Variation in the days supply field for osteoporosis medications in Ontario

We examined pharmacy claims for osteoporosis medications dispensed in the community (78 %) and long-term care (LTC) to determine if days supply values matched expected dosing intervals. Results identify potential reporting errors that can have implications for drug exposure misclassification, particularly in LTC where only 59 % of reported values matched expected values.

The days supply field is commonly used to examine patterns of drug utilization and classify drug exposure, yet its accuracy has received little attention. We sought to describe the days supply reported for osteoporosis drugs and examine if values matched expected therapeutic dosing intervals.

We examined days supply values for osteoporosis medications submitted to the Ontario Drug Benefits program for seniors, 1997-2011. Days supply values were evaluated by dosing regimen and setting (community or long-term care [LTC]) and compared to pre-defined expected values. We defined expected days supply by the therapeutic dosing interval: daily in 7- or 30-day intervals, or as 100 days; weekly in 7- or 30-day intervals; monthly and daily nasal spray in 28- or 30-day intervals; and cyclical etidronate as a 90-day supply.

We identified 17,615,404 osteoporosis prescriptions, with 78 % dispensed in the community. Most daily oral prescriptions were dispensed by an expected therapeutic dosing interval (97 %). Annual IV zoledronic acid was most commonly dispensed as a 1-day supply (62 %). Distinct differences in agreement were observed for other regimens, with the expected days supply more commonly reported in community versus LTC: cyclical etidronate (86 % vs. 40 %), weekly (91 % vs. 60 %), monthly (94 % vs. 35 %), and nasal spray (84 % vs. 40 %).

Results suggest that inaccuracies in the days supply field exist, particularly among prescriptions dispensed in LTC. Inaccurate reporting may have significant implications for osteoporosis drug exposure misclassification.