Overview
What a CGM Measures—and What It Cannot Tell You
A continuous glucose monitor (CGM) uses a small sensor in subcutaneous tissue to estimate glucose in interstitial fluid.
A continuous glucose monitor (CGM) uses a small sensor in subcutaneous tissue to estimate glucose in interstitial fluid. It sends readings to a receiver, smartphone, or insulin pump, creating a glucose pattern rather than isolated finger-stick results. Interstitial glucose generally follows blood glucose, but it does not change at exactly the same moment. The difference matters most when glucose is moving quickly: after a meal, exercise, a correction dose of insulin, or treatment of hypoglycemia. A falling sensor value may reflect a low that is still developing, while a recovering sensor value can lag behind treatment. Some CGMs display readings and alarms continuously. Others require the user to scan the sensor to see the current value and trend. The device type changes how information is obtained, but not the clinical question: Is glucose safe now, and where is it heading? CGM is a monitoring tool, not a replacement for clinical assessment. Obtain a capillary blood glucose measurement when symptoms do not match the sensor value, when glucose is changing rapidly, when a result seems implausible, or when the device instructs...