Mass or molar? Recommendations for reporting concentrations of therapeutic drugs

A working party (WP) from the Australasian Association of Clinical Biochemists, Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists, Royal College of Pathologists of Australasia and Royal Australasian College of Physicians recommends the following: • mass units should be used for reporting therapeutic drug concentrations in Australia and New Zealand; and • the litre (L) should be used as the denominator when expressing concentration. Examples of these units are mg/L and μg/L. Exceptions to these principles include: • drugs for which there is current uniformity of reporting and supporting information using molar units, notably lithium (μmol/L) and methotrexate (μmol/L); • drugs that are also present as endogenous substances, where the units used routinely should continue to be used. This applies to many substances, including minerals (eg, iron; μmol/L), vitamins (eg, vitamin D; nmol/L) and hormones (eg, thyroxine; pmol/L). • drugs for which the denominator is not a volume of fluid and there is international uniformity of reporting (eg, thiopurine metabolites; per 109 red blood cells). These recommendations relate to drugs that are used therapeutically, whether measured for therapeutic drug monitoring purposes or for assessment of overdose. Other substances, such as drugs of misuse, heavy metals or environmental toxins, were not considered by the WP and are thus not covered by this document. These recommendations should also be applied to other supporting documentation such as published guidelines, journal articles and websites. The implementation of these recommendations in New Zealand is subject to local confirmation.