/L) or molar units (eg, μmol/L) by different laboratories, (see Box for a note on the prefix micro [μ]).

Clinical errors may occur if a result in one unit is interpreted using information expressed in a different unit. This situation may occur when a result is separated from its report (for example, telephoned results), or when a clinician is unaware of unit differences when consulting a textbook, journal article, guideline or website. When assessing results in the toxic range it is more likely that external references will be used, as the therapeutic intervals provided on laboratory reports generally do not indicate the severity of possible toxicity. With the rise of health databases where results from more than one laboratory may be combined, uniformity of reporting takes on…