What Is Continuous Glucose Monitoring?

A continuous glucose monitor (CGM) is a tiny glucose sensor inserted under the skin that measures the glucose level in the interstitial fluid (fluid between the cells). A small transmitter attached to the skin sends the results to a receiver or a device such as a smartphone.

CGM reduces the need to poke your fingers for conventional blood glucose checks (or, in some cases, eliminates it completely). Diabetes blogger Kerri Morrow Sparling writes that when she started checking her sugars with an Autolet lancing device back in 1986, “That thing was a beast and blasted my fingertip to bits on a regular basis.”

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Research shows distinct benefits for continuous glucose monitoring in people with Type 1 diabetes. A Belgian study of 515 patients using an insulin pump found that people using CGM had lower glucose levels, higher quality of life, fewer hospital admissions, and less work absenteeism than patients relying on fingerstick glucose measures. The researchers also found that CGM saved money for patients who were motivated enough to learn to use it.

Hospitalization rates for severe low blood sugar (hypoglycemia) and/or diabetic ketoacidosis (DKA) declined from 16 percent the year before starting CGM to 4 percent during a year of using it, a significant change. Quality of life (QoL) improved dramatically for patients who had problems with hypoglycemia. QoL improvement was less in patients not having hypos.

Since CGM is monitoring you all the time, it will find times when sugars are too high or too low, times that self-monitored blood glucose (SMBG) will miss. In a study at University of California at Davis, 34 patients with Type 2 and 27 with Type 1 were put on a Medtronic CGM system. All patients continued SMBG with fingersticks, while a nurse monitored their CGM results.

Over four days, the lowest glucose value reported by patients conducting SMBG was approximately 25 mg/dl higher than that recorded by continuous monitoring. The highest values reported with SMBG were 30 mg/lL lower than that detected by continuous monitoring among patients with Type 1 diabetes, and 55 mg/dL lower in those with Type 2 diabetes.

The lows tended to come while asleep, while the highs were postprandial (after meals). People often don’t test at these times, so they miss glucose changes that can be harmful. A CGM catches those changes.

Our reader Jim B. commented on using a CGM to understand a high morning glucose. “The monitor graphs your levels all night. You’ll know whether you went low and rebounded, or your level just goes up when you wake up” (the dawn phenomenon).

Kerri Sparling says CGM like “Streaming video of my blood sugars versus a [still] picture version.”

For adolescents with Type 1, CGM improves their quality of life. Having to stick yourself for SMBG can be embarrassing, time-consuming, and unpleasant. A study at Ann and Robert H. Lurie Children’s Hospital of Chicago surveyed 1040 adolescents at diabetes camps. They were divided into four categories: using CGM and an insulin pump, using a CGM alone, using an insulin pump alone, or using no technology.

Surveys showed that diabetes distress was lowest among those using CGM alone and highest among the no technology group. Hemoglobin A1C, a measure of average glucose over the previous 2–3 months, was lowest in the CGM-only and CGM-plus-pump groups.

Using CGMs

CGMs have a learning curve. A Cleveland Clinic webpage says, CGMs “require some upfront time to understand their technical aspects. For example, you will need to learn how to insert the sensor properly, how to calibrate the device with fingerstick blood glucose readings, how to set device alarms [for sugars going high or low], how to connect it with your computer or smart phone, (for long-term analysis), and how to respond to and make changes to your care plan based on the collected data.”

(There are videos online to show how all of these are done.)

Different systems’ sensors last for different lengths of time. Some can be worn for up to 10 days. An FDA committee recently voted for approval of a CGM that can last up to 90 days, which may become available fairly soon.

As mentioned earlier, CGMs don’t read the amount of glucose in the blood. Rather, they measure glucose in the interstitial fluid under the skin, and the monitor converts that number to a blood glucose. To learn how your blood glucose relates to your interstitial glucose, most (but not all) CGMS need to be calibrated with fingerstick tests twice a day (or more).

Finding the right adhesive to attach your CGM can take some time. Some people find that some adhesives don’t stick, and others may cause skin reactions. Most patients are able to work with the CGM company and their doctor to find one that works.

Most insurance companies cover CGMs for people with Type 1, and some cover them for Type 2 if the doctor can make a good case for it. Your share of costs for sensors, receivers, and transmitters may be more than you pay for test strips, but whether it’s affordable depends on your income and your insurance.

If you have Type 2 diabetes and are and not using frequent insulin, it might be useful to get a diagnostic CGM, which is usually worn for three days and will give you ideas of how and when your glucose goes up and down. You need to keep thorough logs of foods, activity, and medicines for those three days. Diagnostic CGM is usually done with an endocrinologist.

Whether CGM is worth it for you may depend on what you do with the information you get. If you are thinking about trying one, see the comments on this blog entry I wrote in 2011 or check out a CGM group on Facebook or TuDiabetes.

Want to learn more about CGM? Watch “Continuous Glucose Monitoring” and read “Dexcom G6: The Future of CGM Is Here” and “Sensing the Big Picture With Continuous Glucose Monitoring.”