Refill persistence with chronic medication assessed from a pharmacy database was influenced by method of calculation

Background and ObjectivesIn literature, different methods of calculating persistence are used. In this study, the effect of using these different methods on persistence and the association of patients characteristics and persistence are assessed.MethodsThe PHARMO record linkage system was used to calculate persistence with antihypertensive drugs for a cohort of 14,466 new users of antihypertensives. Three different types of methods were used to define the maximum gap allowed between two prescriptions that a patient may have to be defined as a continuous user, one based on a defined number of days (varying from 9–365 days), the second based on the duration of the last prescription (varying from 0.1–4 times the duration), the third based on a combination of both methods, whichever leads to the lowest number of days.ResultsRefill persistence varied between 19.7–86.4% (method 1), between 27.9–90.2% (method 2), and between 19.7–86.4% (method 3). Furthermore, patient characteristics associated with persistence differed between and within the three different methods.ConclusionThe method used and the variation within a method influenced both persistence and the association between patient characteristics and persistence. Results of persistence studies are highly influenced by the researchers' method of the maximum allowed treatment gap.