Duration of Insulin Supply in Type 1 Diabetes—Is 90 Days Better or Worse than 30 Days?

We evaluated whether the duration of insulin supply was associated with hemoglobin A1c. Data were obtained through the Clinformatics^® Data Mart Database, from 2001-2015. Patients were grouped according to the duration of insulin supply: 30 days, 90 days, and a combination of both 30 day and 90 day prescriptions. Generalized estimating equations which account for repeated measures within patients were used, with robust estimates of variance. Prescription duration was regressed on HbA1c (dependent variable) with alpha=0.(2-tailed). There were 16,725 eligible patients (9362 with 30-day, 1280 with 90-day, and 6083 with both 30- and 90-day prescriptions). The mean number of HbA1c tests/patient was 3.1. Patients who used 30-day prescriptions had HbA1c values that were 0.65 greater than those with 90-day prescriptions (p<0.001). Patients who used a combination of 30-day and 90-day prescriptions had HbA1c values that were 0.37 greater compared to 90-day prescriptions (p<0.001). The predicted HbA1c values were 8.33% (95% CI: 8.31%, 8.36%) for 30-day, 8.05% (95% CI: 8.03%, 8.08%) for the combination, and 7.69% (95% CI: 7.64%, 7.73%) for 90-day prescriptions. Results were similar when stratified by age. For those <20 years of age, the predicted HbA1c values were 8.95% for 30-day, 8.71% for the combination, and 8.33% for 90-day prescriptions. For those ≥20 years of age, the results were 8.26%, 7.96% and 7.64%, respectively. Patterns were similar when stratified by sex. Mailed prescriptions were also associated with lower HbA1c (p<0.001). Predicted HbA1c was 7.58% (95% CI: 7.53%, 7.63%) when all prescriptions were mailed and 8.21% (95% CI: 8.19%, 8.23%) when not. After simultaneous adjustment for age, sex, duration of supply, and mailed status, both 90-day prescriptions and mailed prescriptions were associated with lower HbA1c values (p<0.001 for both). In patients with type 1 diabetes, 90-day insulin prescriptions and mailed prescriptions were both independently associated with lower HbA1c values.