Johns Hopkins Diabetes GuideClinical TestsGlucose Monitoring

Continuous Glucose Monitoring Systems

Thomas Donner, M.D., Ari S. Eckman, M.D., Christopher Saudek, M.D

DESCRIPTION

  • 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.

ASSAYS

  • 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

INDICATIONS

  • 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.

INTERPRETATION

[General]

  • JDRF study suggested more frequent CGM use associated with greater reduction in HbA1c after 6 months[4].
  • 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[5].
  • 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[8].
  • 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.

EXPERT COMMENTS

  • 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.

References

  1. 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 32:1947, 2009  [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.

  2. 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 32:2047, 2009  [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.

  3. Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group: The effect of continuous glucose monitoring in well-controlled type 1 diabetes. Diabetes Care 32:1378, 2009  [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.

  4. Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group et al: Continuous glucose monitoring and intensive treatment of type 1 diabetes. N Engl J Med 359:1464, 2008  [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.

  5. Garg S et al: Improvement in glycemic excursions with a transcutaneous, real-time continuous glucose sensor: a randomized controlled trial. Diabetes Care 29:44, 2006  [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.

  6. Klonoff DC: Continuous glucose monitoring: roadmap for 21st century diabetes therapy. Diabetes Care 28:1231, 2005  [PMID:15855600]

    Comment: Real-time recognition of both the absolute magnitude of glycemia and trend patterns provides enormous, useful information to patient.

  7. Tanenberg R 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 79:1521, 2004  [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.

  8. Boyne MS et al: Timing of changes in interstitial and venous blood glucose measured with a continuous subcutaneous glucose sensor. Diabetes 52:2790, 2003  [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.

  9. Chico A 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 26:1153, 2003  [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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Last updated: August 9, 2013