Chapter 21 - Continuous Glucose Monitoring

Abstract

A continuous glucose monitor (CGM) has three components: (1) a sensor that is inserted under the skin and measures interstitial glucose levels almost continuously, (2) a transmitter that wirelessly transmits the glucose values, and (3) a receiver that displays the glucose values in real time or intermittently with trend arrows to indicate the direction and magnitude of glucose changes. Professional CGM devices are owned by providers and are loaned to users for 1 to 2 weeks, recording blinded glucose patterns or displaying real-time glucose values that can later be downloaded by providers for analysis. Personal CGM devices are owned by users to be worn every day or intermittently. Personal CGM provides people with real-time or intermittent interstitial glucose values; trend arrows showing the direction and magnitude of glucose changes; and alarms that alert for hypoglycemia, predicted hypoglycemia, hyperglycemia, and predicted hyperglycemia. The ambulatory glucose profile (AGP) is generated from the CGM download and consists of the glucose management indicator (GMI), an estimate of the equivalent A1c for the preceding 2-week period; the average glucose value; the percent of time in range (TIR) at various levels; indices of glucose variability; and active CGM wear time. CGM devices measure interstitial fluid glucose and not blood glucose (BG). When BG values are stable, interstitial glucose values are usually very close to BG values. However, when BG levels are changing, interstitial glucose values lag by 15 to 30minutes and thus will give lower glucose readings when BG values are rising and higher glucose readings when BG values are falling. Multiple studies in people with type 1 diabetes, treated with multiple daily insulin injections or insulin pumps, have demonstrated that the use of CGM reduces A1c, increases glucose time in target range, reduces time in hypoglycemia, improves hypoglycemia awareness in people who have hypoglycemia unawareness, and improves quality of life.