At a glance
- Accurate blood glucose data is used to schedule insulin administration, dietary interventions, and exercise.
- Continuous glucose monitoring (CGM) devices measure glucose levels in interstitial fluid rather than blood samples.
- Proponents argue that CGM can provide real-time trend data from which out-of-range events can be anticipated.
- CGM technology reduces, rather than eliminates, the need for finger-stick blood glucose measurements.
- Patients with diabetes-related peripheral neuropathy of the hands may have difficulty manipulating CGM equipment.
Repeated blood glucose monitoring is a mainstay of modern diabetes management, with type 1 diabetes patients testing blood glucose concentrations up to 10 times a day and type 2 diabetes patients testing at least once daily, using finger-stick blood samples. Accurate and timely blood glucose data allow clinicians and patients to identify patterns and trends as well as hyperglycemic or hypoglycemic events—information around which the timing of insulin administration, dietary interventions, and exercise can be scheduled. Intensive diabetes management involves rapid insulin intervention using an insulin pump or rapid insulin self-administration to curb hyperglycemic and hypoglycemic events, which in real-world practice requires multiple finger-stick blood glucose determinations.1
An emerging technology
Continuous glucose monitoring (CGM) technology, while very early in its development, has been optimistically extolled in numerous editorial and technical reviews in the medical literature as “the future of diabetes management.”1,2 CGM device designs are based on miniature electronic transducers that convert glucose concentrations into measurable, proportional electrical currents.
CGM provides patients near-real-time glucose data tracking based on automated testing every minute or every five minutes. Rather than using blood samples, CGM devices measure glucose levels in interstitial fluid. CGM systems consist of disposable subcutaneous sensors that are connected to a radio-frequency transmitter. The transmitter communicates with a receiver with an electronic display that reports the most recent blood glucose level measured and may show glucose trends in near-real time. (Data can also be uploaded from CGM devices to computers for longer-term analysis.1) The transmitter and display unit can be worn on a belt or carried in a pocket.
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CGM proponents argue that blood glucose monitoring provides mere snapshots of sugar concentrations, whereas CGM can provide real-time trend data from which out-of-range glucose events can be anticipated.1 CGM devices even come equipped with alarms for out-of-range events. (Alarms can be sound-based or vibrate to draw the patient’s attention to hyperglycemic or hypoglycemic events.)
However, despite optimistic expectations about the future of CGM devices’ role in improving diabetes management, their utility in current clinical practice is very limited. CGM devices may be used to help clinicians develop patient treatment plans. CGM device purchases require a doctor’s prescription, but currently should not be used as the sole, or even primary, patient self-monitoring source of glucose information. The American Diabetes Association’s official position is that CGM’s utility is limited to supplemental self-monitoring for type 1 diabetes patients using traditional blood glucose monitoring.1,3 While advocates contend that type 2 diabetes patients will benefit from CGM as well, empirical evidence of the clinical utility and efficacy of CGM for these patients is sparse, and the ADA does not currently recommend its use for these patients.3,4
Promise vs. practical realities
Calibration with traditional blood-based glucose sampling means that even when CGM is prescribed to a patient, the technology reduces, rather than eliminates, the need for finger sticks. CGM readings must be confirmed at least twice daily using finger-stick blood measurements.1 Changes in glucose levels are evident in blood before they show up in interstitial fluid, creating a lag time in accurate CGM readings of up to five minutes typically (and sometimes longer).5 CGM can therefore yield misleading data during sudden hypoglycemic or hyperglycemic events, and patients must be educated both to monitor the current glucose level and to anticipate out-of-range glucose levels through attention to short-term upward and downward trends. For example, sharp drops in glucose should prompt finger-stick blood sample spot-checking of real-time glucose levels.
CGM devices have not been widely adopted, partly because they represent a young technology to which many clinicians have not yet become accustomed, but also because they are expensive, and costs are not widely reimbursed by insurance companies.1,6 Sensors and receivers have variable but limited useful life spans (Table 1). Sensors in particular must be replaced relatively frequently—in some cases twice per week. Typical costs for CGM equipment range from $5 to $10 per day.1 Medicaid billing schedules exist for the equipment, but clinicians cannot currently be compensated directly for the time they must spend educating patients about the use of CGM devices, creating an economic disincentive for the widespread adoption of these systems in clinical practice.
While proponents believe “any individual who experiences hypoglycemia will benefit” from CGM devices,1 practical limitations exist regarding which patients should be prescribed this type of equipment. For example, patients with diabetes-related peripheral neuropathy of the hands may experience difficulty manipulating CGM equipment and inserting sensors.1 Elderly patients or patients with cognitive impairment may also have difficulty handling the devices.
FDA approval of new devices requires only evidence of safety and efficacy equivalent to existing technologies. Although three FDA-approved CGM devices are available, empirical evidence demonstrating that they live up to the optimistic expectations of proponents is scant.
Several studies have shown limited CGM device sensitivity for the detection of hypoglycemic events.1 Data from five clinical trials of CGM devices for childhood diabetes management in children were reviewed and subjected to meta-analysis in 2008 by Polish researchers, one of whom disclosed having received speaker’s fees from the CGM device manufacturer.7 Even when data were pooled, the sample size was small, potentially weakening the statistical power of the analysis and its ability to discern small benefits: Data were available for only 131 children with type 1 diabetes. Meta-analysis of the pooled data failed to show a significant diabetes management benefit in this patient population, despite a significant increase in the number of altered insulin doses per month among patients using CGM devices.7
