What’s the difference between blood glucose and interstitial fluid?

One of the FreeStyle Libre FAQs we get often is about the difference between blood glucose testing (what you do with a fingerstick test) and interstitial fluid – what you get from a flash glucose monitor like the FreeStyle Libre. Here’s a simple explanation of the difference.

Understanding the difference between capillary blood glucose testing and the FreeStyle Libre Flash Glucose Monitoring System

Fingerstick blood glucose readings and sensor glucose readings won’t always match and in fact are likely to be different. That’s because sensor glucose readings come from the interstitial fluid (ISF), a thin layer of fluid that surrounds cells in the tissue below the skin, not from capillary blood.

A 5 to 10-minute delay

There is a 5 to 10-minute delay in interstitial fluid glucose response versus changes in blood glucose. Glucose readings in ISF have been proven to reliably reflect capillary blood glucose levels.

Capillary blood glucose testing provides an instant reading for current glucose levels without providing any information on its progression or trend. The FreeStyle Libre Flash Glucose Monitoring System provides a complete image of the changes in glucose levels during the day and night.

Not all readings are alike

There are a few situations where interstitial fluid glucose levels may not accurately reflect blood glucose levels:

• Hypoglycaemia (low blood sugar)
• Rapidly changing glucose levels
• Postprandially (after a meal)
• During or after participation in sporting activities

Here’s a simple video explanation:

Remember: A fingerstick test using a blood glucose meter is required during times of rapidly changing glucose levels when interstitial fluid glucose levels may not accurately reflect blood glucose levels or if hypoglycaemia or impending hypoglycaemia is reported by the system or when symptoms do not match the system readings.

Tell us your experiences with the FreeStyle Libre Flash Glucose Monitoring System on Facebook – Diabetic South Africans.

What to read next?

What is a CGM? Flash glucose monitoring and CGM: Not sure what the difference is, or what these words mean? Read this article.

We did it! How to claim the Discovery CGM Benefit: A step-by-step guide to claiming the new CGM Benefit.

Your options on the Discovery CGM Benefit: There are 3 CGM / flash glucose monitors available in South Africa. Here’s what they are, and how much they cost.