Background: Timing of insulin administration in persons using multiple daily injections (MDI) is self-reported. New technology enabling tracking and logging of insulin doses, combined with continuous glucose monitoring (CGM), may provide insight into the relationship between insulin administration and glucose levels. Methods: We performed a prospective observational study using Bluetooth-enabled pen caps, along with CGM, in participants with type 1 diabetes (T1D) on MDI to objectively assess the relationship between the timing of insulin administration and its impact on pre- and postprandial glucose levels for a 2-week period. Results: We evaluated 50 participants (age 40.3 ± 19 years; A1c 8.2% ± 1.5%, duration of T1D 20 ± 15 years). Thirty-seven percent of total boluses resulted in persistent hyperglycemia (glucose >180 mg/dL 3 h postprandially), while 10% resulted in clinically significant hypoglycemia (glucose <55 mg/dL 3 h postprandially) on CGM. Preprandial glucose levels at the time of the bolus did not correlate with postprandial glucose levels. Late boluses, defined as a rise in glucose of ≥50 mg/dL before a bolus, were seen two times/patient/week. Missed boluses, defined as a rise in glucose of ≥50 mg/dL without a bolus within 2 h, occurred 17 times/patient/week. Late and missed boluses were associated with worse glycemic control (A1c; R² = 0.1, P = 0.02; R² = 0.1, P = 0.02). Conclusions: The use of Bluetooth-enabled pen caps, with CGM, in persons with T1D on MDI can illustrate the relationship between insulin bolus timing and postprandial glucose. These data may help clinicians and patients understand the impact of timing of insulin doses on glucose levels and glycemic control.

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