ISF is a reasonable alternative for blood. Glucose can be measured in the ISF–the fluid around the body’s cells where glucose freely diffuses from capillaries to the interstitial space.¹  The physiological delay in ISF glucose with respect to changes in blood glucose is about 5-10 minutes2  which is unlikely to impact routine day-to-day treatment decisions. The average lag time of the FreeStyle Libre system is approximately 2 minutes.³  

1. Rebrin K, Steil GM. Can interstitial glucose assessment replace blood glucose measurements? Diabetes Technol Ther. 2000;2(3):461-472. 

2. Rebrin K, Sheppard NF Jr, Steil GM. Use of subcutaneous interstitial fluid glucose to estimate blood glucose: revisiting delay and Sensor offset. J Diabetes Sci Technol. 2010;4(5):1087-1098. 

3. Data on File, Abbott Diabetes Care Inc, Clinical Report: Evaluation of the Accuracy of the Abbott Sensor-Based Interstitial Glucose Monitoring System 2014.