WHERE SENSORS SIT

Not in the blood

A CGM filament is a hair-thin wire that sits just under the skin. It is not in a blood vessel. It sits in interstitial fluid, the thin layer of fluid that surrounds your cells.

A CGM measures glucose in tissue fluid, then reports it as if it were blood glucose.

INTERSTITIAL FLUID

Glucose takes a short trip

Source Glucose leaves tiny blood vessels called capillaries

Route It diffuses through the capillary wall into the fluid between cells

Result Tissue-fluid glucose follows blood glucose, but not instantly

Diffusion is fast over tiny distances, but it still takes time.

LAG

A few minutes behind

The physiological lag between blood and tissue fluid is commonly reported as roughly 5 to 15 minutes. The sensor's own signal processing can add a little more.

The CGM curve has the same shape as blood glucose, shifted later in time.

WHEN LAG MATTERS

Fast changes show the gap

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Rapid change

After meals, exercise, or insulin, glucose moves quickly and the CGM and blood differ more

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Steady

When glucose is flat, such as overnight, the two agree closely

Lag matters most exactly when glucose is changing fastest.

TREND ARROWS

Direction as well as value

Because of the lag, most CGMs show an arrow for the rate and direction of change. A reading of 100 mg/dL that is falling fast means something very different from 100 that is steady.

If symptoms do not match the CGM number, a fingerstick is the usual way to double check.

UNIT 2 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered where a CGM sits, how glucose gets there, and why its numbers trail blood glucose.