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Continuous glucose monitoring: the facts

Melanie Weiss
RGN
Diabetes Research Nurse
The Bournemouth Diabetes
and Endocrine Centre (BDEC)
Bournemouth Hospital

The Diabetes Control and Complications Trial (DCCT) in 1993 clearly demonstrated that improved glycaemic control through intensive therapy and self-monitoring of blood glucose significantly reduced microvascular complications caused by diabetes.(1) Up to now patients have had to rely on blood glucose meters to monitor their diabetes intermittently. Current recommendations from the American Diabetes Association suggest that patients with type 1 diabetes test their blood glucose at least 3-4 times daily.(2) However, capillary blood glucose monitoring is a crude method that gives only a snapshot picture. Many patients are reluctant to monitor due to inconvenience and pain, leading to occasional inappropriate treatment adjustments.(3)
Although there have been advances in glucose monitoring technology over the last 20 years it remains a problem for many patients, especially as their frequency of monitoring may depend on their current control. Obtaining tight glycaemic control often requires repeated glucose measurements.
Problems associated with finger-stick tests can include:

  • Difficulty in making appropriate adjustments to certain treatment regimens on the basis of only 3-4 blood glucose results a day.
  • Undetected postprandial hyperglycaemia.
  • Undetected glucose excursions between testing.
  • Pain.
  • Time and effort required from patient.
  • Hidden nocturnal hypoglycaemia.

There are many reasons for fluctuations in blood glucose levels, such as poorly timed insulin administration, food consumption and type, alcohol, insulin absorption and exercise.(4)There is also an increased risk of hypoglycaemia when trying to achieve tight blood glucose control.1 Despite the availability of new therapies, glycaemic control can still remain a problem. Improving patients' knowledge of diabetes and having more information available may ultimately improve glucose levels.(5)
Over the past few years, advances in technology have brought about the introduction of continuous glucose monitoring, providing insight into glycaemic patterns in a way that was previously not possible. The Continuous Glucose Monitoring System (see Figure 1) continuously measures subcutaneous glucose levels in interstitial fluid.
The CGMS consists of four components:

  • A monitor the size of a pager.
  • A small glucose sensor.
  • A cable - connects the sensor to the monitor.
  • A com-station - allows the data to be downloaded to a computer using specific software.

[[NIP25_fig1_36]]

The sensor is inserted into the subcutaneous tissue, usually around the abdomen, using a special insertion device called a Sen-serter. This is a minimally invasive technique that measures the changes in glucose in interstitial fluid. This information is then sent to the monitor, which is connected by the cable. Interstitial glucose levels are recorded every 10 seconds and averaged over five-minute intervals, providing up to 288 readings per day. The CGMS is designed to provide continuous glucose measurements for up to 72 hours at a time. A minimum of three finger-stick measurements a day are required to calibrate the system. These measurements are required as without calibration continuous readings may be inaccurate.(6)
It is possible to enter information into the monitor such as insulin administration, exercise and meal times. This information is then stored and can be downloaded later for analysis by specialist software. The CGMS does not display real-time values, but a retrospective approach of the data can then be viewed in several formats:

  • The software is able to calculate the number of readings, averages, standard deviations and range of glucose measurements.
  • It is possible to view daily graphs (see Figure 2), which range from midnight to midnight so a whole 24-hour period can be viewed at a time. CGMS and finger-stick levels are plotted in the profile, as well as events such as insulin administration and meal times, showing exactly what effect these events had on glucose levels.
  • A modal day graph shows three days of measurements superimposed onto one graph in different colours. This is helpful in identifying any patterns or trends in glucose during any one day.
  • A sensor summary report (see Figure 3) allows you to see the number of times and duration that glucose levels were in or out of target range.

[[NIP25_fig2_38]]

[[NIP25_fig3_38]]

These graphs are very easy to understand, and charts can be printed out or displayed. Having these 72-hour glucose profiles makes it easier to detect postmeal excursions and unrecognised hypoglycaemia.
Continuous glucose monitoring has potentially many implications for use (see Box 1) and may be a useful tool for detecting low glucose levels, especially during the night when periods of hypoglycaemia are often prolonged and go unnoticed. Most children and teenagers only measure premeal blood glucose levels during the day and rarely measure their levels during the night.(7) This is the time of greatest vulnerability to hypoglycaemia. Finger-stick testing during this time is often difficult or inappropriate.

[[NIP25_box1_38]]

The DCCT study revealed that teenagers had a higher HbA(1c) and a greater risk of hypoglycaemia than adults, and very young children with type 1 diabetes have been shown to be at greater risk from symptomatic nocturnal hypoglycaemia.(1) There is also the neurological risk of hypoglycaemia, especially in children.(8) Therefore, by applying continuous glucose monitoring to this group, nocturnal and postmeal profiles can be analysed.
Continuous glucose monitoring can assist in designing a more holistic individual diabetes programme based on detailed glycaemic profiles. This is a method that can enhance collaboration with patients and more importantly can be a useful educational tool that may improve motivation and patient empowerment.
Therefore continuous glucose monitoring can be seen as a diagnostic tool for healthcare professionals, allowing insight into glycaemic patterns of hidden hyper- and hypoglycaemia.
Patients can be educated and are more willing and motivated in trying to achieve better diabetes control when they have seen the actual effects that their actions have on their glucose control, and they will have a better understanding of the link between daily activities and blood glucose levels. The most important use of continuous blood glucose monitoring is to facilitate adjustments to therapy in order to improve control.
Continuous glucose monitoring provides a more thorough and robust picture of glucose control by identifying trends rather than isolated glucose values.
Real-time continuous glucose monitoring technology, Guardian RT, has recently been developed for patient use with real-time glucose readings and alarms around the clock. Guardian RT is a wireless system with glucose measurements relayed every five minutes from a transmitter to the Guardian monitor. This device has an adjustable alarm system that alerts patients when their glucose levels become too high or too low. Patients are then able to understand how fast, and in what direction, their glucose levels are heading, and immediate intervention can then be taken to reduce the severity and duration of glucose excursions.
By recognising how diet, exercise, medication and lifestyle affect their glucose levels, patients can make more informed decisions and achieve more confidence in managing their diabetes.
Studies have shown that having this information can facilitate and empower patients to take action before hyper/hypoglycaemia becomes serious, therefore giving them more confidence in managing their diabetes and peace of mind.(9)

Conclusion
Continuous glucose sensing has been a major advantage in the management of diabetes. The advantages of having real-time readings and an alarm system that can offer protection in preventing both hyper- and hypoglycaemia are extremely promising for anyone with diabetes.
Continuous glucose monitoring certainly can play a role in research, especially when analysing the effectiveness of new insulins and treatments and for use with adolescents or in other circumstances where it would be preferable to have blinded or in-depth data. Real-time glucose monitoring can be used by patients or healthcare professionals who wish to assess and improve glucose control.
Although at present these devices are used on selected patients due to NHS budget constraints, hopefully because of the wealth of data attainable and with the advent of the Guardian RT, we will see use of continuous glucose monitoring becoming more widespread.

References

  1. The Diabetes Control and Complications Trial Research Group. N Engl J Med 1993;329:997-1003.
  2. American Diabetes Association. Diabetes Care 1996;19(S1):S1-S118.
  3. Bode BW, Sabbah H, Davidson PC. Postgrad Med 2001;109(4):41-4, 47-9.
  4. Richardson T, Weiss M, Thomas P, Kerr D. Diabetes Care 2005;28(7):1801-02.
  5. Panja S, Starr B, Colleran KM. J Investig Med 2005;53(5):264-6.
  6. Klonoff DC. Diabetes Care 2005;28:1231-9.
  7. Boland E, Monsod, et al. Diabetes Care 2001;24:1858-62.
  8. Jeha G, Karaviti L, Anderson B, et al. Diabetes Care 2004;27:2881-6.
  9. Ludvigsson J. Prac Diab Int 2003;20(1):7-12.