People Choose CGMs For:
- Better Glucose Management
- Avoiding frequent hypoglycemia
- Preventing hypoglycemia unawareness
- Understanding the effects of particular foods on after-meal glucose levels
- Overnight glucose safety
- Living alone
- Frequent driving, travel, high-risk professions
- A young child unable to report hypoglycemia
- Tight control before and during pregnancy
- Avoiding diabetes complications
A continuous glucose monitor reveals short-term trends in the blood sugar as they happen. The monitor reads out a constant stream of glucose data every 1 to 5 minutes. You can see the direction your blood sugar is taking in the last 1, 3, 6, 9, 12, or 24 hours, depending on what times your specific monitor offers. This gives 288 to 1440 readings a day compared to the 4 to 8 fingerstick readings you rely on with a meter.
A CGM acts like a diabetes coach who shows you how boluses, exercise, carb intake, and treatment of lows and highs affect glucose readings. Once undesirable patterns are identified, you can work toward preventing them with insulin and lifestyle adjustments.
Parts of a Continuous Glucose Monitoring System
CGMs have 3 basic components: a sensor, a transmitter and a receiver. The receiver and transmitter are durable devices that are reused each time the sensor is changed. Sensors are disposable and approved for 3 to 7 days of use depending on the manufacturer. In practice, sensor life varies from person to person and sensors can sometimes be used beyond their stated life. The main concern with longer use is infection, but the risk of this appears to be low compared to infusion sets. At some point during extended use, readings will become less accurate. When inaccurate readings occur, you can try calibrating the sensor a couple of times over an hour or so, but don’t continue to use it if readings remain inaccurate. Be sure to discuss this with your physician. Irritation at the site from the sensor or tape is unusual, but check for this on removal.
CGM Site Tips
- Approved for the abdominal area, many people wear their CGM on the back of the arms, outer thighs, upper buttocks, or love handles.
- Avoid scar tissue, shave areas where hair may prevent good adhesion.
- Pinch up the skin a bit during insertion to prevent the insertion needle from going too deep, BUT make sure it goes deep enough to get into fat tissue.
- If you use an adhesive barrier under the sensor, such as IV 3000, make sure to cut a hole where the sensor goes through.
- If you use Skin Tac or IV Prep pads to improve adhesion, make sure to avoid gumming up the skin near the point where the sensor will go into the skin.
A sensor is a thin, flexible wire inserted beneath the skin with a needle and an insertion device. The sensor rests in the fatty layer below the skin and is usually not felt by the person wearing it. Sensor wires vary from 1/4 to 3/5 inch (6 to 15 mm) in length. Current introducer needles vary from less than 26 gauge to 22 gauge. Sensors contain enzymes that react with glucose in the interstitial fluid around cells. A chemical reaction produces an electric signal that the transmitter sends to the CGM receiver where it is interpreted as a glucose level.
Any site at least two or three inches away from both the last sensor site and the infusion set can be used. Choose any fleshy site without scars or bruises. Swab the site with alcohol and insert the sensor (Pump Preparation). Some sensors can also be inserted by hand. Once the sensor is in place, the inserter and needle are removed. Unlike pump infusion sites, infections are rare with sensor sites because nothing is infused into the body. Bleeding can occur, so check the site for bleeding and wipe it clean if needed. If significant bleeding occurs, use another sensor in a different site. Check periodically for redness, tenderness, or swelling.
Once the sensor is inserted, a reusable waterproof transmitter is seated into a plastic pod on top of the sensor or clicked into the side of the sensor. Transmitters vary in size and shape from 1/4 to ½ inch (6 to 12 mm) thick and from a thumbnail to a half dollar in circumference.
Transmitters vary in how far they can send readings, currently about 5 to 10 feet (2 to 3 meters). Cell phones and other electronic devices can occasionally interfere with transmission of data.
A battery-powered receiver or a display on the pump or pump controller displays glucose information received from the transmitter. Separate receivers are about the size of a small cell phone and can be carried in a pocket, backpack, or purse, or worn on a belt. With pump receivers, the user does not carry a separate receiver.
Current U.S. insulin pumps don’t use CGM readings to adjust insulin delivery, but clinical trials are underway that will gradually add more closed loop features to pumps. Some trials are trying to close the loop with the wearer giving an approximate bolus for a meal to reduce post-meal glucose spiking. This allows slower bolus insulin to get started before the meal, with the CGM and pump handling the smaller dose adjustments that would then be required.
The receiver displays glucose values every five minutes, shows trend graphs of past glucose values over the last several hours (1 to 24 hours), and has arrows and trend lines that show whether the glucose is rising or falling.
Various companies have already released continuous monitors, with more companies developing theirs every day. Significant differences in accuracy can be seen in one individual when two different continuous monitors are worn at the same time.
MiniMed received FDA approval for it's CGMS system in June of 1999 for 3-day use in a physician's office. A wireless transmitter sends information from a glucose sensor to the Guardian Monitor for readings every 5 minutes, 24 hours a day, and provides helpful alerts for peace of mind around the clock.
In 2006, some 14 years after beginning its CGMS project, MiniMed received FDA approval for sale of its Paradigm REAL-Time System. This system integrates the x22 series of Paradigm pumps with a continuous glucose monitor. The monitor's accuracy is not good enough to give bolus doses from and it is certainly not accurate enough to "close the loop" which still appears to be several years off. But it does help determine trends in blood sugar levels and can provide early warning for when a blood sugar test may be warranted for a rise or fall in the blood sugar.
DexCom, headquartered in San Diego, California, is focused on developing technology for continuous glucose monitoring to improve the lives of people with diabetes. Their DexCom STS system was approved by the FDA on March 27, 2006. Since then, Dexcom has upgraded their system to the new Dexcom SEVEN Plus.
The Food and Drug Administration (FDA) has approved the FreeStyle Navigator® Continuous Glucose Monitoring System in the United States for people with diabetes. Designed to discretely and continuously measure glucose levels through a sensor in the back of the upper arm or abdomen, this system provides minute-by-minute information about which way and how quickly blood sugar levels are changing. This information can lead to adjustments that can result in tighter glucose ranges. As with all current continuous monitors, a traditional fingerstick glucose test should be performed before making any treatment decisions.