What does a CGM actually measure, and is it the same thing as a blood sugar test?

A continuous glucose monitor measures interstitial glucose, the glucose dissolved in the fluid between your cells, not blood glucose directly. The two track closely, with a lag of about 5–10 minutes, so what you see on the app reflects where your blood sugar was a few minutes ago, not exactly where it is right now.

The sensor sits in the subcutaneous fat of your arm or abdomen. A filament detects glucose via an electrochemical reaction and transmits readings to your phone every few minutes around the clock. A standard finger-stick measures capillary blood glucose directly at the moment of the draw; the CGM is continuous, passive, and slightly delayed. In the 70–200 mg/dL range most non-diabetic users operate in, the difference is clinically insignificant for behavioral decisions. Where it matters is in rapid rises or drops, where the lag can misrepresent the actual peak by a small margin. (Dexcom G7 accuracy data, FDA 510(k) clearance summary, 2022)

Cardiologist's calibrated position, Promising (2). Interstitial CGM is well-validated against venous glucose at rest, meaning the readings are reliable enough to guide dietary decisions. They are not precise enough to replace a formal OGTT or fasting glucose for diagnostic purposes. Use it for behavior, not for diagnosis.

What to do: Wear the sensor for 14 days without changing behavior first. Your baseline reveals your real metabolic responses before you start optimizing. Post-meal walks, protein-first meals, and reduced refined carbohydrates are the three interventions with the most consistent glucose-lowering effects.

For the full picture, read The CGM on Your Arm Doesn't Know Your Arteries.