Variation in pharmacy prescription refill adherence measures by type of oral antihyperglycaemic drug therapy in seniors in Nova Scotia, Canada

Objective: To examine the association between pharmacy prescription refill adherence by type of oral antihyperglycaemic medications used in seniors in Nova Scotia, Canada. Research design and methods: Pharmacy and health care utilization data from April 1993 to March 1996 for Nova Scotia Seniors' Pharmacare beneficiaries treated with 1st and 2nd generation sulphonylureas and biguanides was analysed. Refill adherence was quantified by two proportions: number of days beneficiaries had a medication surplus compared with the total period of observation and gaps in treatment compared with the total period of observation. Analysis examined association of type of oral antihyperglycaemic agent and dosing on refill adherence, after adjustment for age, gender and hospital use. Results: A total of 3358 beneficiaries met the study criteria. The mean refill adherence rate [continuous multiple-interval measure of medication availability (CMA)] was 86 +/- 0.4% SE and continuous measure of medication gaps (CMG) was 16 +/- 0.4% SE. Use of biguanides was associated with lower odds of having a medication surplus. The use of 2nd generation sulphonylureas and biguanides, and use of agents with a dosage frequency of more than one dose per day was associated with medication gaps. Conclusions: Many beneficiaries taking antihyperglycaemic agents adhered well to prescribed therapy. The proportion of days not covered by medications averaged 16%. Beneficiaries taking medications once a day were more likely to have good refill adherence. Further work is needed to compare prescription refill adherence rates with other adherence measures and clinical outcomes. These methods are useful for establishing baseline adherence, monitoring the success of programmes designed to improve adherence, and determining cost-effectiveness of drug regimens.