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Continuous Glucose Monitoring Systems
- Real time monitoring of interstitial fluid glucose is available, with continuous display of glucose level for up to 3-7 days before changing sensor.
- Displays results on an external palm-held device or else on an insulin pump.
- Calibrated by self-monitored blood glucose, used to indicate immediate interstitial glucose as well as patterns of high and low glucose throughout the day.
- Consists of sensor, transmitter and receiver providing real time readings, graphs, trends, high and low glucose and projected glucose alarms directly to patient.
- Glucose reported every 5-10 minutes, with capability of every minute for some CGM’s.
- Small, flexible glucose oxidase sensor inserted under skin usually in abdomen or arm measuring interstitial glucose concentrations; water resistant transmitter sits on skin, sends glucose readings wirelessly to receiver; values downloaded to personal computer, glycemic profiles generated.
- Portion of membrane polymer remains in skin after sensor removed. Long term effects of this not yet determined, although no health effects initially reported in clinical studies.
- Devices currently available: Abbott Free Style Navigator Continuous Glucose Monitor, DexCom SEVEN Plus, Dexcom G4, Medtronic Guardian Real -Time Continuous Glucose Monitoring System, and MiniMed Paradigm Real-Time System
- Specific indications are yet to be established, but may be indicated for patients with unstable diabetes for purpose of improving diabetes management.
- May be useful for patients with type 1 diabetes who use intensive insulin therapy, with or without insulin pump, to help patient recognize fluctuations in glycemia and their causes.
- Also used to evaluate glucose control and in specific clinical situations such as gestational diabetes or intensive care units
- Useful in patients with hypoglycemia unawareness, repeated severe hypoglycemic episodes or undetected hypoglycemia
- JDRF study found that children and adolescents used it less regularly, and with limited use there was no benefit.
- JDRF study suggested more frequent CGM use associated with greater reduction in HbA1c after 6 months.
- Adults (> 25 years old) with diabetes associated with greater CGM use compared to children and adolescents.
- With regular use, more time within target glucose range 71-180 mg/dL.
- Patients using CGM may spend less time in hypoglycemic and hyperglycemic range, and may have less nocturnal hypoglycemia.
- Valuable in guiding therapy adjustments: changing mealtime bolus dosage, adjusting basal insulin rate, changing insulin-to-carbohydrate ratio, etc.
- Used to diagnose and prevent postprandial hypoglycemia
LIMITATIONS OR CONFOUNDERS
- Results are not as accurate as with SMBG. Mean error about 15%.
- Physiological lag between capillary blood glucose data and interstitial fluid sensor data can be as much as 4-10 minutes, depending on rate of glucose change.
- Not approved as replacement for SMBG; abnormally high or low reading should prompt SMBG before acting upon CGM result.
- Can have inflammation, slight bleeding or, rarely, infection at glucose sensor insertion site.
- Sensor may dislodge, new sensor must be inserted; sensors needs to be changed every 3-7 days, depending on CGM brand.
- Receiver must be within 5-20 feet of sensor for wireless range, depending on CGM used.
- No data collected during warm up period (can be between 2 and 10 hours depending on CGM device), required before 1st calibration each time new sensor inserted.
- Calibrations only permitted when blood glucose levels not changing rapidly, so calibrate after overnight fasting or at least 2-3 h postprandially.
- Not good choice for people who are technically challenged, and not adapted for visually impaired.
- Expensive; confirm insurance coverage prior to initiating CGM.
- CGM may enhance management of diabetes in highly motivated people, who are technically capable to incorporate it into personal daily diabetes management.
- Provides complete picture of glycemic control, by increasing number of glucose values available to make appropriate changes to insulin therapy, food intake, and activity in patients with diabetes
- CGM useful for detecting unrecognized hypoglycemia in type 1 and type 2 diabetic subjects
- Alarms may prevent severe, potentially dangerous hypoglycemic events.
- Useful in self-education of motivated patients, showing them what self-care events (insulin doses, diet, exercise) cause highs and lows.
- Valuable in controlling daily fluctuations in blood glucose, which may not be reflected in HbA1c levels.
- Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group, et al. "Factors Predictive of Use and of Benefit From Continuous Glucose Monitoring in Type 1 Diabetes." Diabetes Care, vol. 32, no. 11, 2009, pp. 1947-53. [PMID:19675206]
Comment: Factors associated with greater CGM use was age > 25 years and more frequent self reported pre study blood glucose meter measurements per day. More frequent CGM use associated with greater reduction in HbA1c after 6 months, in all age groups.
- Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group, et al. "Sustained Benefit of Continuous Glucose Monitoring On A1C, Glucose Profiles, and Hypoglycemia in Adults With Type 1 Diabetes." Diabetes Care, vol. 32, no. 11, 2009, pp. 2047-9. [PMID:19675193]
Comment: Evaluated long term effects of CGM in intensively-treated adults with type 1 diabetes. CGM use and benefit sustained for 12 months in this population.
- Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group. "The Effect of Continuous Glucose Monitoring in Well-controlled Type 1 Diabetes." Diabetes Care, vol. 32, no. 8, 2009, pp. 1378-83. [PMID:19429875]
Comment: Study examined CGM benefits for patients with type 1 diabetes who have already achieved HbA1c levels < 7.0 %. Most outcomes, including those combining A1c and hypoglycemia, better with CGM group.
- Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group, et al. "Continuous Glucose Monitoring and Intensive Treatment of Type 1 Diabetes." The New England Journal of Medicine, vol. 359, no. 14, 2008, pp. 1464-76. [PMID:18779236]
Comment: Landmark study evaluating the value of CGM in management of type 1 diabetes mellitus. Results suggested CGM can be associated with lower HbA1c levels in adults with T1DM.
- Bailey, Timothy, et al. "Improvement in Glycemic Excursions With a Transcutaneous, Real-time Continuous Glucose Sensor: a Randomized Controlled Trial." Diabetes Care, vol. 29, no. 1, 2006, pp. 44-50. [PMID:16373894]
Comment: Study revealed patients using CGM spent less time in hypoglycemic and hyperglycemic range, more time at target glucose range, and had less nocturnal hypoglycemia; no difference in A1C levels.
- Klonoff, David C.. "Continuous Glucose Monitoring: Roadmap for 21st Century Diabetes Therapy." Diabetes Care, vol. 28, no. 5, 2005, pp. 1231-9. [PMID:15855600]
Comment: Real-time recognition of both the absolute magnitude of glycemia and trend patterns provides enormous, useful information to patient.
- Bode, Bruce, et al. "Use of the Continuous Glucose Monitoring System to Guide Therapy in Patients With Insulin-treated Diabetes: a Randomized Controlled Trial." Mayo Clinic Proceedings. Mayo Clinic, vol. 79, no. 12, 2004, pp. 1521-6. [PMID:15595336]
Comment: Study revealed fewer hypoglycemic events per day (1.4 + 1.1 vs. 1.7 + 1.2; p value .30) as well as a shorter duration of the event (49.4 + 40.8 minutes per event vs. 81.0 + 61.1 minutes per event; p value .009) in a group of patients using the CGM as compared to a control group using SMBG.
- Boyne, Michael S., et al. "Timing of Changes in Interstitial and Venous Blood Glucose Measured With a Continuous Subcutaneous Glucose Sensor." Diabetes, vol. 52, no. 11, 2003, pp. 2790-4. [PMID:14578298]
Comment: Physiological lag between capillary blood glucose data and interstitial fluid sensor data can be as much as 4-10 minutes, depending on rate of glucose change.
- Chico, Ana, et al. "The Continuous Glucose Monitoring System Is Useful for Detecting Unrecognized Hypoglycemias in Patients With Type 1 and Type 2 Diabetes but Is Not Better Than Frequent Capillary Glucose Measurements for Improving Metabolic Control." Diabetes Care, vol. 26, no. 4, 2003, pp. 1153-7. [PMID:12663589]
Comment: CGM useful for detecting unrecognized hypoglycemias in type 1 and type 2 diabetic subjects; but not better than standard capillary glucose measurements for improving metabolic control of type 1 diabetic subjects.
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