T1D glossary

Continuous glucose monitor (CGM)

Also called sensor, continuous glucose monitoring.

A CGM is a small sensor worn under the skin that estimates glucose every few minutes and keeps track of it over time. It measures glucose in the fluid between cells, which tracks blood glucose but can lag when glucose rises or falls fast. Most prescribed CGMs now send a continuous stream of values with adjustable alarms; over-the-counter models have no alarms. (NIDDK (NIH); ADA Standards of Care 2026, section 7)

Worked example

A sensor that reports every 5 minutes gives 288 values a day and about 4,032 over the 14 days that time in range is usually measured on.

Related terms

• Time in range
• MARD
• Sensor warm-up
• Signal loss
• Follower
• Closed loop

Sources, quoted

• NIDDK (NIH): “A continuous glucose monitor (CGM) estimates what your glucose level is every few minutes and keeps track of it over time.”
• NIDDK (NIH): “CGM sensors estimate the glucose level in the fluid between your cells, which is very similar to the glucose level in your blood.”
• ADA Standards of Care 2026, section 7: “CGM measures interstitial glucose (which correlates well with plasma glucose, although at times, it can lag if glucose levels are rising or falling rapidly).”
• ADA Standards of Care 2026, section 7: “now most available prescribed CGM devices have similar characteristics: no swiping required, continuous stream of data, and adjustable alarms and alerts.”
• ADA Standards of Care 2026, section 7: “There are two categories of CGM devices generally available for personal use: those prescribed for diabetes management (rtCGM) and over-the-counter CGM (OTC-CGM), which lacks alarms and alerts and can be used by people not taking diabetes medications that increase the risk of hypoglycemia.”

Every quote above is checked against its source page before each update of this site; last checked 2026-10-06. The worked example is our own arithmetic. This page explains a term; it is not advice for a person's treatment.