What is hypoglycemia in patients with well-controlled type 1 diabetes treated by subcutaneous insulin pump with use of the continuous glucose monitoring system?

Objective: To analyze 72-hour interstitial glucose concentrations measured by a glucose sensor in adult subjects. Methods: We compared glucose levels in 10 patients with type 1 diabetes mellitus (DM-1) and 10 normal control subjects with use of the Medtronic Continuous Glucose Monitoring System (CGMS). Hypoglycemic events identified by the CGMS, the patients, or both were compared in the patients with DM-1. In addition, the results between two glucose sensors placed at similar sites in the same subject were evaluated. Results: Hemoglobin A1c and mean glucose values were higher in the DM-1 group than in the control group (P<0.01), whereas time spent at glucose levels of less than 60, 60 to 79, and 121 to 150 mg/dL were similar between the two study groups. Time spent at glucose levels above 150 mg/dL was greater in the DM-1 group than in the control group (P<0.05). Of the 74 total hypoglycemic events in the patients with DM-1, the sensor and the patient recognized 14%, only the patient recognized 9% (confirmed by readings of <55 mg/dL on a glucose meter), and only the sensor detected 77%. Of note, 41% of the patient-identified symptomatic hypoglycemic events were missed by the CGMS. When one patient with DM-1 and one control subject wore two glucose sensors simultaneously, one and not the other sensor often identified "hypoglycemia," and the correlation was only fair to good by intraclass correlation analysis. Conclusion: Many asymptomatic "hypoglycemic events" identified by the CGMS may not be actual hypoglycemia. Hypoglycemia is both overreported and underreported with use of the CGMS. Thus, adjustment of insulin doses to reduce asymptomatic hypoglycemia identified by the CGMS alone may not be warranted.