Accuracy of Dexcom G6® and FreeStyle Libre® Sensors in Standardized Hypoxia Conditions: A Randomized Controlled Trial—FOX Study

BACKGROUND: Continuous glucose monitoring (CGM) systems predominantly rely on oxygen-dependent enzymatic electrochemistry. While their accuracy is well established in normoxic conditions, little evidence exists regarding their performance during hypoxia, a situation encountered during altitude exposure, air travel, or specific medical conditions. This study assessed the accuracy of two widely used glucose-oxidase CGM systems (FreeStyle Libre® and Dexcom G6®) during controlled hypoxia. METHODS: In a randomized controlled study, healthy volunteers and participants with diabetes were exposed to standardized normobaric hypoxia (fraction of inspired oxygen of 14.5%). Participants simultaneously wore FreeStyle and Dexcom sensors. Venous plasma glucose, analyzed using the hexokinase method, served as the reference. To induce glycemic excursions, participants consumed a standardized mixed meal and performed moderate-intensity cycling exercise. The primary end point was the mean absolute relative difference (MARD) comparing CGM values with reference glucose. Secondary assessments included consensus error grid analysis (CEGA). RESULTS: . Among participants with diabetes, 53% had type 1 diabetes and 47% type 2, with a median diabetes duration of 16.3 [IQR, 11.3-21.6] years and HbA1c of 7.5% [IQR, 6.6-7.8]. During hypoxia MARD values were 21.2% for FreeStyle and 41.8% for Dexcom in healthy volunteers and 11.8% and 17.5%, respectively, in participants with diabetes. CEGA showed that 97% of FreeStyle and 87% of Dexcom readings fell within zones A or B during hypoxia. CONCLUSIONS: Both CGM systems showed reduced accuracy under hypoxia, particularly during dynamic glycemic changes. Awareness of these limitations and selective confirmation with capillary testing can assist safe use when clinically relevant decisions are required. The implications for automated insulin delivery systems warrant further dedicated evaluation.