Traditional blood glucose meters measure the amount of glucose in a drop of blood. CGMs measure the amount of glucose in interstitial fluid. As a result, the glucose readings from your CGM might be delayed by up to 15 minutes compared to manual blood glucose readings. While blood glucose and interstitial fluid readings usually correlate very well, there is a chance that your CGM can’t get a good reading. Both the FreeStyle Libre and Dexcom apps indicate when that’s the case and recommend using a regular glucose meter before making any treatment decisions.