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CGM：How to interpret and use the data

Time：2016-12-28 0

Unlike self-monitor blood sugar test, continuous glucose monitoring(CGM) can provide continous glucose level information to optimise glycaemic control. With the improving accurancy and reliability of the sensors and the inducing cost of the technology, CGM has become more user-friendly and spaned clinical usage. To realise improvements in patient outcomes when using CGM, both health care professionals and patients need to have ability to interpret the numuous data obtained from CGM. For instance, the Poctech CT-100B CGM can provide one glucose data every 3 minutes and 3360 sensor glucose data for 7 days. In this article, I will demonstrate how to analyse CGM data with examples to spur thinking in clinical practice.  

1.      The glucose daily overlay

The interstitial glucose levels for each day are superimposed upon each other, which allows identification of repetitive patterns.

There is repetitive pattern overnight, with hypoglycemia from midnight until about 03:00, followed by a small degree of rebound. An adjustment of the nocturnal basal insulin dose should be rectify the problem.

This glucose curve is from a 34-year-old woman on multiple daily injections, using a once-daily insulin analogue. The following conclusions can be draw:

1)      Glucose levels remain stable overnight but rise from about 05:00 to 10:00. Adjustment include：Increase basal insulin dose, but this will increase the risk of nocturnal hypoglycemia. Increase bolus insulin dose with breakfast. Change to insulin pump.

2)      Glucose levels fall rapidly after lunch resulting in hypoglycemia between 14：00 and 16：00, which cause resultant hyperglycemia before dinner time . Adjustment: decrease bolus insulin dose with lunch.

PS: The adjustment on the dose of insulin should be confirmed with fingertip BG or venous BG.

2.      Assess the effect factors of blood sugar, e.g. exercise

When look at each day glucose plot, we can assess the effect of exercise, the food intake, insulin dose and other factors on glucose levels.

This is an example from a 36-year-old man who suffer a sharp fall in blood sugar after excise resulting in unaware hypoglycemia.In figure1 a high starting glucose protects against hypoglycemia while infigure 2 he experiences a period of hypoglycemia(<3.9mmol/l) in 14：00-18:00 andlate in 20：00-22：00. Adjustment include：check blood sugar before exercise, take carbohydrate if necessary,avoid dangerous activity such as driving, and watch out delayed hypoglycemia.

3.      The glucose meal overlay

The meal overlay view is particularly helpful when trying to assess whether insulin dosing is adequate.

In this example from an insulin pump user, there is a late fall in glucose levels after breakfast, a well control glucose after lunch but tending to rise before dinner although after dinner back to normal. Adjustment include：a decrease  in basal rate before lunch and an increase before dinner. We can also find an anomalous glucose curve after dinner on Saturday. Perhaps a mistake in insulin counting before dinner, or snacking, stress happened at that time.

In 2012 USA International Diabetes Center panel discussed recommendations for standardization of analysis and presentation of CGM data, and advised to use The Ambulatory Glucose Profile(ACP) in clinical practice. CGM is comparatively considered as a useful educational, diagnostic and therapeutic tool. The health care professionals should analyse the CGM data with the impact factors such as diet, exercise and medicine to offer advices.

References：

1.      Continuous glucose monitoring:the clinical picture. How to interpret and use the data. Peter Hammond,Practical Diabetes 2012; 29(9):364-368.

2.      Recommendations for standardizing glucose reporting and analysis to optimize clinical decision making in diabetes: The Ambulatory Glucose Profile (ACP). Richard M. Bergenstal, Andrew J. Ahmann, Timothy Bailey, et al. Diabetes Technology &Therapeutics 2013;15(3):198-211.

Tracy Yu

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