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Diabetes Spectr. 2019 May;32(2):139-144. doi: 10.2337/ds18-0054.

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

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Department of Internal Medicine, University of Michigan, Ann Arbor, MI.
Institute of Healthcare Policy and Innovation, University of Michigan, Ann Arbor, MI.
Pediatric Endocrinology, Child Health Evaluation and Research Unit, University of Michigan, Ann Arbor, MI.



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.


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.


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 (P <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 (P <0.001). Results were similar when stratified by sex. For this within-person comparison, the results remained significant for those ≥20 years of age (n = 1,536, P <0.001), but not for youth (n = 176, P = 0.972).


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.

[Available on 2020-05-01]

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