Can Continuous Glucose Monitoring Systems Predict Glycemia?

Background: We investigated whether the arrow on a continuous glucose monitoring system (CGMS) screen predicts the course of the capillary glucose level 15 min later. Subjects and Methods: Twenty-three patients with type 1 diabetes (age, 40 +/- 13 years; diabetes duration, 19 +/- 12 years; hemoglobin A1c, 8.5 +/- 1.5%) admitted for education in the use of a CGMS performed 242 observations: the arrow was noted at time 0, and the interstitial and capillary glucose levels were noted at time 0 and 15 min later. Results: The capillary glucose courses were -15 +/- 28 mg/dL after a descending arrow (n = 55), + 1 +/- 23 mg/dL after a stable arrow (n = 147) (P < 0.001 vs. descending), and + 2 +/- 23 mg/dL after an ascending arrow (n = 40) (P < 0.01 vs. descending), with similar findings for the 67 observations after an interstitial glucose level < 100 mg/dL. There were 4.5% grossly erroneous arrows: six descending with later increasing and five ascending with later decreasing capillary glucose. Conclusions: Although there is a large room for improvement, the arrow on the CGMS screen does predict the decline in capillary glucose 15 min later.