When the sensor “lies”: why the CGM reads too high, then too low

The core: blood first, tissue after

A CGM measures glucose not in the blood but in the fluid between the cells. Blood sugar changes first, the sensor glucose follows, the lag is, depending on the situation, roughly 2 to 20 minutes.¹ As long as the value barely moves (rate of change under about 1 mg/dl per minute), blood and tissue are practically the same.² Only when it goes fast does the gap become visible.³

You can see exactly this on my beer-garden evening

After the unexpected sugar surge (the detective work is here) my sensor showed 205 mg/dl at 19:42 with a steeply rising arrow. Later in the day, in hindsight, with a smoothed curve, the peak reaches about 190. Then the curve falls steeply, shoots below the target band to about 78 mg/dl, and afterwards settles around 100, where it stays calm for hours.

This is exactly the pattern that unsettles many: the sensor overshoots on a fast rise and undershoots on a fast fall, before it catches itself. That’s not a malfunction, it’s the physics of the tissue lag plus the smoothing algorithms in the device.

When the deviation is largest

Typical situations with fast changes, and therefore the largest deviation:¹

• After eating, especially with fast carbs (my beer-garden evening)
• During and right after exercise
• After treating a hypo, the sensor still reads low although the blood is already rising
• After medications that move blood sugar fast

The other gap: the sensor change

On 13 July my notes simply say: “gap due to sensor change”. That’s the mundane but important kind of gap: between the old and new sensor there’s simply no data. On top of that, a fresh sensor is often not yet up to operating temperature in the first hours: the values can deviate more at the start.

How I handle it: mark the gap as a note. Otherwise weeks later I puzzle over why there’s nothing there, or worse: I interpret a gap as an event.

What I make of it

• Trend beats single value. With a steep arrow the direction matters more than the exact number.
• Don’t react to the swing, but to the pattern. The undershoot to 78 was no cause for panic – the curve caught itself.
• If feeling and display contradict each other: confirm with a finger-prick. If symptoms don’t match the sensor value or the value is implausible, the fingertip measurement according to the manufacturer’s instructions applies.
• Note the gaps so the later analysis stays honest.
• In calm phases the sensor is most accurate – that is where looking at absolute numbers pays off most.

Article ID: SWF-P-004Please quote this ID for corrections or additions.