Optimizing Use of CGM
It is important to have the right expectations about what you can and can't gain from using a CGM. Below are some key facts regarding the systems, as well as strategies for interpreting the data and making each system function as well as possible.
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What They Can't Do
Let's first discuss the limitations of CGMs. For starters, CGM data is not meant to completely replace fingerstick values. In fact, CGM systems require calibration by way of entering fingerstick blood glucose values into the receiver an average of twice daily. Even with appropriate calibration there is, on average, a 15% difference between CGM data and simultaneous fingerstick measurements.
There is also an issue known as "lag effect". CGM systems take frequent measurements and average them of a period of time to generate single data points. This produces data that is, on average, a couple of minutes old. And because CGMs are measuring interstitial fluid (and not blood), there is an inherent 5-10 minute delay between fingerstick values and what appears on the CGM display. Changes in blood glucose concentrations take time to make their way into the interstitial fluid. Thus, if blood glucose levels are rising quickly, the CGM data point can be significantly lower than the actual blood glucose. Likewise, if the blood glucose level is dropping quickly, the CGM data point can be significantly higher.
The radio transmitters can also be troublesome at times. Even though the sensor/transmitter apparatus attached to the skin is waterproof, radio signals do not travel well through water. So data can be lost while swimming or bathing. Signal interference or erratic signals can, at times, cause erroneous/unreliable readings as well as false high/low alarms on the CGM.
What They Can Do
So what are CGMs really good for? Plenty. Even though the specific glucose values displayed on the receiver are not as precise as fingerstick readings, they are usually close enough to get a feel for approximately where the blood glucose is. More importantly, the trends on the graphs are fairly reliable - if they show a rise, the glucose is likely rising. If they show a drop, the glucose is almost sure to be dropping.
Perhaps the most important feature of the CGM is the high/low alert. The CGM will capture most highs and lows long before the wearer can "feel" them on their own. This is extremely important to those with hypoglycemia unawareness who could otherwise be oblivious to potentially dangerous low blood glucose. The low alert makes it considerably safer to work, drive, exercise, and generally aim for tighter glycemic control. It can also provide a much-needed respite for spouses and parents.
Likewise, the high alert allows the user to act in a timely manner in order to avoid prolonged bouts of hyperglycemia and extreme high values. It enables more aggressive management of after-meal blood glucose levels, and serves as an effective early warning system to prevent ketoacidosis.
Other specific uses for CGM include:
- Use of trend arrows to "forecast" where the blood glucose level will be in the next 30-60 minutes and determine the need for extra food or insulin/medication.
- Revealing the post-meal effect of various food types. Did the blood glucose level spike very high soon after the meal? Did it drop and then rise a few hours later? Or was it fairly stable? This type of information can help with the timing/distribution of mealtime insulin doses, as well as appropriate food selection.
- Showing patterns related to exercise and stress. Does the blood glucose fall or rise? When does it happen? Is there a delayed effect several hours later?
- Maintaining tight control for optimizing performance in sporting events, exams, high-risk jobs, or any time blood glucose control is important to success.
- Evaluating and fine-tuning basal insulin (taken via injection or pump) so that glucose levels can remain steady between meals and overnight.
- Measuring the time it takes for rapid-acting insulin to finish working, also known as "active insulin time" or "duration of insulin action". This is important for determining insulin-on-board or bolus-on-board.
Strategies for Success
Experts in the field of diabetes care have developed a number of practical recommendations for improving the performance of CGM systems and hopefully enhance your CGM experience:
Calibrate properly. Putting inaccurate or insufficient fingerstick glucose data into the system will lead to inaccuracy. Follow the manufacturer's recommendation for the frequency and timing of calibration, and make sure your fingerstick readings are precise: clean your hands, code your meter properly, use proper testing technique, store your strips correctly, do not use expired strips, and enter the values immediately into your CGM.
- The simple act of looking at your CGM has its rewards. Checking the receiver approximately once an hour (10-20 times per day) will supply you with sufficient information for making smart decisions.
- Don't over-react to your data. Changes to your program should be made based on repeated patterns/trends, not individual events. It is always best to take a confirmational fingerstick reading before acting on a high CGM value, and always take "insulin on board" into account.
- Plan to wear the CGM on an ongoing, consistent basis. Intermittent use of CGM has not been shown to improve control. Regular use has been correlated with improvements in HbA1c and reductions in hypoglycemia.
- Minimize the "nuisance alarms". High and low alert thresholds are not the same as your target blood sugar levels. When first starting to use a CGM, set the high alert well above what you would consider "acceptable". You can always lower the alert threshold later. Likewise, set the "snooze" feature for at least a couple of hours.
- When setting the low alert, keep the "lag effect" in mind. When the blood glucose level is dropping, the CGM value is likely to be a bit higher than your actual blood sugar. So it is a good idea to set the low alert threshold a bit higher than what you would consider "hypoglycemia".