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Evaluating the Clinical Accuracy of Two Continuous Glucose Sensors Using Continuous Glucose–Error Grid Analysis

¹ Division of Pediatric Endocrinology, Department of Pediatrics, University of Virginia Health System, Charlottesville, Virginia
² Division of Endocrinology, Department of Internal Medicine, University of Virginia Health System, Charlottesville, Virginia
³ Division of Behavioral Medicine, Department of Psychiatric Medicine, University of Virginia Health System, Charlottesville, Virginia

Address correspondence and reprint requests to William L. Clarke, MD, Department of Pediatrics, University of Virginia Health System, Box 800386, Charlottesville, VA 22908. E-mail: wlc{at}virginia.edu

OBJECTIVE—To compare the clinical accuracy of two different continuous glucose sensors (CGS) during euglycemia and hypoglycemia using continuous glucose–error grid analysis (CG-EGA).

RESEARCH DESIGN AND METHODS—FreeStyle Navigator (Abbott Laboratories, Alameda, CA) and MiniMed CGMS (Medtronic, Northridge, CA) CGSs were applied to the abdomens of 16 type 1 diabetic subjects (age 42 ± 3 years) 12 h before the initiation of the study. Each system was calibrated according to the manufacturer’s recommendations. Each subject underwent a hyperinsulinemic-euglycemic clamp (blood glucose goal 110 mg/dl) for 70–210 min followed by a 1–mg · dl^–1 · min^–1 controlled reduction in blood glucose toward a nadir of 40 mg/dl. Arterialized blood glucose was determined every 5 min using a Beckman Glucose Analyzer (Fullerton, CA). CGS glucose recordings were matched to the reference blood glucose with 30-s precision, and rates of glucose change were calculated for 5-min intervals. CG-EGA was used to quantify the clinical accuracy of both systems by estimating combined point and rate accuracy of each system in the euglycemic (70–180 mg/dl) and hypoglycemic (<70 mg/dl) ranges.

RESULTS—A total of 1,104 data pairs were recorded in the euglycemic range and 250 data pairs in the hypoglycemic range. Overall correlation between CGS and reference glucose was similar for both systems (Navigator, r = 0.84; CGMS, r = 0.79, NS). During euglycemia, both CGS systems had similar clinical accuracy (Navigator zones A + B, 88.8%; CGMS zones A + B, 89.3%, NS). However, during hypoglycemia, the Navigator was significantly more clinically accurate than the CGMS (zones A + B = 82.4 vs. 61.6%, Navigator and CGMS, respectively, P < 0.0005).

CONCLUSIONS—CG-EGA is a helpful tool for evaluating and comparing the clinical accuracy of CGS systems in different blood glucose ranges. CG-EGA provides accuracy details beyond other methods of evaluation, including correlational analysis and the original EGA.

Abbreviations: CG-EGA, continuous glucose–error grid analysis • CGS, continuous glucose sensor • P-EGA, point–error grid analysis • R-EGA, rate and direction of blood change–error grid analysis

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