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What’s new in continuous glucose monitors?
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Currently available continuous glucose monitoring systems include the iPro Recorder, the Guardian REAL-Time and the MiniMed Paradigm REAL-Time Systems by Medtronic, the SEVEN Plus by DexCom and the FreeStyle Navigator by Abbott Laboratories.
Accuracy
A study conducted by Garg et al compared the accuracy of the SEVEN Plus and the FreeStyle Navigator. Fourteen patients wore both sensors for three consecutive five-day periods. Laboratory reference measurements of venous blood glucose were taken every 15 minutes for eight hours on days five, 10 and 15 in a clinic using the YSI STAT Plus glucose analyzer. The mean absolute relative difference (ARD) for CGM vs. YSI laboratory glucose was 16.8% for SEVEN vs. 16.1% for Navigator (P=.38). When evaluating the hypoglycemic region (<80 mg/dL), the mean ARD for CGM vs. YSI laboratory glucose was 21.5% for SEVEN compared with 29.8% for Navigator. The SEVEN system provided better agreement with the YSI laboratory glucose measurement within the range of 40 mg/dL to 80 mg/dL (P<.001). There were a total of 12 (27%) adhesive site reactions and three (4.2%) insertion site reactions reported with SEVEN vs. 32 (58%) adhesive site reactions and 36 (65%) insertion site reactions reports with Navigator.
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A similar study was conducted in 16 patients using the Guardian REAL-Time system. Values from the Guardian REAL-Time system were compared with reference YSI glucose values taken every 30 minutes in the clinic. The mean absolute percent difference between the Guardian REAL-Time glucose values and the YSI values was 19.7. The calculated bias, or overall difference between the two, was –15.0 mg/dL. Of 3,941 total paired glucose measurements, 96% fell in clinically acceptable Clarke error grid zones A or B. For low glucose values between 40 mg/dL to 80 mg/dL, 76.1% of readings fell in zones A or B and for high glucose values >240 mg/dL, 86.8% of readings fell in zones A or B. Accuracy in the hypoglycemic range declined as the time increased from insertion of the sensor.
Patient selection
Most CGM devices are only FDA approved for use in adults and only available with a prescription and letter of medical necessity from the prescriber. Guardian REAL-Time and the MiniMed Paradigm REAL-Time systems are available in pediatric models. CGM may be particularly useful in patients being switched from multiple insulin injections to insulin pumps or current pump users and patients with hypoglycemia, including hypoglycemic unawareness, severe episodes of hypoglycemia and those who keep blood glucose levels above their goals due to fear of hypoglycemia. Other patients who may benefit from CGM include those with glycemic variability or those who have not achieved their HbA1c goal and want to be proactive. Although none of the current systems are approved for use in pregnant women, they would likely benefit from the use of CGM systems because of the tight glucose control required during pregnancy.
Clinical implications
Currently, CGM systems are only approved for use in conjunction with traditional blood glucose testing. Implications for a particular CGM system depend on how data are transmitted to patients and clinicians. Because traditional blood glucose meters only provide snapshots of glucose levels, achieving glycemic control in some patients can be difficult. Snapshots do not provide information regarding whether the patient’s glucose is increasing or decreasing, and the amount of data generated are obviously dependent on the patient’s willingness and ability to self monitor their glucose. However, traditional glucose monitoring remains useful for determining insulin doses when CGM results do not correspond to symptoms the patient may be experiencing, prior to driving and for assessing glucose after treating hypoglycemia.
CGM systems utilize sensors that measure interstitial glucose every one to five minutes without patient intervention and then transmit that data to a storage device. CGM systems are able to detect glycemic fluctuations throughout the day, which may be an independent risk factor for the development of complications. More precisely, several studies have shown a relationship between postprandial glucose fluctuations and macrovascular disease. Lastly, an analysis of data from the DCCT showed that HbA1c, mean blood glucose and glycemic variability were independent risk factors for severe hypoglycemia. Thus, reducing glycemic fluctuations may reduce risk for severe hypoglycemia.
Data obtained from a CGM could be used to adjust medications or diet based on real-time glucose, identify glucose trends and aid in pattern management by providing retrospective, nearly continuous glucose values. One study evaluated the benefit of using a continuous glucose monitor in 91 patients receiving insulin. All patients wore the monitor but were randomized to a control group (continuous glucose data not provided to patient) or display group (continuous glucose data not provided during period 1 but displayed during periods 2 and 3). Each period lasted 72 hours. The results indicated that the display group spent 21% less time hypoglycemic, 23% less time hyperglycemic and 26% more time in the target glycemic range. Nocturnal hypoglycemia was also significantly reduced in the display group. These improvements were seen despite no prescribed plan to adjust therapy based on glucose readings.
CGM systems allow users to enter “events,” such as information about meals ans exercise. This can provide the patient with information to make changes in response to glucose patterns. These changes may include adjusting the prandial or basal insulin dose, modifying insulin correction algorithm and other such interventions. Short-term periodic use of a CGM in a patient with type 2 diabetes may identify times when the patient needs more frequent self-testing or guide further therapy selection.
Future implications
There are some barriers that need to be addressed before CGM use will become widespread. Some may have concerns about the accuracy of CGM systems. Clinical trials will help address this issue. CGM users need to realize that there can be a lag time between interstitial and blood glucose changes when the glucose is rapidly changing. Patients and health care providers should confirm glucose values with a standard glucose meter before making adjustments to therapy. Widespread insurance coverage and reimbursement should follow the evidence indicating improved outcomes. Patients and health care providers will need to be educated on the proper use of CGM systems and how to apply the data. Patients will need to understand action time courses of their insulin so that they can make appropriate adjustments. Health care providers will also have to find ways to incorporate the use of CGM systems into their already busy clinical practice. Things like integrating the data into electronic medical records or downloading data prior to the office visit may streamline the process.
James R. Taylor, PharmD, CDE, is a Clinical Associate Professor in the Department of Pharmacy Practice at the University of Florida.
For more information:
- Garg S. Diabetes Care. 2006;29:44-50.
- Garg S. Diabetes Technol Ther. 2009;11: 65-72.
- Hirsch IB. Diabetes Technol Ther. 2008;10:232-44.
- Medtronic Diabetes. A frequent sample accuracy evaluation of the Medtronic Diabetes Telemetered Glucose Monitoring System II (TGMS II) in subjects with diabetes mellitus. 2004. Accessed on July 30, 2009.
- Weinstein R. Diabetes Care. 2007;30:1125-1130.