Duration of Insulin Supply in Type 1 Diabetes: Are 90 Days Better or Worse Than 30 Days?

Objective: There have been few studies regarding the duration of insulin prescriptions and patient outcomes. This study evaluated whether A1C varied with the duration of insulin prescription in patients with type 1 diabetes.

Methods: We conducted a longitudinal investigation (from 2001 to 2015) within a nationwide private health insurer. A cohort study was first used to compare A1C after 30-day only, 90-day only, and a combination (30-day and 90-day) of insulin prescriptions. Second, a self-controlled case series was used to compare A1C levels after 30-day versus 90-day prescriptions for the same person.

Results: In the cohort study, there were 16,725 eligible patients. Mean A1C was 8.33% for patients with 30-day prescriptions compared to 7.69% for those with 90-day prescriptions and 8.05% for those who had a combination of 30- and 90-day prescriptions ( <0.001). Results were similar when stratified by age and sex. Mean A1C was 7.58% when all prescriptions were mailed versus 8.21% when they were not. In the self-controlled case series, there were 1,712 patients who switched between 30- and 90-day prescriptions. Mean A1C was 7.87% after 30-day prescriptions and 7.69% after 90-day prescriptions ( <0.001). Results were similar when stratified by sex. For this within-person comparison, the results remained significant for those ≥20 years of age ( = 1,536, <0.001), but not for youth ( = 176, = 0.972).

Conclusion: There was a statistically significant but clinically modest decrease in A1C with 90-day versus 30-day insulin prescriptions in adults. A mailed 90-day insulin prescription may be a reasonable choice for adults with type 1 diabetes.