Why is interstitial CGM measurement lagged, and how should this affect treatment decisions?

• A. CGM is always more accurate than SMBG, so SMBG is unnecessary.
• B. Interstitial glucose lags behind blood glucose by ~5-15 minutes; treat based on trend and confirm with SMBG when readings are abnormal.
• C. CGM provides exact blood glucose values without lag.
• D. CGM measurements do not require calibration and are not affected by lag.

Answer: (B)

Interstitial CGM readings lag behind what's in the blood because the sensor measures glucose in the interstitial fluid, which takes time to equilibrate with the bloodstream. That delay is typically about 5–15 minutes and can be larger during rapid changes, such as after meals or during exercise.

Because of this lag, a single CGM value might not reflect the current blood glucose. The important idea for treatment is to focus on the direction and rate of change shown by the trend, not just the number at one moment. Use the trend to anticipate where glucose is headed and adjust decisions accordingly. If a reading is abnormal or you’re unsure, confirm with a finger-stick SMBG before acting, especially when symptoms don’t match the CGM reading. Also keep calibration requirements in mind, since some CGMs need calibration and lag effects can vary with sensor performance.