*Geek Box: Plasma Glucose vs. CGMs
The standard reference method for measuring glucose levels in the body, either fasted or in response to food, is a measurement of blood glucose, specifically plasma glucose concentrations. Glucose also circulates systemically in the arteries, and arterial glucose levels are higher compared to plasma, particularly in the postprandial state. Arterial glucose is higher than plasma due to the diffusion of glucose from plasma into interstitial fluid [IF].
IF is the fluid between blood vessels and cells and provides a medium through which nutrients, including glucose, are shuttled to cells. Levels of glucose in IF are determined by the rate at which glucose diffuses from plasma into IF and the rate at which glucose is then taken up in cells. CGMs contain sensors that are inserted directly in the interstitial fluid of SAT.
It should be noted that interstitial glucose and plasma glucose should not be considered equivalent compartments, and there is evidence of discordance between the values obtained from plasma glucose measures [the “gold standard”] and interstitial glucose as measured by CGMs. In particular, differences in postprandial glucose levels suggest the potential for measurement error from CGMs. Thus, it is important to note that CGMs remain a technology in development.