Can continuous glucose monitoring improve insulin therapy?

Photo of a man resting his chin on his hand (PantherMedia / Phovoi R.)

Monitoring the levels of sugar in body tissue (continuous glucose monitoring, CGM) in addition to measuring blood sugar levels can improve therapy outcomes in people with insulin-dependent diabetes: CGM helps to reliably lower blood sugar without leading to more episodes of severe hypoglycemia (low blood sugar).

In people who have insulin-dependent diabetes, the pancreas produces either too little or no insulin. Injecting this missing hormone into the body is the only way to lower their blood sugar levels enough. People with type 1 diabetes are insulin-dependent. Some people with type 2 diabetes are dependent on insulin as well.

Insulin can either be injected several times a day or it can be supplied continuously from an insulin pump. It is essential to avoid getting too much or too little insulin: When too much insulin is supplied there is a risk of hypoglycemia (very low blood sugar levels). When too little is given, blood sugar levels rise quickly, resulting in hyperglycemia (very high blood sugar levels). Keeping blood sugar at an optimal level helps to prevent damage to the small blood vessels in your eyes, kidneys and nerves and to avoid long-term complications.

In intensive insulin therapy the amount of insulin used is adjusted based on the body’s needs: A fixed amount of insulin is injected to provide the body with a basic supply. Additional injections are given based on your blood sugar levels, the amount of food you eat and your physical activity. This means that blood sugar levels need to be measured several times a day.

Continuous glucose monitoring in body tissue

Another way to monitor glucose levels is to use devices that can continuously monitor the amount of glucose in your body – but in tissue below your skin known as subcutaneous fat tissue, not in your blood. This is called continuous glucose monitoring (CGM) or continuous interstitial glucose monitoring. “Interstitial” refers to the spaces between cells or different layers of tissue.

A CGM system is made up of the following:

  • A sensor with a small transmitter that is placed on your skin. The thread-like sensor is inserted into subcutaneous fat tissue, for instance in your belly, using a special insertion device. The sensor needs to be replaced regularly.
  • A small device that you might wear on your belt which receives and stores the data sent by the transmitter. The information is displayed on its screen. 

Illustration: CGM system: Continuous glucose monitoring in transcutaneous tissue – as described in the articleCGM system: Continuous glucose monitoring in transcutaneous tissue

The system measures the sugar levels in the body’s tissue every few minutes and can alert you if the readings get too high or too low.

Unlike conventional blood sugar monitoring, CGM devices keep tabs on your sugar levels 24 hours a day. They let you know how your blood sugar levels are developing over time, which can help you make adjustments to prevent hypoglycemia or hyperglycemia. The data the device saves can be sent to a computer and printed out. This allows doctors to check how well your blood sugar is regulated and, if necessary, make changes to improve it.

But people who use a CGM system still need to measure their blood sugar. Because CGM measurements do not correspond directly to your blood sugar levels, CGM systems are calibrated on a regular basis – every twelve hours, for example – using the information from the blood sugar readings.

Also, there is a delay in how quickly the CGM device can display blood sugar fluctuations, such as those that may be caused by doing sports or eating a meal: It takes about 5 to 20 minutes for the changes in blood sugar levels to be detected in subcutaneous fat tissue. So you will have to measure your blood sugar levels if you want to know immediately how much insulin you need at a certain time or whether you are at risk of hypoglycemia.

Many CGM systems can be used together with an insulin pump. Sometimes both of the devices can be controlled using the same display. One of these combination devices can stop the insulin pump whenever sugar levels fall below a predetermined threshold, in order to prevent hypoglycemia. This is known as the low glucose suspend function, or LGS function for short.

 Illustration: CGM system with an insulin pump – as described in the articleCGM system with an insulin pump

Research on continuous glucose monitoring

What advantages and disadvantages does additional continuous glucose monitoring have in comparison with the conventional ways of measuring blood sugar levels? Can it actually help to optimize insulin therapy better than blood sugar monitoring on its own? Researchers from the Institute for Quality and Efficiency in Health Care (IQWiG, Germany) took a closer look at these questions.

They searched for studies comparing both ways of monitoring sugar levels:

  • Continuous tissue glucose monitoring (in combination with blood sugar monitoring) using devices that display the measurements in real time (real-time CGM)
  • Conventional blood sugar monitoring

The researchers found 13 randomized controlled trials testing whether additional continuous monitoring of glucose levels in tissue led to better results than blood sugar monitoring on its own. These studies covered a period of at least 24 weeks. Nearly all of the participants had type 1 diabetes. Most of them were middle-aged or younger adults, and some school-aged children also took part. All of the participants were using intensive insulin therapy. Each was randomly assigned to one of the two treatment groups.

Advantages of monitoring sugar levels in the body’s tissue

These studies confirm: The additional monitoring of sugar levels in tissue improved the regulation of blood sugar levels. Compared with conventional blood sugar monitoring, continuous glucose monitoring made it possible to lower HbA1c without more episodes of severe hypoglycemia occurring. Serious cases of hypoglycemia can be dangerous because you cannot act on your own to correct the situation and are dependent on help from others. HbA1c levels show how high your blood sugar levels have been over the last two to three months.

This benefit mostly applies to adults with type 1 diabetes and their risk of developing severe hypoglycemia. But the results of the studies suggest that additional glucose monitoring could help children with type 1 diabetes better regulate their blood sugar levels too. There was not enough research to determine whether patients with type 2 diabetes also benefit from additional continuous monitoring of tissue glucose.

Long-term effects unknown

These studies did not last long enough to be able to compare the effects of the two approaches on cardiovascular disease, number of deaths, or complications of diabetes caused by damage to small blood vessels. It is also not clear whether quality of life can be improved by continuous glucose monitoring. The different studies came to different conclusions here.

As far as other possible complications are concerned, the studies either didn’t look into them properly or didn’t find that either of the two approaches had any advantages or disadvantages over the other. Such complications include things like problems caused by longer-lasting hyperglycemia or the effects of serious metabolic imbalances caused by hyperglycemia, including coma.

The researchers did find one possible drawback:  The studies suggest that people who use continuous glucose monitoring are more likely to have minor skin irritation where the sensor is placed on the skin compared with people who monitor their blood sugar normally.

Different ways of monitoring glucose in tissue

The researchers also looked into what happens when the levels of glucose in the body’s tissue are monitored only some of the time, for example every other week instead of constantly. And they wanted to find out whether combining a CGM system and an insulin pump with an LGS function has any advantages over conventional blood sugar monitoring.

There was not enough good-quality research to be able to find reliable answers. It is therefore still not clear what advantages and disadvantages these different approaches may have.