OBJECTIVES: To compare traditional, dispensation-based estimates of Proportion of Days Covered (PDC) with estimates using linked prescription and dispensation information. We illustrate and quantify their differences by estimating real-world, long-term medication secondary adherence in a population-based cohort of osteoporotic patients in the region of Valencia.

METHODS: We provide PDC with only dispensation data in two ways: a) censoring patients only in case of death or loss to follow-up due to disenrollment, where the assessment period (almost) coincides with the follow-up period (“fixed interval”) and b) censoring the assessment periods also at the moment of the last dispensation within the follow-up period ( “last-refill interval” ). We calculated the prescription-adjusted PDC with two different operational definitions: c) considering a minimum prescription gap of three months to adjust PDC (meaning that, when there is a gap of three months or longer in prescription, this period is censored and is not accounted as patient non-adherence), and d) considering any prescription gap (meaning that, when there is any gap in prescription, this period is censored and excluded from the calculation of patient non-adherence).

RESULTS: Mean PDC at 12 months for new users was 63.1% using only dispensation data and a fixed interval, 86.0% using only dispensation data and a last-refill interval, 81% using linked dispensation and prescription data censoring any period without prescription, and 78,3% when using linked prescription and dispensation data but censoring when prescription gaps are 3 months and over. For experienced users, figures were 80.0%, 88.9%, 83% and 81%, respectively.

CONCLUSIONS: Linking prescription and dispensation data allows for the accounting of primary and early non-adherence and of physician interruption when estimating PDC, resulting in a refined and more accurate estimation of secondary adherence versus using dispensation only data. This methodological improvement may be useful and applied to every chronic disease where effective medication is available.