How to assess and compare the accuracy of continuous glucose monitors?

Continuous glucose monitors may be valuable tools for improving glycemic control and avoiding hypoglycemia in patients with diabetes. To this goal, sensor readings must adequately reflect the actual blood glucose, emphasizing the need for solid accuracy assessment methods for continuous glucose sensor readings. Analysis of continuous glucose data is challenging, and despite many efforts there still is no all-embracing method to overcome the obstacles in the assessment of continuous data. In this review we disclose the weaknesses of currently available methods and propose a guideline for sensor accuracy assessment and comparison. For accuracy assessment it is best to first plot the sensor readings against the reference values and draw a line of identity, visualizing the degree of agreement. Thereafter data pairs should be given in a Bland-Altman plot to detect a possible relationship between the difference and the mean. The next step is to calculate the absolute relative difference over all paired readings together and per glucose range. A possible lag time between the measurements of both methods can be detected by combined curve fitting. Finally, sensitivity and positive predictive value for detecting hypoglycemia are important indicators of the sensors' performance. For comparing the accuracy between different glucose sensors it is best to use indirect comparison against blood glucose, rather than direct comparison methods, since none of the current glucose sensors is accurate enough to be considered the reference value