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Continuous Glucose Monitoring: Making Sense of Your Numbers

by Gary Scheiner, MS, CDE

A1c. HDL. LDL. Fasting blood glucose levels. Postprandial blood glucose levels. Grams of carbohydrate. Insulin-to-carbohydrate ratios. Correction factors.

Ever feel like you’re drowning in numbers? Diabetes care is chock full of numerical data, all designed (presumably) to help a person manage his blood glucose levels, prevent complications, and live life to the fullest. But judging by the problems diabetes still causes, perhaps there needs to be a better way.

And perhaps there is one: continuous glucose monitoring (CGM), which lets people know their blood glucose level practically all of the time, not just when they take a fingerstick reading. The trouble is, CGM systems produce even more numbers, charts, and graphs that can seem like an endless ocean of data unless you know what to do with all that information. And that’s a shame, because even with their shortcomings, CGM systems can be an extremely valuable resource for guiding daily decision making. They can generate information that, if you know how to use it, will allow you to fine-tune your (or your child’s) diabetes management program and stay out of harm’s way.

CGM basics

There are now three CGM systems available that provide real-time blood glucose estimations and warning systems to guard against highs and lows. The two that have been around the longest are the DexCom Seven and the Medtronic Guardian REAL-Time System (which can be used alone or as an integrated part of the MiniMed Paradigm 522 or 722 insulin pump). A third system, the FreeStyle Navigator, from Abbott Diabetes Care, was approved by the Food and Drug Administration in March of 2008. Only the Medtronic Guardian System has received FDA approval for use by children, but any system can be used by a child if prescribed by a physician.

All three systems use a thin metallic sensor inserted just below the skin to detect glucose in the fluid between the cells (called the interstitial fluid) in the fatty tissue just under the skin. Spring-loaded insertion devices make sensor insertion quick and relatively painless, and the sensors do not tend to cause skin irritation the way pump infusion sets can.

The information from the sensor is transmitted by radio signal to a receiver/monitor, which displays an estimate of the current glucose concentration. While the transmitter and receiver/monitor are “durable,” meaning they should last a long time, the life expectancy for the glucose sensors ranges from 3 days to about 14 days.

The DexCom and Medtronic receiver/monitors provide an updated glucose reading every five minutes and can be set to sound an alarm if the glucose level goes above or below a target range set by the user. The Guardian system can also warn the user of glucose levels that are nearing the upper or lower end of the target range. To get the most accurate results, both systems require the user to take fingerstick blood glucose readings at least twice a day and enter those readings into the system. Both systems are generally accurate to within 15% to 20% of fingerstick readings. Both provide line graphs showing glucose trends over the past several hours, and both have compatible software that permits detailed analysis of glucose trends over longer time intervals. (For a fuller look at CGM basics, see the article “Continuous Glucose Monitoring” online or in the March/April 2007 issue of Diabetes Self-Management.)

Immediate numbers

Go back to the paragraph above. See that “15% to 20%” figure? That means the data generated by CGM are usually only a rough estimate — too rough, in fact, to use for making most insulin-dosing decisions. That’s not to say that CGM users never use their data to determine insulin doses, but you need a sound knowledge of the nuances of the system before doing so.

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