Anatomy of a Continuous Glucose Monitoring Sensor

By: 
Zona Batacchi, MD

If you don’t know what a CGM (continuous glucose monitor) is, you probably have heard someone mention this alphabet soup recently, perhaps at your doctor’s office or in the news. A CGM is a small, wearable device that monitors your blood sugar (glucose) levels by measuring the glucose level of the fluid in the tissue under the skin, which is called “interstitial fluid.” The CGM sensor measures the glucose levels frequently, with an average determination made every five minutes. It then transmits the information to a recording device that displays the blood glucose levels. The levels are shown as numbers and a graph. Conveniently, the system is wireless, and the sensor can be worn under clothing but has to be changed, depending on the brand, from every several days to every week or slightly less often. To be sure that the device is working as it should, it is recommended that a traditional finger stick for blood glucose levels using a traditional glucose meter be performed at least twice daily to calibrate the system.

What are the different types of sensors available?

Currently, there are two kinds of CGMs: one for diagnostic use and one for personal use. Your healthcare provider may suggest the use of a diagnostic CGM to look at patterns and trends in your daily blood glucose fluctuations that might not be clear from your self-monitoring or traditional finger stick blood glucose (SMBG) log.

Examples of fluctuations include frequent low blood glucoses (hypoglycemia) or difficult-to-explain changes in your blood glucoses. The diagnostic CGM, depending on the brand, is usually worn from three to 14 days. At the end of this period, the information collected from the CGM is processed through computer software and reviewed along with a diary of your meals, physical activity and insulin doses. While being collected, the glucose information is not visible to anyone...that is, it is “blinded,” so the data must be reviewed retrospectively. Therefore, it is very important that meal content and timing, as well as diabetes medication (usually insulin doses and timing), be correctly recorded in the daily diary that you will be asked to keep.

The personal CGM allows you, the wearer, to see your glucose changes. You insert the sensor yourself, a relatively easy task using a spring-loaded device specific to the sensor brand you are using. And you can even have the blood sugar information transmitted to your phone, which minimizes the number of care-related items you have to carry around. You can also share the information (or not) with family, spouses, or others, if you wish. Or if you have a history of having low blood sugars and not being aware that they are going low, it can certainly be comforting to know that your spouse or family member could be watching your blood sugars as you exercise.

High blood sugar or low blood sugar alert settings can be chosen, with an alarm that goes off when blood glucoses don’t fall within a pre-determined range. The live information showing blood glucose changes or trends which predict when you are about to go out of your personal blood sugar target range gives you an opportunity to take action, so seeing trends when eating different foods or food amounts, or exercising, allows you to better manage your diabetes.

The CGM device may be “stand-alone” or “linked” to an insulin pump system. For the linked CGMs, the information directly transmitted to your pump can help calculate doses for insulin boluses, a dose of insulin that is specifically taken at meal times to keep blood glucose levels under control following a meal.

In addition to high and low alerts, some pumps have a “low glucose suspend” function: there is an automatic suspension of insulin delivery if the glucose falls lower that than a pre-set value, and the wearer of the pump/sensor is not aware that blood sugar has dropped so low.

How accurate is the CGM?

If your blood glucose is steady, glucose in the interstitial fluid is about the same as in your blood. But if your blood glucose is rapidly changing, there can be a lag in the change in the interstitial fluid glucose level up to 15 minutes behind the blood glucose level. The CGM is less accurate once glucose falls below 70mg/dL (milligrams per deciliter) or if the glucose level is changing rapidly. For this reason, finger stick glucose checks are recommended, although the technology in sensor chemistry has been improving. In fact, the Food and Drug Administration (FDA) just recently approved one sensor system as being accurate enough to use to make insulin dosing decisions without a corroborating finger stick value. Progress!

Are there recent scientific studies about CGM use?

Until recently, most scientific studies on CGM and diabetes mellitus were focused on patients that used insulin pumps. In January 2017, two studies put a spotlight on CGM use in persons not using insulin pumps, but using multiple injections of insulin. Both studies explored how CGM would impact blood glucose control compared to usual treatment.

In the DIAMOND trial at the Jaeb Center for Health Research in Tampa, Florida Roy Beck, MD, PhD, and his team conducted a 24-week trial that randomly assigned 158 study participants to either a CGM group or finger stick glucose group (but they were required to check blood sugar four times daily). All participants used multiple daily insulin injections, but were not using insulin pumps. Blood sugar control was significantly better in the group using CGM, both in terms of an average overall blood sugar level as well as fewer ups and downs, and less variation, in daily blood sugar levels.

In the GOLD trial, Marcus Lind, MD, PhD, of the Diabetes Outpatient Clinic at Uddevalla Hospital in Sweden, and his team conducted a study of 161 participants who were using multiple daily insulin injections. In this study, the participants switched between using a CGM and not using one. Again, blood sugars were significantly better with CGM, both in overall average, as well as less fluctuations in high to low sugars.

Together, these studies support that CGM can improve diabetes control when using multiple daily doses of insulin, not just when paired with an insulin pump.

Would a CGM system be helpful for me?

CGM is most beneficial if you are willing to:

  • Wear the device daily
  • Understand what the glucose trends suggest (your blood sugar is going up or down), and
  • Understand how to treat your blood sugar changes (change meal time or content, change insulin doses, go for a walk or exercise)

In January 2017, the Centers for Medicare and Medicaid Services (CMS) announced that “therapeutic” CGM systems were newly classified as “durable medical equipment,” which is covered under Medicare Part B. This was a great victory for persons wanting to get the CGM systems covered under Medicare. But for now, the Dexcom G5 Mobile system is the only CGM system that falls within this classification.

There are CGM devices on the market that require minimal or no calibration, but are only available for personal use outside of the United States at this time. However, scientists are also working on an implantable device with long-term use capability. Stay tuned for developments, as this is the future!