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Glucose monitoring has come a long way, from dipstick measurements of urine glucose, to portable blood glucose meters and strips, to the present era in monitoring: continuous glucose monitoring or "CGM."
Using the evolving CGM technology, people can now move beyond that "snapshot" analogy outlined earlier in the chapter, into an era of "video" perspectives on glucose patterns -- truly watching the patterns of glucose rise and fall during the day, and seeing the influence of the various factors that can affect glycemic control.
CGM has made its own advancements from retrospective to real-time (RT) CGM. Retrospective, or professional CGM, still has a role in diabetes management, however. Professional CGM is geared towards clinical use. It's meant to be used for brief periods of time to assist the clinician and the patient in fine-tuning the diabetes treatment plan, especially when information from blood glucose meter readings and A1C aren't sufficient in and of themselves to pinpoint problem areas, such as overnight hypoglycemia, for example. With professional CGM the results are blinded to the patient; results are only available after downloading the device. For patients who desire to continuously monitor and refine their diabetes management, RT-CGM (or patient CGM), is now available. It's expected that RT-CGM will revolutionize diabetes management for providers and patients alike. A key feature of RT-CGM is its ability to alert the patient, via trending arrows and alarms, to impending hypo- and hyperglycemia; the patient is then in a better position to take necessary measures to prevent glucose levels from dropping or rising further.
Indications for both professional and patient CGM include frequent hypoglycemia, hypoglycemia unawareness, elevated A1C despite multiple adjustments in the treatment plan, frequent visits to the emergency room or hospital due to issues with glycemic control, pregnancy and presence of diabetes complications that may be impacting glycemic control (e.g., gastroparesis). Insurance coverage for RT-CGM is limited and is mainly approved for those patients with documented glycemic control issues. Also, at this time, RT-CGM is primarily used by patients on physiologic insulin regimens.
RT-CGM uses a disposable sensor that measures interstitial glucose every few minutes continuously, over the course of 24 hours. The patient inserts the sensor just underneath the skin and the sensor must be replaced every three to seven days. Attached to the sensor is the transmitter; this sits on top of the skin and relays glucose readings from the sensor to the receiver. The receiver is a small, pocket-sized device that receives and displays glucose readings from the transmitter. All CGM devices have software that enables both the patient and the healthcare team to download reports. These reports can be invaluable to guide the provider in making treatment plan decisions with the patient.
As more and more patients begin using RT-CGM, it's important for the healthcare provider to understand an important distinction between this technology and SMBG. A blood glucose meter measures capillary glucose, whereas a CGM sensor measures interstitial glucose. Because of this, SMBG and sensor readings may vary. It can take up to 30 minutes for glucose to pass from capillary blood into interstitial fluid. This delay is known as "sensor lag." This does not mean that the sensor is inaccurate, but it does mean that the patient needs to base treatment decisions, such as insulin boluses or hypoglycemia treatment, on fingerstick readings rather than sensor readings.
Provider and patients alike must have realistic expectations of RT-CGM. While RT-CGM provides minute-by minute readings of glucose levels and alerts for high and low glucose, and displays glucose fluctuations and trends that are missed with intermittent SMBG, at this time, it does not eliminate the need for SMBG, nor does the continuous glucose monitor make treatment decisions. In addition, both the provider and the patient must understand the device's technology, along with its subsequent limitations, such as possible issues with sensor adherence, skin irritation and bleeding, and the potential tendency for patients to overreact to high readings seen on their receiver by overbolusing insulin. Finally, patients (and clinicians!) must be prepared for the seemingly overwhelming amount of information that a CGM device provides. Patients now have numerous glucose readings 24 hours a day rather than the usual four fingerstick readings obtained from a blood glucose meter. Initial excitement can quickly turn to frustration and anxiety unless realistic expectations are discussed prior to using RT-CGM. The diabetes care team plays an important role in guiding patients to properly use RT-CGM and interpret results, with the end result being improved diabetes management. This is a process that requires expertise, time and patience for all involved.
Clinicians interested in using either retrospective or RT-CGM with their patients have the option of initiating CGM in their own office, or referring their patients to a diabetes center. Patients must first determine coverage through their health plan. Also, a letter of medical necessity and documentation of clinical need for CGM is required from the patient's physician. CGM requires training and education, and given time and resource constraints, it may be expeditious to refer patients to a local diabetes program or contract with a local diabetes educator who is trained in CGM to come to the office to initiate the training. The CGM device companies can also provide training, albeit more of the "technical" aspects of training, such as how to insert a sensor and how to set alarms. Company representatives generally do not provide diabetes education and treatment plan adjustment around the results; result interpretation is a critical component of CGM use and should be provided by qualified clinicians.
—Interpreting CGM
Interpreting the glucose patterns obtained through CGM takes practice, and the more you do the more adept you become. Generally, an approach would be to view 3-7 days of data at one time. Trying to absorb more than that in one review is not helpful and may be overwhelming.
As you look over the data, think about factors that may lead to high and low glucose. Low glucose may be caused by too much insulin, too little food, unexpected or unplanned physical activity, and alcohol. Factors that may lead to high glucose include too little insulin, too much food, not enough physical activity, illness, and stress. Then, look for lows or highs that occur at the same times on different days and ignore one-time, single excursions that may not reflect patterns. As you would when looking at blood glucose log books, look for patterns (when multiple lines are closest together on the report). Focus on addressing the lows first, starting with overnight and then progressing throughout the day, and try to fix those problems initially.
For example, many of the glucose "excursions" will be food related; this entails possibly adjusting the I:carb ratio, reinforcing more precise carbohydrate counting, choosing lower glycemic index foods and/lower fat foods, use of alcohol, etc. Also, the correction factor may need adjusting, or patients may need to learn how to adjust insulin for exercise, for example, if lows are an issue. Involve the patient and inquire as to why they think they are spiking or going low, and then review possible treatment options, such as choosing a breakfast with a lower glycemic index.
As the use of these devices becomes more widespread, educational programs to further support the development of skills in using and interpreting results from these devices will undoubtedly be developed. Until that time, if you have a patient who would benefit from the use of CGM but you and your practice do not have the time and resources to develop the capabilities to mange it yourself, referral to a diabetes specialty center might be the best option. Through these consultative interactions, you will begin to learn more about the use and interpretation of this new technology which might help with the development of your own capabilities in this area at some point in the future.
Suggested Reading
Standards of Medical Care in Diabetes–2007, American Diabetes Association Diabetes Care 2007 30: S4-41 (Updated annually -- seek newest version)
Diagnosis and Classification of Diabetes Mellitus, American Diabetes Association Diabetes Care 2007 30: S42-47 (Updated annually -- seek newest version)
Diabetes Epidemiology: Collaborative Analysis of Diagnostic Criteria in Europe (DECODE). Lancet 1999; 354:617–621.
Tominaga M, et al. The Funagata Diabetes Study. Diabetes Care 1999; 22:920–924.
The DECODE study group. Glucose tolerance and mortality: comparison of WHO and American Diabetes Association diagnostic criteria. Lancet 1999; 254: 617–621.
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