It’s completely normal for a CGM (Continuous Glucose Monitor) to show different numbers than a standard blood glucose meter. The difference comes down to where glucose is measured in the body.
CGM: measures glucose in the interstitial fluid under the skin.
Blood glucose meter: measures glucose directly in the blood from the fingertip.
Because glucose levels in interstitial fluid lag about 10–20 minutes behind blood levels, readings can differ – especially when glucose levels change rapidly, for example:
after eating,
during physical activity,
or after insulin intake.
This means the CGM doesn’t always show the same exact value as a blood test, but it provides valuable insight into the trend and direction – whether your glucose is rising, stable, or falling.
In other words, a blood glucose meter shows a snapshot, while a CGM shows a continuous movement over time.
Why does CGM sometimes show low readings?
It’s quite common for CGMs to show temporarily low readings, especially at night or during rest. This doesn’t always mean your blood glucose is truly that low. Because CGMs measure in interstitial fluid, the sensor can sometimes pick up delayed or temporary fluctuations that don’t appear in the blood.
Pressure on the sensor (for example, lying on it while sleeping) can also cause false low readings.
If you get repeated low readings that don’t match how you feel, you can always double-check with a traditional blood glucose meter to confirm the level.