Interstitial vs. Capillary Glucose Lag
CGMs measure interstitial fluid rather than capillary blood, resulting in a physiological lag time of 5–15 minutes that creates discrepancies during rapid glucose changes.
Reviewed by CGM AFIA Editorial Team. Last reviewed: 2026-01-24.
Author: CGM AFIA Editorial Team
Part of: Accuracy and Interstitial Lag Time
Understanding the Difference
Continuous Glucose Monitors measure interstitial fluid (ISF) rather than capillary blood, resulting in a physiological lag time of 5–15 minutes.
What Causes the Lag?
The delay is caused by the time required for glucose to diffuse from blood vessels into the tissue fluid. This is a physical process that cannot be eliminated, only compensated for.
The Push-Pull Phenomenon
During rapid glucose changes, this creates a discrepancy:
- When blood glucose rises: The CGM reads lower than a fingerstick
- When blood glucose falls: The CGM reads higher than a fingerstick
Safety Implications
This "push-pull" phenomenon poses a safety risk during rapid hypoglycemia. The sensor may report safe levels after blood glucose has already reached critical lows.
Compensation Strategies
Modern devices use predictive algorithms to mathematically compensate for this delay. However, the physical diffusion limits remain immutable—no algorithm can completely eliminate the physiological lag.