For example, I find that my CGM data tend to be unreliable during the first 12–24 hours after I insert a new sensor. It often takes time for a sensor to become acclimated to being below the skin, and for fingerstick readings to allow the sensor to “hone in” properly. The CGM data seem to become more precise and reliable the longer a sensor has been in place. Until I start to see a couple of sensor values that are very close to simultaneous fingerstick readings, I just don’t trust the CGM system for immediate decision making.
The accuracy of CGM values must also be questioned if there have been frequent or prolonged data gaps on the receiver/monitor. Signal interference or inconsistent signals may cause erroneous and unreliable information.
There is also an issue known as “lag effect” that must be considered. CGM systems take multiple glucose readings over a five-minute period and average them to generate a single glucose level. This means that, on average, the level you read on your display is 2½ minutes old. And because CGM systems measure the glucose level of interstitial fluid (not blood), there is an inherent 5- to 10-minute delay between fingerstick values and what appears on the monitor’s display. Therefore, if blood glucose levels are rising quickly, the CGM level can be significantly lower than a blood glucose reading done with a fingerstick at the same time. Conversely, if the blood glucose level is dropping quickly, the CGM level can be significantly higher.
Because of this, I only recommend using CGM numbers for immediate insulin dosing if the following are true:
- The last couple of fingerstick values have matched the sensor values closely (within 10%). For example, if the fingerstick reading is 120 mg/dl, the CGM value should be between 108 and 132 mg/dl.
- Your current glucose level is not rising or dropping rapidly. (A look at the graph on your display screen can tell you if this is the case.)
- The sensor has not generated any error messages or significant data gaps for at least the past hour.
Whether or not you use your CGM data for insulin-dosing purposes, the simple act of looking at your monitor has its rewards. Research has shown that checking the monitor 10–20 times per day and wearing the system most of the time (rather than intermittently) tends to produce improvements in HbA1c level (a measure of average blood glucose control) and a reduction in episodes of hypoglycemia (low blood glucose).
Perhaps more valuable than the immediate glucose value is the direction it is headed. If you checked your blood glucose at bedtime and it was 95 mg/dl, you would react differently if you knew it was on the rise rather than if you knew it was dropping. Similarly, seeing a level of 188 mg/dl before exercise would mean more to you if you knew whether your level was rising, falling, or remaining steady.
The Medtronic CGM systems display “up” or “down” arrows to denote both the direction and the magnitude of glucose changes over the past 20 minutes. The DexCom Seven system displays a one-hour trend graph with similar information. Use this information to your advantage! The ability to forecast where your blood glucose will be over the next hour or two can help keep you from straying too far out of your desirable blood glucose range.
For instance, if you see that your glucose level is close to or slightly above normal but dropping quickly, you can either have a snack immediately or be prepared to have a snack soon to prevent hypoglycemia. Similarly, if you see that your glucose level is near or slightly below normal and dropping gradually, you know to have a snack now and to check your monitor again soon.