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Self-Monitoring of Glucose in Diabetes

In conjunction with the American Society of Endocrine PAs, Clinician Reviews will be bringing you practical information about topics in endocrinology that may help you to better understand and manage patients with diabetes, thyroid disorders, and other metabolic conditions. In this month’s column, Joe Largay, PA-C, CDE, Clinical Instructor in the School of Medicine at the University of North Carolina Diabetes Care Center, discusses whether self-glucose monitoring is necessary and how continuous glucose monitoring fits into the clinical picture.

2011;21(2):

Despite therapeutic advances in diabetes management, the majority of patients with diabetes are unable to achieve glycemic targets proven to reduce the burden of the disease. This burden not only involves the quality of life of patients with diabetes who experience the complications of this disease; it also includes the burden to society. One out of every five health care dollars is spent on caring for someone with diabetes—the majority on treating the complications.1

Major barriers to patients’ ability to achieve glycemic goals include the need to make behavioral changes, lack of awareness of glycemic levels, and fear of hypoglycemia.2

Q: Is self-monitoring of blood glucose worthwhile in diabetes?

Studies have shown a benefit from self-monitoring of blood glucose (SMBG) in patients using insulin but not in those taking oral antidiabetic drugs. However, the American Diabetes Association recommends that patients with diabetes monitor their glucose once daily if they are being treated with noninsulin therapy and at least three times daily if they are taking insulin.3

Guidelines from the American Association of Clinical Endocrinologists (AACE) state that patients taking noninsulin or once-daily insulin therapy who have not achieved A1C targets should monitor at least twice daily, while those at target should monitor at least once daily. Those taking multiple daily injections should perform SMBG at least three times per day. If patients experience frequent hypoglycemia, AACE suggests monitoring glucose more often.4

The A1C test provides the “big picture,” the average daily glucose level during the previous 90 to 120 days, and correlates with end-organ impact. It does not identify glycemic variability, hypoglycemia, or hyperglycemia.

By contrast, SMBG patterns provide day-to-day data that can be used to select and manage glucose control programs and ultimately optimize a patient’s A1C. SMBG provides a measure of the specific pharmacologic impact of medications and, through feedback, allows design and implementation of physiologic insulin-replacement programs.

One example of SMBG is to have patients monitor glucose in pairs (ie, pick a meal each day and do a premeal and two-hour postmeal reading) and ask them to keep a log or download the data from their meter in the office. This type of monitoring can be enlightening and self-empowering for the patient in that it can provide valuable information regarding the glycemic response to the particular meal.

Intensive glycemic management has been shown to reduce the incidence and progression of diabetic complications. However, it is associated with an increase in severe hypoglycemia. This is worrisome for both patients and providers, as severe hypoglycemia has been associated with an increase in risk for mortality. SMBG can assist patients in understanding how their lifestyle affects their diabetes, as well as identifying hypoglycemia for those who may have hypoglycemia unawareness (ie, who lack the relevant symptoms).

Q: What is continuous glucose monitoring (CGM)?

CGM devices give real-time readouts of current glucose levels. They utilize a subcutaneous sensor that is inserted in the abdomen and worn for 3 to 7 days (depending on which device is used). The sensor sends an electronic signal to a receiver worn by the patient.

There are three major CGM devices that have been approved by the FDA and are available for both personal and professional use. Health care providers can purchase the units and have patients wear them for retrospective analysis; this is a reimbursable expense. All available CGM devices measure glucose values in the interstitial fluid. The sensor reads electrical current produced by the same glucose-oxidase reaction that is utilized by glucose meters that patients use to perform fingersticks for home monitoring.

Currently available CGM systems need to be calibrated at least twice daily. Sensor calibration entails the pairing of the fingerstick value with the sensor value from the interstitial space. Calibration confirms sensor accuracy during various points by “teaching” the sensor the glucose value that corresponds with the electrical current signal.

There is a known physiologic lag time that occurs between fingerstick and sensor values. This lag time is typically up to 15 minutes but is increased with rapidly changing glucose values.

Q: What are the benefits of CGM?

Recent studies have shown CGM can improve A1C without increasing the incidence of hypoglycemia.5

CGM systems have both low and high glucose threshold alarms that can be set to alert once the threshold is reached. The newest generation devices can also predict hypoglycemia or hyperglycemia by tracking rate of change, and users can be alerted to a potential event. This would then allow them to take appropriate action, such as consuming food or carbohydrates or taking insulin as necessary. (Before taking any action, the glucose should first be confirmed by SMBG.)

Software programs allow for review of glucose data, which can assist in identifying trends not appreciated by typical SMBG testing (such as nocturnal hypoglycemia and meal-time excursions). This allows for adjustment of insulin regimens to reduce the incidence of these events.

Q: Can CGM replace SMBG?

While CGM can provide much more detail regarding glucose trends and patterns, it is not a replacement for SMBG. CGM should not be used as a replacement for SMBG to dose insulin for meal- or activity-related adjustments. All dosing decisions should be based on the SMBG.

Currently, CGM is indicated for patients 18 or older, in conjunction with SMBG for the purpose of improving glycemic control:

• to identify and aid in management of glycemic patterns not recognized with typical SMBG

• to prevent glycemic excursions of hypoglycemia and hyperglycemia.

Its use is supported by ADA and AACE guidelines for glucose monitoring.

Q: Who would benefit from CGM?

Suitable candidates for CGM include those with a high degree of glycemic variability, those with hypoglycemic unawareness, shift workers, patients who use insulin pumps, athletes, and women who are planning to become or are pregnant. Patients should work closely with their health care team and perform regular SMBG.

It has been suggested that patients need comprehensive training and follow-up visits to fully understand the large amount of data that they can be confronted with, in order to fully benefit from these devices.6 While the accuracy is improving, there are a few limitations to this technology, including false alarms. Studies have also shown a positive correlation between sensor wear time (hours per week) and greater reductions in A1C.5

Conclusion
Glucose monitoring is a necessary tool—for patients as well as providers—that assists in identifying how patients’ lifestyles affect their diabetes.

References
1. American Diabetes Association. Economic costs of diabetes in the US in 2007. Diabetes Care. 2008;31(3):596-615.

2. Hirsch IB, Armstrong D, Bergenstal RM, et al. Clinical application of emerging sensor technologies in diabetes management: consensus guidelines for continuous glucose monitoring (CGM). Diabetes Technol Ther. 2008;10(4):232-246.

3. American Diabetes Association. Standards of medical care in diabetes—2011. Diabetes Care. 2010;34(suppl 1):S11-S61.

4. American Association of Clinical Endocrinologists. Medical guidelines for clinical practice for the management of diabetes mellitus. Endocrin Prac. 2007;13(suppl 1):1-68.

5. Bergenstal RM, Tamberlane WV, Ahmann A, et al; STAR 3 Study Group. Effectiveness of sensor-augmented insulin-pump therapy in type 1 diabetes. N Engl J Med. 2010;363(4):311-320.

6. Fabiato K, Buse J, Duclos M, et al. Clinical experience with continuous glucose monitoring in adults. Diabetes Technol Ther. 2009;11(suppl 1):S93-S103.