Introduction
Continuous glucose monitoring (CGM) is another tool that is used to obtain optimal diabetes control particularly in those with consistent glucose control problems on insulin therapy (1). It is useful to provide greater insight into glucose levels throughout the day, supply trend information, determine benefits (or otherwise) of medication changes and may help identify and prevent unwanted periods of hypo and hyperglycaemia (2,3).
A typical system consists of
- a disposable glucose sensor which is placed under the skin and worn for a few days
- a monitor which records and/or displays the glucose levels
- a wire that connects the sensor and the monitor (however some are wireless)
There are currently three systems available on the market in the United Kingdom
- Guardian REAL-Time
- Minimed Paradigm REAL - Time (insulin pump and CGMS combined)
- CGMS system Gold
All three of these are made by Medtronic however Abbott produce the Freestyle Navigator CBG system and are looking to come to the UK in the future as is Dexcom who make the Dexcom Seven system which is already available in the USA and is likely to come to the UK in conjunction with Animas pumps in the future.
The two real time devices currently available in the UK (Guardian REAL- Time and Minimed Paradigm REAL-Time) measure interstitial fluid every five minutes 24 hours a day and display the result onto the monitor. The results can have up to a 10 minute lag time in which it takes to display the recording during periods of rapidly changing blood glucose. In the two systems the readings are transmitted wirelessly via a transmitter (the MiniLink). The information is then displayed on the monitor in the Guardian REAL- Time and on the insulin pump on the Minimed system.
The CGMS system Gold does all of the above however it is not wireless and instead of displaying the information on the monitor it is downloaded after three days and supplies the person with retrospective results.
All three of the systems have the ability to download the data and supply the person with trend information of their glucose readings.
However, it must be made clear in all of the systems if a person is wearing a separate insulin pump to their monitor or if their monitor and insulin pump are combined (Minimed system) the pump and sensor do not link and communicate to each other and at present the monitor does not replace capillary blood glucose testing and the person being responsible for the administration of insulin via a pump (2) (not a closed loop system).
It is important to note that when comparing CGM and blood glucose results that the measurement from the CGM is measuring interstitial fluid not capillary glucose. Increasing glucose changes are seen first in the capillary glucose and therefore the sensor has a lag time of up to 10 minutes depending on how quickly the glucose level is changing. It is also important when using CGM that the inaccuracies are explained. Studies have shown there are inaccuracies between the CGM and the blood glucose (4, 5) reading this is due to the measurement of interstitial fluid but there can also be calibration inaccuracies. For example if the calibration is performed when blood glucose is rising and due to the lag period there will be a discrepancy between the capillary blood glucose reading and the interstitial reading.
Where is it useful?
CGMS and REAL-Time monitors are often used for different purposes. CGMS is mostly used as a diagnostic tool and used for a short period of time whereas REAL-Time monitors are mostly used in those who are willing to undergo intensive insulin management and/or hypoglycaemia control.
CGMS is useful for intermittent use by clinicians and patients to look at interstitial glucose profiles and identify trends. It is particularly useful in adults and children to identify overnight glucose profiles, postprandial hyperglycaemia and therefore aid in adjustment therapy rather than looking at absolute values (2,6). Published literature would suggest that HbA1c reduction is similar or slightly better than using repeated 7-point profiles, but there is less hypoglycaemia.
REAL – Time glucose monitoring can be used for the above and also for longer periods of time (two to three months) in those that want intensive therapy and education. It has the added value of adding an extra dimension to intensive therapy for diabetes as the interstitial glucose values are immediately available to patients in the absence of the person’s health care professional. It must be explained to the person that there is up to a 10 minute lap period between the reading and when it is displayed and that all readings must be calibrated before taking action due to the percentage of error that the monitors can have against blood glucose readings (4).
People with diabetes and their health care professional can learn how blood glucose levels react to insulin, physical activity, food and different medication types and doses (2) aiding in better self management.
Studies have shown that across all HbA1c levels through the use of REAL – Time monitors there may be improvements in time spent within the blood glucose target range(7). REAL-Time monitors have the ability to reveal hyperglycaemia (8) show reductions in hyperglycaemia without associated increased hypoglycaemia (9, 10) and improvements in HbA1c may also occur (3, 10).
In addition to the above it should be noted that CGM can also be useful for reasons such as the below
- Aid in better glycaemic control in pregnant women and avoid the duration of hypo and hyperglycaemia (11). Good glycaemic control prior to and during pregnancy reduces the risk of adverse outcomes (12).
- May help in athletes to help diagnose nocturnal late onset hypoglycaemia (13).
- Aid in better self management.
- Clinical situations such as surgery, intensive care and acute medical emergencies however these studies are still ongoing and there is no conclusive evidence on this.
How does it work?
Is a tiny glucose sensor that sits under the skin. It measures the amount of glucose in the person’s interstitial fluid not in the blood. This means that this measurement and the measurement of blood glucose can often be different, particularly when there is a rapid change of glucose.
In REAL – Time pumps, the interstitial reading is then displayed on the meter or on the persons pump. In other monitors the reading will be downloaded onto the computer when the person returns to their diabetes team.
In all of the CGM systems the information is downloadable onto a computer and can plot the trends in the person’s blood glucose levels over a 24 hour period. These can then be matched with a persons capillary blood glucose tests and also with their diary. This can then help in providing information on hypo unawareness, night time hypoglycaemia, insulin intake, medications, activity levels, dietary intake, stress levels etc.
The REAL-Time monitor provides an alarm which can be set by the patient and their healthcare professional to alert them if their blood glucose levels go outside of their individually set target range. However the person will still need to calibrate their blood glucose levels with a capillary blood glucose test due to the delay in the monitor displaying the information and the accuracy of the interstitial fluid reading. The alarm systems may reduce the duration of hypo and hyper glycaemic excursions by alarming at set glucose targets.
Current usage and availability in the UK
Diabetes UK advises against purchasing from outside the UK as the person will not receive the education and support they require from their healthcare professional if purchased online or independently of their healthcare team. It is also advisable to have the healthcare professional involved to help with interpretation of data and to use the sensor to its maximum benefit.
Adults and children who have persistent problems with hypoglycaemia unawareness or repeated hypoglycaemia or hyperglycaemia should be offered CGM (1) to help determine where and why this is happening and therefore aid in management and education. In these circumstances these devices should be available through their local diabetes services. It is recommended in a report by the Committee for Evaluation and Diffusion of Innovative Technologies (CEDIT) that an estimated 5-8 monitors would allow follow up of 220 patients in one year (14).
In England, money is available to help the care of people who have exceptional needs, from ‘Specialist treatment advisor committees’. Examples would include those with severe hypoglycaemia unawareness interfering with the persons or their carers quality of life, the sports persons need and pregnancy (with complicating issues such as hypoglycaemia unawareness or previous miscarriage).
Monitors take time to be able to learn to use and a one off three day period may not be enough for the patient to learn how to use the system and to gather enough data and learn from the results (12). People using CGM need to be well motivated and have a good basic understanding of their Diabetes (2). CGM need to be used in conjunction with good education and support. Clinical decisions should be based on HbA1c levels, capillary glucose levels and CGM information not solely on CGM alone.
There are significant potential cost savings to also consider when using CGM in those with impaired or absent hypoglycaemia awareness and/or previous history of admission due to severe hypoglycaemia. The possible side effects need to be outlined to the person and these are bleeding, bruising and skin irritation from the glucose sensor under the skin (12)
Future
In the long term it is hoped that the ultimate diabetes management system will be developed into a closed loop artificial pancreas that will monitor glucose levels and dispense insulin accordingly. However in the mean time CGM provide information to the person with diabetes and their healthcare professional that can aid in improvement of HbA1c levels and diabetes control (3,8,7,9,10).
References:
(1) NICE CG15, 2004
(2) David Kerr, K Fayers Continuous real-time glucose monitoring systems: time for a closer look. Practical Diabetes Int 2008; 25(1)
(3) Deiss, D., Bolinder, J., Riveline, J-P., Battelinom, T., Bose., Tuviana-Rufi, N., Kerr, D and Phillip, M. Improved glycaemic control in poorly controlled patients with type 1 diabetes using real-time continuous glucose monitoring. Diabetes Care, 2006. 29 (12): 2730-2732
(4) The Diabetes Research in children network (DIRECNET) study group. The accuracy of the CGMS in children with type 1 diabetes: results of the diabetes research in children network (DirectNet) Accuracy Study. Diabetes Technology and Therapapeutics. Volume 5, Number 5, 2003
(5)Kovatchev B, Anderson S, Heinemann L, Clarke W. Comparison of the Numerical and Clinical Accuracy of Four continuous Glucose Monitors. Diabetes Care Publish ahead of print, online March 13, 2008
(6) Couper JJ. Prins JB. Recent advances in therapy of diabetes. Medical Journal of Australia 179(8) 441-7, 2003 Oct 20
(7) Garg S, Jovanovic L. Relationship of Fasting and Hourly Blood Glucose Levels to HbA1c Values. Safety, accuracy, and improvements in glucose profiles obtained using a 7-day continuous glucose sensor. Diabetes Care, Volume 29, Number 12, December 2006
(8) Boland E, Monsod T, Delucia M, Brandt C, Fernando S, Tamborlane W. Limitations of conventional Methods of Self-Monitoring of Blood Glucose. Lessons learned from 3 days of continuous glucose sensing in pediatric patients with type 1 diabetes. Diabetes Care 24:1858-1862, 2001
(9) Garg SK, Schwartz S, Edelman SV. Improved glucose excursions using an implantable real-time continuous glucose sensor in adults with type 1 diabetes. Diabetes Care 27(3):734-738. 2004 (##) Diabetes Care. 2004 Mar;27(3):734-8. Garg SK, Schwartz S, Edelman SV.
(10) Bailey, M.D, Howard C, Zisser, M.D, and Satish K. Garg, M.D. Reduction in Hemoglobin A1c with Real-Time Continuous Glucose Monitoring: Results from a 12-Week Observational Study. Diabetes Technology and Therapeutics. Volume 9, Number 3, 2007
(11) NICE CG63, 2008
(12) CEMACH 2003
(13) Iscoe KE, Campbell JE. Jamnik V, Perkins BA. Riddell MC. Efficacy of continuous real time blood glucose monitoring during and after prolonged high-intensity cycling exercise: spinning with a continuous glucose monitoring system. Research Support, Diabetes Technology and Therapeutics. 8(6):627-35, 2006 Dec
(14) The Committee for Evaluation and Diffusion of Innovative Technologies Study.
(15) Focus group held at Bradford Diabetes centre with 6 people with Type 1 Diabetes on insulin pumps. 21st February 2008
September 2008