Many CGM systems use a surrogate for blood glucose--tissue glucose or interstitial glucose. Interstitial fluid (ISF) is the medium for these approaches. A physiological lag in ISF glucose relative to capillary blood glucose has been noted, probably due to relatively low blood perfusion at the sampling site. 9 This physiological lag can result in an accuracy error, manifesting as a significant bias between blood and tissue glucose, particularly during periods of rapid glucose change. This lag error might be particularly meaningful when glucose is rapidly dropping towards hypoglycemia.
