OBJECTIVE -The purpose of this study was to compare the accuracy of measurements of glucose in interstitial fluid made with the FreeStyle Navigator Continuous Glucose Monitoring System with Yellow Springs Instrument laboratory reference measurements of venous blood glucose.RESEARCH DESIGN AND METHODS -Fifty-eight subjects with type 1 diabetes, aged 18 -64 years, were enrolled in a multicenter, prospective, single-arm study. Each subject wore two sensors simultaneously, which were calibrated with capillary fingerstick measurements at 10, 12, 24, and 72 h after insertion. Measurements from the FreeStyle Navigator system were collected at 1-min intervals and compared with venous measurements taken once every 15 min for 50 h over the 5-day period of sensor wear in an in-patient clinical research center. Periods of high rates of change of glucose were induced by insulin and glucose challenges. RESULTS -Comparison of the FreeStyle Navigator measurements with the laboratory ref-erence method (n ϭ 20,362) gave mean and median absolute relative differences (ARDs) of 12.8 and 9.3%, respectively. The percentage in the clinically accurate Clarke error grid A zone was 81.7% and that in the in the benign error B zone was 16.7%. During low rates of change (ϽϮ1 mg ⅐ dl Ϫ1 ⅐ min Ϫ1 ), the percentage in the A zone was higher (84.9%) and the mean and median ARDs were lower (11.7 and 8.5%, respectively).CONCLUSIONS -Measurements with the FreeStyle Navigator system were found to be consistent and accurate compared with venous measurements made using a laboratory reference method over 5 days of sensor wear (82.5% in the A zone on day 1 and 80.9% on day 5).

The analytical methodologies of the continuous glucose monitors (CGMs) commercialized to date are described. The devices include two noninvasive products: the GlucoWatch (Cygnus, Redwood City, CA) G2 Biographer uses iontophoresis to obtain a sample and electrochemistry for glucose detection, and PENDRA (Pendragon Medical Ltd., Zurich, Switzerland) utilizes impedance spectroscopy. Three minimally invasive systems employ subcutaneous amperometric sensors-Guardian REAL-Time (Medtronic MiniMed, Sylmar, CA), DexCom STS-7 (DexCom, San Diego, CA), and FreeStyle Navigator (Abbott Diabetes Care, Alameda, CA). The GlucoDay (A. Menarini I.F.R. S.r.l., Florence, Italy) collects a sample with a minimally invasive microdialysis fiber and measures the glucose electrochemically.

Even with a high rate of FSN-BG testing, hypoglycemia detected by FSN-CGM was verified by patients with T1DM very infrequently. A high rate of hypoglycemia detection with a moderate rate of unnecessary alarms can be attained using FSN-CGM.

The warm-up period for the second-generation FreeStyle Navigator CGM was reduced from 10 to 1 h, with minimal interruption of glucose reporting and without sacrificing clinical performance.

Factory calibration of sensors for continuous subcutaneous glucose monitoring is feasible with similar accuracy to standard fingerstick calibration. Additional data are required to confirm this result in subjects with diabetes.