Glucose levels can be measured from the bloodstream, or from the interstitial fluid (ISF), which surrounds the body’s cells. There is a 5- to 10-minute delay in ISF glucose response to changes in blood glucose,1 which is unlikely to impact routine day-to-day treatment decisions. The average lag time of the FreeStyle Libre system is approximately 2 minutes.2   ISF glucose and BG measurements taken simultaneously won’t always match, and, in fact, are likely to be different.
1Rebrin 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.  2Data on File, Abbott Diabetes Care Inc, Clinical Report: Evaluation of the Accuracy of the Abbott Sensor-Based Interstitial Glucose Monitoring System 2014.