How Does a Continuous Glucose Monitor Work?

Medically Reviewed by Michael Dansinger, MD on February 13, 2024
3 min read

Glucose meters are a great tool, but sometimes you need to keep a closer eye on your blood sugar levels. That's where a device called a continuous glucose monitor (CGM) can help. This FDA-approved system tracks your blood sugar levels day and night. It collects readings automatically.

It can help detect trends and patterns that give you and your doctor a more complete picture of your diabetes. The data can help you find ways to better manage your condition.

Several devices are available for adults and children. You need a prescription from your doctor to get one.

CGM measures the amount of glucose in the fluid inside your body. Different devices collect the information in different manners using tiny sensors. In some cases, the sensor is placed under the skin of your belly in a quick and painless fashion or, it can be adhered to the back of your arm. A transmitter on the sensor then sends the information to a wireless-pager-like monitor that you can clip on your belt.

The monitor displays your sugar levels at 1-, 5-, 10-, or 15-minute intervals. If your sugar drops to a dangerously low level or a high preset level, the monitor will sound an alarm.

In the past, only doctors could see the readings CGM systems collected. Now anyone can use the devices as part of at-home diabetes care. You can download data on your computer, tablet, or smartphone to see patterns and trends in your sugar levels. The information can help you and your doctor make the best plan for managing your diabetes, including:

  • How much insulin you should take
  • An exercise plan that’s right for you
  • The number of meals and snacks you need each day
  • The correct types and doses of medications

CGM doesn’t replace traditional home monitors. You’ll still need to measure your blood sugar with a regular glucose meter a few times a day to help the monitor stay accurate. Most monitors still require a finger stick and you should also replace the sensor under your skin every 3 to 7 days.

If you use an insulin pump, you can also link it to your CGM system for continuous care. You won’t need to manually program the pump as with the other finger-prick methods. This is called a “sensor-augmented pump.”

Unlike traditional glucose meters, CGM records your blood sugar levels throughout the entire day and night, showing your highs and lows throughout the whole week. The systems can help:

  • Record dangerously low overnight blood sugar levels, which often go undetected
  • Track high levels between meals
  • Show early morning spikes in blood sugar
  • Evaluate how diet and exercise affect you
  • Determine if your treatment plan works on a day-to-day basis

CGM isn't right for every person with diabetes, though. They’re more expensive than glucose meters and your insurance or Medicaid might not cover one unless you take insulin medication. You may also need some extra training and practice to use the technology correctly. Talk to your doctor about whether CGM is a good fit for you.

Your doctor may recommend CGM if you have:

  • Major highs and lows in your blood sugar levels for no clear reason
  • Gestational diabetes, which happens during pregnancy
  • An insulin pump
  • Blood sugar levels that are very low, called hypoglycemia, or very high, called hyperglycemia

The device can be used by adults and children ages 2 and older. The FDA recently approved smartphone apps to pair with the CGM. Information on blood glucose is shared immediately. It’s expected to be a great help to parents and caregivers who can’t always be in the same place with the person who has diabetes.

Scientists are testing new and better kinds of CGM systems in clinical trials. The technology is also a key part of researchers’ efforts to build an artificial pancreas, which could mimic the body’s natural process of controlling insulin.