Using Pharmacy Data and Adherence to Define Long-Term Bisphosphonate Exposure in Women

Area of Science:

• Pharmacoeconomics and Pharmacoepidemiology
• Observational Study Design
• Medication Adherence Research

Background:

• Accurate drug exposure assignment is crucial for observational studies on bisphosphonate (BP) treatment using pharmacy data.
• Understanding medication adherence is key to interpreting treatment effectiveness and patient outcomes.

Purpose of the Study:

• To evaluate how different adherence levels, defined by the proportion of days covered (PDC), influence the assignment of BP exposure.
• To assess the impact of varying PDC thresholds on classifying patients as continuously on or off BP therapy.

Main Methods:

• A cohort of 10,381 women initiating oral BP therapy was analyzed over a 5-year treatment period followed by up to 5 years of follow-up.
• Adherence was assessed in 90-day intervals using predefined PDC thresholds (≥ 0.5, ≥ 0.6, ≥ 0.7).
• Women were classified as 'on treatment' or 'off treatment' based on PDC, and their continuous adherence status was tracked.

Main Results:

• At year 2, the proportion of women continuously on BP therapy varied from 48% (PDC ≥ 0.5) to 36% (PDC ≥ 0.7).
• By the end of follow-up, continuously treated women ranged from 18% (PDC ≥ 0.5) to 12% (PDC ≥ 0.7).
• The percentage of women continuously off therapy by the end of follow-up was 4% across all PDC thresholds (0.5, 0.6, 0.7).

Conclusions:

• A proportion of days covered (PDC) threshold of 0.6 provides a practical balance for defining bisphosphonate (BP) adherence.
• Adjusting the PDC threshold between 0.5 and 0.7 resulted in only modest changes in the classification of continuous BP therapy.
• The choice of PDC threshold has a discernible but not drastic effect on categorizing long-term bisphosphonate exposure in observational studies.