Robust determination of the optimal continuous glucose monitoring length of intervention to evaluate long-term glycaemic control

Objective Consensus Continuous Glucose Monitoring (CGM) guidance includes a recommendation that a minimum of 14 days of CGM data are used to report times in ranges. The previously employed approaches to determine the optimal duration for CGM data have limitations. Here, we present a robust approach to define the minimum duration of CGM data to report times in ranges, as well as other glycaemic metrics. Methods The approach is based on the median absolute percentage error and employs a sliding time window to reduce the impact of the inter-time interval variability, hence allowing smaller datasets to be used. A 10% and 5% threshold were employed to assess the optimal duration of CGM data for a set of commonly employed metrics to assess quality of glycaemic control and glycaemic variability. To evaluate the impact of the dataset size and type of intervention, data from two randomised controlled trials involving participants with type 1 diabetes were used (n=236 and n=25). Results Results suggest that mean glucose reaches the 5% threshold for mean absolute percentage error within 2 weeks, while percentage time in target 70-180 mg/dL, mean absolute glucose, standard deviation, and coefficient of variation reach the same threshold within 4 weeks in both datasets, suggesting that these metrics can be robustly assessed from CGM data over a 4 week period, while some other metrics require much longer window lengths, especially those evaluating hypoglycaemia. Conclusions Our data suggest that there is no optimal duration for CGM data to robustly assess all outcomes and that the duration required for a robust outcome depends on the population being studied, the sampling frequency, and the primary outcomes selected.