CGM explained: what your glucose sensor shows you 24/7

Measure around the clock: without constant finger pricks.

CGM stands for “continuous glucose monitoring”. A small sensor is worn on the skin (usually the upper arm); a fine filament reaches into the tissue beneath. Instead of one-off finger pricks, the system delivers a value every few minutes and draws a continuous curve from them.

What exactly is measured

Important to understand: a CGM doesn’t measure sugar directly in the blood, but in the fluid between the cells (interstitial fluid). From this signal an algorithm calculates the estimated glucose value.¹

Why the curve “lags” slightly

Because sugar has to pass from the blood into the tissue first, the sensor value follows blood sugar with a small delay. Studies put this physiological lag at about 5 to 10 minutes.¹² It’s most noticeable during fast changes: for example right after eating or during exercise.

Fig. 1: The sensor measures in interstitial fluid, not in blood. That’s why the green curve follows the blue one with about a 5–10 minute lag.

How accurate is a CGM?

Accuracy is often given as MARD (mean absolute relative difference from a reference value). Modern systems reach a MARD of about 10%.¹³ That’s good for everyday use and spotting trends, but in critical situations it doesn’t replace a blood measurement prescribed by your doctor.

Time in Range: the key everyday metric

From CGM data you can read how much time the glucose value spends in the target range: the “Time in Range” (TIR). The 2019 international consensus recommends a target range of 70–180 mg/dl for most people with diabetes, in which you should spend at least 70% of the day (about 17 hours).⁴ Below 70 mg/dl should be under 4%, below 54 mg/dl under 1% of the time.⁴

Fig. 2: The target range and time goals per the international Time-in-Range consensus. 70% TIR corresponds to roughly an HbA1c of 7%.

Key takeaways

A CGM measures glucose in interstitial fluid, not directly in blood.
This creates a physiological lag of about 5–10 minutes: mainly during fast changes.
Accuracy of modern systems is around a MARD of 10%.
Time in Range: target 70–180 mg/dl, at least 70% of the day.
TIR and HbA1c complement each other – daily distribution plus long-term average.

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Sources

Sources as of: 21 Jul 2026.

CGM measures interstitial fluid; physiological lag ~5–10 min; MARD. Clinical Diabetes / review. diabetesjournals.org (opens in a new window)
Interstitial vs. blood glucose lag; kinetics. Study. pmc.ncbi.nlm.nih.gov (opens in a new window)
CGM accuracy and MARD of modern systems. Review. ncbi.nlm.nih.gov (opens in a new window)
Time in Range international consensus 2019: 70–180 mg/dl, ≥70% of day; targets. Diabetes Care. diabetesjournals.org (opens in a new window)