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This study investigated whether glucose readings from a sensor sampling in interstitial fluid differ substantially from blood glucose (BG) values measured at the same time.We have evaluated the relationship between BG and glucose extracted from interstitial fluid using the GlucoWatch (Cygnus, Redwood City, CA) biographer, a device that collects glucose from subcutaneous interstitial space through intact skin by application of a low electric current. We evaluated the relative change in the interstitial glucose (IG) signal (IGS) as measured by the biographer versus BG using a normalized two-point sensitivity index (NSI).The results show that biographer measures of IG differ in time and magnitude from the corresponding BG values. In particular, the biographer values were shifted in time due to instrumental and physiological lag. Results show an average total lag of 17.2 ± 7.2 min for all subjects evaluated. The instrumental lag was 13.5 min, suggesting that physiological lag is ∼5 min. In addition, when glucose was increasing, the change in IGS was less than that in BG, while when BG was decreasing, the change in IGS was greater than that in BG.Similar results have been reported for other measures of IG, suggesting that differences reflect physiological variation in glucose uptake, utilization, and elimination in blood and interstitial space. This further evidence of the difference between IG and BG should be considered when interpreting glucose measurements from devices that sample interstitial fluid.