What are the differences between interstitial fluid (ISF) and blood glucose (BG) readings?

Glucose levels can be measured from the bloodstream, or from the interstitial fluid (ISF), which surrounds the body’s cells. There is a delay in ISF glucose response to changes in blood glucose1, which is unlikely to impact routine day-to-day treatment decisions. The average lag time of the FreeStyle Libre systems* is approximately 2.4 minutes for adults and 2.1 minutes for pediatric population2

ISF glucose and BG measurements taken simultaneously won’t always match, and, in fact, are likely to be different.

1. 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.
2. Alva, S. J Diabetes Sci Technol (2020): DOI: 10.1177%2F1932296820958754
* FreeStyle Libre and FreeStyle Libre 2 systems are part of the same family of products.