Ask the Pharmacist

Q. I am happy to hear that there is a glucose monitor on the market that does not require me to prick my finger anymore. Can you tell me more about it?

A. You are referring to a new method of measuring your blood glucose called continuous glucose monitoring (CGM). There are a few CGMs on the market and they are collectively broken down into Professional CGMs (usually used short term by a prescriber) and Personal CGMs (for long-term use at home).

For this article, we will obviously focus on the personal CGMs of which there are two main types. One type is integrated with insulin pumps and are therefore only used by people who receive their insulin via these devices. The other type are stand-alone products that both insulin pump users and non-insulin pump users can use.

CGM is achieved by inserting a sensor into the skin in a non scarred, non tattooed area of body while avoiding the waistline due to restrictiveness. The ideal spot depends on the brand of CGM chosen; FreeStyle Libre sensors are applied to the back of the upper arm whereas others might be applied on the abdomen such as the Dexcom G5. This sensor reads the glucose level in the interstitial fluid which is found just below the skin’s surface.

This is different from traditional finger prick type glucose monitors measurements since they are reading the glucose levels in your blood. As such, the reading from a CGM may be as much as a few minutes (5-10) delayed as compared to a blood reading. Blood readings are more indicative of what’s actually occurring within the rest of your body so these will still need to be performed, probably irregularly, as I will explain further below.

Now to continue with in(s) and out(s) of CGMs, depending on the system used, the sensor needs to be changed every 6 to 14 days or 90 days for Senseonics Eversense (one of the Professional CGMs implanted by a healthcare provider). The system also requires the use of a transmitter that needs to be attached to the sensor. The transmitter is responsible for sending the glucose value to a receiver or a reader which will display and store your readings.

It is suggested for FreeStyle Libre sensors to scan for a reading at least 3 times a day (or every 8 hours). For the techy people, you can download an app on your smartphone to scan for your glucose results instead of using the reader. As I noted earlier, a CGM does NOT replace the need for using the more traditional glucose monitors that you likely are using right now and as such, occasionally you still do need to prick those fingers.

Most CGM’s need to be calibrated using the more traditional devices at least once every 12 hours although this is not required with the FreeStyle Libre and Dexcom G6. It is suggested to prick your finger more frequently when you are;

  • experiencing rapid changes of glucose (recall from above that interstitial readings do not give a true reflection of blood glucose)
  • sensing that your CGM reading is off
  • wanting to confirm low blood sugar
  • feeling unwell
  • wanting to make a treatment decision based on CGM (need to confirm blood values before changes are made to therapy)

This new type of monitoring would be a great choice for people that may be more prone to low blood sugar such as the elderly, people that are very active, people who are not in-tune with their hypoglycemic symptoms and people with an impaired kidney system. On the flip side, they are not a great choice for pregnant women, critically ill patients or those requiring dialysis due to the lack of known safety.

There are a few medications that can interfere with the reading acquired by CGM’s.

  • acetaminophen may give a false increased reading with Dexcom G5 and Medtonic Guardian Connect
  • vitamin C (ascorbic acid) may give a false increased reading with the FreeStyle Libre sensors
  • salicylic acid (ASA, Aspirin) may give a false decreased reading with FreeStyle Libre sensors
  • albuterol, atenolol, acetaminophen and lisinopril are others that may interfere with readings from the Dextrose 4G Platinum sensors

Once a CGM sensor is inserted, depending on the brand of sensor, it takes anywhere from 2 to 24 hours before glucose values can be read. When it is time to change the sensor, it is removed as you would remove a bandage and then disposed of in a sharps and biohazard container. The sensors are water resistant so once the sensor is in place they can be exposed to water. The FreeStyle Libre are water resistant up to 3 feet (1 metre) for up to 30 minutes. The Dexcom G5 Mobile sensors are water resistant for to 8 feet (2.4 metres) for up to 24 hours.

The CGM sensors can give someone an average glucose level which can then be used to calculate a Glucose Management indicator (GMI). This GMI gives information about how well (or not well) the glucose is controlled in the person. It is similar to the A1C your healthcare practitioner looks at every few months except that it is for a much shorter duration (14 days) as opposed to the 2 to 3 month average an A1C provides. For more information on this or any other topic, contact your pharmacist.