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, Beck RW, Buckingham B, et al. Factors predictive of use and of benefit from continuous glucose monitoring in type 1 diabetes. Diabetes Care. 2009;32(11):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, Bode B, Beck RW, et al. Sustained benefit of continuous glucose monitoring on A1C, glucose profiles, and hypoglycemia in adults with type 1 diabetes. Diabetes Care. 2009;32(11):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. 2009;32(8):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, Tamborlane WV, Beck RW, et al. Continuous glucose monitoring and intensive treatment of type 1 diabetes. N Engl J Med. 2008;359(14):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.
- Garg S, Zisser H, Schwartz S, et al. Improvement in glycemic excursions with a transcutaneous, real-time continuous glucose sensor: a randomized controlled trial. Diabetes Care. 2006;29(1):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 DC. Continuous glucose monitoring: roadmap for 21st century diabetes therapy. Diabetes Care. 2005;28(5):1231-9. [PMID:15855600]
Comment: Real-time recognition of both the absolute magnitude of glycemia and trend patterns provides enormous, useful information to patient.
- Tanenberg R, Bode B, Lane W, et al. Use of the Continuous Glucose Monitoring System to guide therapy in patients with insulin-treated diabetes: a randomized controlled trial. Mayo Clin Proc. 2004;79(12):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 MS, Silver DM, Kaplan J, et al. Timing of changes in interstitial and venous blood glucose measured with a continuous subcutaneous glucose sensor. Diabetes. 2003;52(11):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 A, Vidal-Ríos P, Subirà M, 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. 2003;26(4):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.