Your CGM numbers
Why Doesn’t My CGM Match My Finger-Stick? Sensor Lag Explained
A meter and a CGM don’t measure the same thing. A meter reads glucose in a drop of blood. A CGM sensor reads glucose in the fluid between your cells, called interstitial fluid. That fluid trails your blood, so the two numbers can differ, especially when glucose is changing.
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Why the lag exists
Glucose has to move from your blood into the interstitial fluid before a sensor can read it. A 2007 paper in the Journal of Diabetes Science and Technology (Nichols and Klonoff) puts the delay at as little as a few minutes or as much as 10 to 20 minutes, depending on the sensor and on whether glucose is rising, stable or falling.
When the gap is biggest
When glucose is steady, the two readings tend to be close. When it is moving fast, after a meal, after insulin or during exercise, your blood has already moved on and the sensor is still catching up. A meter and a CGM taken at the same moment can then disagree.
Other things that matter
Lag is only one factor. Sensor type, how long the sensor has been worn and where it sits can all affect how closely a reading tracks your blood. Your device’s instructions describe its accuracy and say when to confirm a reading with a meter.
Which one to trust
Neither is simply “right”: they measure different fluids and each has its own margin of error. When to act on which belongs to your device’s instructions and your care team. If a reading doesn’t match how you feel, follow the guidance you’ve been given.
How Dia handles it
Dia works with a CGM or with finger-stick checks. With a CGM it describes your state and how long it has held. With a meter it shows each check as a dot rather than drawing a line between them, because a line would imply knowledge of the hours in between. Dia is a companion to your CGM or meter, not a replacement for either.
Questions
Is a CGM less accurate than a finger-stick?
They measure different fluids, and each has its own error range, so neither is a perfect reference. Your device’s documentation lists its accuracy.
How long is CGM lag?
A few minutes to 10 to 20 minutes, depending on the sensor and on the direction glucose is moving, according to Nichols and Klonoff.