Interstitial fluid (ISF) surrounds cells of the body.1 Glucose can freely move from your blood vessels to this fluid, making it a reasonable alternative to blood for measuring glucose.2
Glucose levels in the ISF can be different from blood glucose levels and may mean that sensor glucose readings are different from blood glucose. You may notice this difference during times when your blood glucose is changing quickly; for example, after eating, taking insulin or exercising.3,4 Changes in blood glucose can take a few minutes (“lag time”) to be seen in the ISF.5 However, this is unlikely to impact routine day-to-day treatment decisions. The average lag time of the FreeStyle Libre system is approximately 4.5 minutes.5
1. National Cancer Institute. Interstitial Fluid. National Institutes of Health website. Accessed December 15, 2022. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/interstitial-fluid.
2. Basu A, et al. Time lag of glucose from intravascular to interstitial compartment in humans. Diabetes 2013;62(12):4083-4087. https://doi.org/10.2337/db13-1132.
3. FreeStyle Libre User’s Manual.
4. FreeStyle Libre 2 User’s Manual.
5. Bailey T, et al. The performance and usability of a factory-calibrated flash glucose monitoring system. Diabetes Technol Ther 2015;17(11):787-794. https://doi.org/10.1089/dia.2014.0378.