Accuracy of a Novel “Factory-Calibrated” Continuous Glucose Monitoring Device in Normal Glucose Levels: A Pilot Study

Free Style Libre Pro, a novel continuous glucose monitoring (CGM) device for retrospective assessment of glycemic excursion, does not require calibration during the measurement period. In this study, the accuracy of FreeStyle Libre Pro in normal glucose levels was investigated. Two meal tests, on the 7th day and 9th day of CGM usage, were performed in five non-diabetic adults. Venous blood samples were obtained at baseline, and at 2 hours, 4 hours and 6 hours after the meal. The plasma glucose levels and the glucose levels measured by FreeStyle Libre Pro were compared. Subjects were aged 41.4±7.6 years, 40% male, with body mass index 21.7±2.6, HbA1c 5.6±0.3% and a fasting plasma glucose level of 95.4±2.1 mg/dl. The plasma glucose levels and the glucose levels measured by FreeStyle Libre Pro were significantly different; 94.4±1.8 vs. 111.2±8.5 mg/dl (P <0.01) at baseline, 95.6±7.0 vs. 122.6±8.8 mg/dl (P <0.01) at 2 hours, 90.1±4.3 vs. 111.2±13.0 mg/dl (P <0.01) at 4 hours, and 88.7±3.2 vs. 105.5±7.1 mg/dl (P <0.01) at 6 hours. The absolute difference between the two methods was 17.7±7.5 (minimum 5 maximum 27) mg/dl at baseline, 27.0±9.4 (minimum 8 maximum 40) mg/dl at 2 hours, 21.1±11.3 (minimum 7 maximum 42) mg/dl at 4 hours, 16.8±5.4 (minimum 7 maximum 23) mg/dl at 6 hours, and 20.7±9.3 (minimum 5 maximum 42) mg/dl for all time points, respectively. The absolute relative difference between them was 18.7±7.8 (minimum 5.3 maximum 28.7)% at baseline, 28.7±10.7 (minimum 7.7 maximum 45.5)% at 2 hours, 23.3±12.2 (minimum 7.9 maximum 45.2)% at 4 hours, 18.9±6.0 (minimum 8.0 maximum 26.2)% at 6 hours, and 22.4±10.0 (minimum 5.3 maximum 43.5)% for all time points, respectively. The ratio of values measured by FreeStyle Libre Pro within the interval of ±15 mg/dl in less than 100 mg/dl or within the interval of ±15% in not less than 100 mg/dl plasma glucose levels was 30.0% at baseline, 10.0% at 2 hours, 40.0% at 4 hours, 50.0% at 6 hours and 32.5% for all time points, respectively. These study results suggest FreeStyle Libre Pro may not be sufficiently accurate in normal glucose levels.