CGM readings are not blood glucose
A CGM measures the fluid between your cells, not blood, so it lags when glucose is moving fast.
Your CGM is not reading your blood. It reads the fluid between your cells, and that fluid runs a few minutes behind your actual blood glucose, so the number on screen is where you were a little while ago, not exactly where you are now.
- A CGM (continuous glucose monitor, the sensor on your arm) measures glucose in interstitial fluid, the fluid around your cells, not in your bloodstream.
- There’s a lag of roughly 5 to 15 minutes, so when your glucose is moving fast the sensor number is genuinely behind the truth.
- The trend arrow matters as much as the number: it tells you which way you’re heading and how fast.
- If a reading doesn’t match how you feel, or before you treat a low, check with a finger-prick blood test, which reads blood directly.
What the lag actually changes for you
Interstitial fluid is the fluid sitting between your cells, just under the skin where the sensor filament sits. Glucose reaches it a little after it’s in your blood, so the sensor is always reporting a slightly older number. When your glucose is steady, that gap barely matters and the CGM and a finger-prick will line up closely.
The gap matters most when things are moving fast: after eating, after a correction dose, and during and after exercise, when a low can come on quickly during the session or straight after and again hours later, including overnight. If your glucose is dropping quickly, the screen number is higher than your blood actually is right now. So a CGM showing 4.5 with a straight-down arrow is not a “you’re fine” reading. Your blood may already be lower, and it’s still falling. That’s the moment to trust the direction, not just the digit.
Reading the arrow, not just the number
Two people can both see 6.0 mmol/L and be in completely different situations. One has a flat arrow and is genuinely steady. The other has a sharp down arrow and is heading for a hypo within minutes. The number alone can’t tell you apart, which is why the arrow is doing half the work.
A rough way to read it: a flat or gently sloping arrow means you’ve got time to think. A steep arrow, up or down, means change is happening faster than the screen can keep up with, and you should act on where you’re going, not where you are. This is why treating off the arrow, not just the value, is the habit that keeps you out of trouble.
False lows, compression and confirming with blood
Sometimes a CGM reads low when your blood glucose is fine. The common culprit is a “compression low”, where you’ve lain on the sensor overnight and squashed the fluid around it, so it reports a false drop. These often show up as a sudden low at 3am that recovers the moment you roll over. Sensors can also read oddly in the first day after insertion, or if the filament is disturbed.
The rule that keeps you safe: if a reading doesn’t match how you feel, or if you’re about to treat a low, do a finger-prick first. A blood test reads glucose directly, with no lag, so it settles the argument. If you feel hypo but the CGM says you’re fine, treat the symptoms and check with blood, because how you feel is real information too.
What changes how far off the reading is
The faster your glucose is changing, the bigger the gap between the sensor and your blood, in either direction. Physical activity is a big one: exercise can lower your glucose during the session and for hours afterwards, including overnight, so a CGM can lag behind a fall you can’t feel yet. New sensors sometimes read off for the first day, and overnight compression lows are common. None of this means your device is broken. It means the number is an estimate that’s more reliable when things are calm and less reliable when they’re moving.
How this plays out
Examples, not instructions or doses.
Your CGM shows 4.5 mmol/L with a straight-down arrow after a walk. Because of the lag, your blood is likely already lower and still falling. This is a moment to treat, not to wait and watch the number.
You wake to an alarm saying 3.2, but you feel fine, and by the time you sit up it’s climbing back to 5. If you’d been lying on the sensor, that can be a compression low. A finger-prick would tell you what your blood was actually doing.
You feel shaky and sweaty but the CGM reads 6.0 with a flat arrow. Your symptoms are real data. Do a finger-prick before deciding: if the blood test agrees you’re fine, look at what else might cause those feelings; if it’s low, treat it.