Aims The nature and incidence of errors in prescribing and giving medicines have previously been estimated by trained observers, or by retrospective analysis of incidents in which patients have come to harm. We have examined prospectively in routine clinical practice the concentrations of intravenous infusions of a drug (acetylcysteine) which is given according to a complicated dosing schedule. Methods We prospectively collected samples before and, where possible, after the infusion of acetylcysteine in 66 anonymous patients requiring treatment for acetaminophen (paracetamol) overdose in four centres in the United Kingdom. We measured the concentration in each infusion bag, and deduced from the weight of the patient the percentage of the anticipated dose that had actually been given. Results The experimentally determined dose was within 10% of the anticipated dose in 68 of 184 individual bags (37%), and within 20% of the anticipated dose in 112 bags (61%). Doses in 17 bags were more than 50% from the anticipated doses. In three patients, values in all three bags appeared to be systematically wrong by 50% or more; in a further seven cases, individual bags differed by 50% or more from the anticipated value. The median difference between pre-and post-infusion samples was 0% [interquartile range x5.2% to +14.6%], but 9% showed a disparity of greater than t50%. Conclusions Our data suggest that there is large random variation in administered dosage of intravenous infusions. Systematic calculation errors occur in about 5% [95% con®dence interval 2, 8%] of cases, and major errors in drawing up in a further 3% [1, 7%], with inadequate mixing in 9% [4, 14%]. While we have no evidence that patients were adversely affected, and while the regime of administration of the drug studied (acetylcysteine) is complicated, these data suggest that the delivered dose often deviates from the intended dose, and that methods of quality control are needed.

A comparison was made between the results of acetylator phenotyping by isoniazid (INH) half-life measurements based on samples taken for 6 h after a single oral dose (200 mg

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Plasma concentrations of isoniazid and N-acetylisoniazid were measured at varying times after addition of both compounds to plasma (5 ,ug/ml) and storage at several different temperatures (-70°C, -20°C, 4°C and 20°C). The concentration of both compounds declined log-linearly with time and the half-life of decline was shorter with increasing temperature of storage. Both compounds were stable for at least 5 weeks, however, when stored at -70°C. We conclude that this temperature should be used to prevent loss of both compounds during storage and transport of samples.