Accuracy and Interstitial Lag Time

Introduction to Lag Time

Continuous Glucose Monitors (CGMs) measure glucose in the interstitial fluid (ISF). This results in a lag time of 5–20 minutes. Two components contribute to this lag:

• Physiological transport delay: The time it takes for glucose to diffuse from capillaries to tissue
• Instrumental processing delay: Algorithm smoothing and signal processing

Key Accuracy Concepts

MARD (Mean Absolute Relative Difference)

The industry standard for accuracy. Modern sensors achieve a MARD of <10%, comparable to home blood glucose meters.

Directional Divergence

During rapid glucose rise, CGMs read lower than blood; during rapid drops, they read higher. This "push-pull" phenomenon is important to understand for safe CGM use.

Interference Factors

Accuracy can be compromised by:

• Compression lows: Pressure on the sensor restricting blood flow
• Substances: Vitamin C and Acetaminophen (though modern sensors use selective membranes to mitigate this)

Clinical Implications

While highly accurate for trending and steady states, the physiological lag necessitates caution during rapid glucose excursions. Fingerstick confirmation remains the safety standard during these times.