In the linked retrospective cohort study (doi:10.1136/bmj.a2752), Zhang and colleagues explore which factors known to be associated with admissions for adverse drug events may place older people at an increased risk of readmission.1 Their retrospective study of older Australians showed that readmissions as a result of adverse drug reactions increased over a 13 year period and by 2003 comprised a third of the total admissions for these reactions.2 The mortality rate was 59.3% in first time admissions and 68.1% in recurrent admissions during follow-up (mean 4.2 years (SD 4.3)).

To study recurrent admissions for adverse drug reactions, all admissions need to be identified and then first admissions must be distinguished from recurrent ones. Zhang and colleagues identified recurrent admissions by scrutinising the 10 year period before the start of the study. Rigorously identifying all admissions is difficult when using retrospective data.

Correct coding of admissions for adverse drug reactions is complex and errors can occur at several stages. Knowledge of …