Continuous Glucose Monitoring Systems - Frequently Asked Questions

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What is CGMS?
CGMS stands for Continuous Glucose Monitoring System. Essentially, there are two kinds: diagnostic - a) used by professionals to gather continuous results to sort out a problem e.g. frequent hypos and b) patient managed - where the sensor is attached to a device which provides continuous 'live' information about glucose levels.

How does CGMS work?
A variety of approaches have been tried but both systems in common use in the UK involve inserting a tiny sensor under the skin (placed exactly like a giving set for an insulin pump) and this uses a chemical reaction like a test strip to measure the glucose level. It is very important to note that what is being measured is NOT the blood glucose but that of the interstitial fluid (IF, tissue juice) because they are not the same. The IF glucose generally is about 12-15 minutes behind the blood glucose but can be affected by factors such as temperature.

If it's not measuring blood glucose, what is the point of CGMS?
The benefit of CGMS is in detecting trends so a rising or rapidly falling glucose can be acted upon. It can also provide a lot of information between meals, overnight etc. which can be used to adjust insulin therapy.

How is CGMS used by patients?
It is possible to use CGMS stand alone i.e. in conjunction with injection regimens but most use by patients is linked with an insulin pump. There are two systems in use which work with different pumps but more are in development.

The Medtronic Guardian REAL-Time (http://www.medtronic-diabetes.co.uk/product-information/guardian-real-time/index.html) links with the Paradigm Veo or Medtronic 640g pump via the MiniLink REAL-Time transmitter. Glucose levels are visible on the pump screen as numbers and a graph. This information is used directly by the pump to calculate bolus doses for food and corrections. It is also possible to set something on the pump called Low Glucose Suspend which, exactly like it sounds, temporarily suspends the pump when the glucose falls below a preset value. Additionally, on the Medtronic 640g pump, there is a facility called 'Predictive Low Glucose Management' which can automatically turn the pump off when a low glucose is predicted in the next 30mins. This has proved to be especially valuable in preventing night-time hypoglycaemia.

Dexcom produces the G4 system which is integrated with the Animas Vibe pump and the G5 system which is for use by people on Multiple Dose Injection therapy. The latter allows the results to be displayed on a mobile phone or smart-watch and offers the opportunity for trusted others to see results (http://www.dexcom.com/en-gb)

Does CGMS mean no more finger prick testing?
No.  Although use of the CGMS greatly reduces the need to do finger prick tests, the systems still need regular input of blood values for calibration. Blood ketone testing also still needs a finger prick. Failure to do regular calibration checks is dangerous.

CGMS sounds brilliant so why isn't everyone using it?
The theory is that giving patients continuous information about their blood glucose should result in much better control of their diabetes. Of course it's not so simple. There have now been a number of studies of CGMS which have provided evidence that they work (result in small reductions in HbA1c of 0.2-0.7%) but, not surprisingly, they don't work when patients don't insert the sensors and in the adolescent age group this is frequent. Also, the larger falls occurred in people starting an insulin pump at the same time and we know that, on average, a pump alone can reduce HbA1c by about 0.3%. In other words, in HbA1c terms, the difference is not huge. Furthermore, the best improvement occurs in previously poorly controlled patients and this group will generally improve, albeit temporarily, with almost any change that involves more contact with the diabetes team. The evidence on hypoglycaemia is not very strong. (See www.cochrane.org).

How much does it cost?
This is not a straightforward question because it depends upon factors such as device and continuous versus intermittent use. However, a ballpark figure for continuous use for a year is £3000-4000.

Is it available through NHS Scotland?
Yes and no. CGMS for diagnostic use (data collected for later analysis by diabetes team for a few days) is used routinely throughout Scotland. However, patient managed systems are not used widely yet.  In specific circumstances such as diabetic pregnancy or patients especially vulnerable to hypoglycaemia special arrangements are made via Health Boards. In June 2017, the Scottish Government wrote to Health Boards announcing £10m funding over 5 years to increase insulin pump use and encourage the implementation of CGMS. While welcome, this is not a great deal of money spread across the whole country and Boards still have to make decisions about prioritisation. However, there is no doubt that we can expect to see a lot more CGMS use in the next few years.

Is Libre the same as CGMS?
The Freestyle Libre is similar to CGMS in that it captures glucose measurements from a sensor under the skin. However, the readings are not sent to a screen until the user swipes (or 'flashes') their reader over the sensor worn on the upper outer arm. This means that there are no warning alarms. Also the device cannot be linked directly to a pump. So while, Libre is a welcome step forward in providing more information on which to base changes to insulin and food, it is different and not to be confused with CGMS.

Kenneth Robertson
Updated September 2017

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