Lag time in Continuous Glucose Monitoring (CGM)
When using a continuous glucose monitor (CGM), timing matters. CGM systems measure glucose in interstitial fluid - the fluid found in the spaces around cells - not directly in the blood. When blood glucose changes quickly, interstitial glucose follows with a time delay1. This delay is called lag time.
Since glucose moves from the bloodstream into this interstitial fluid with a slight delay, your CGM readings will trail behind your actual blood glucose levels.
Using your CGM and figuring out lag time is like driving a car with your eyes closed, relying only on navigation software that updates with a delay. On straight roads or gentle turns, this delay isn’t a big issue. But on sharp curves, you need to react faster and anticipate changes ahead to stay on track. Just like managing glucose levels with readings that lag behind actual changes.
How time delay affects your glucose readings
In steady conditions, the lag time between your blood glucose levels and the interstitial fluid is about 5 minutes to 20 minutes based on the placement of the sensor. But when glucose levels are increasing or decreasing quickly— after eating, exercising, or taking insulin — lag time can stretch to 12-24. Depending on the glucose sensor, the lag time can reach up to 40 minutes That means:
- After eating: Your CGM may still show a low glucose reading even though your blood glucose is rising.
- During exercise: Your CGM might lag behind actual drops in glucose, increasing the risk of a delayed response.
- When treating a low: Your CGM won’t instantly reflect the carbs or glucose you’ve consumed, so it’s best to wait before eating more.
- When calibrating your CGM: Calibrate your CGM only when glucose levels are stable, such as before meals or during periods of little activity. If you calibrate during rapid changes — like after eating or exercising — the lag can lead to inaccurate adjustments.
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Lag time can lead to reacting too soon or too late, which may cause unnecessary corrections or fluctuations.
If you have a clear understanding of lag time, you can confidently assess your glucose levels and take appropriate action.
References:
1. Schmelzeisen-Redeker G, Schoemaker M, Kirchsteiger H, Freckmann G, Heinemann L, Del Re L. Time Delay of CGM Sensors: Relevance, Causes, and Countermeasures. J Diabetes Sci Technol. 2015 Aug 4;9(5):1006–15.
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