Continuous Glucose Monitoring System

A Guide for Beginners

Content

• Chapter 1: What is Continuous Glucose Monitoring System (CGMS)?
• Chapter 2: What are the components of CGMS?
• Chapter 3: What are the advantages and disadvantages of using CGMS?
• Chapter 4: Who should use CGMS?
• Chapter 5: Do I still need to do finger-prick tests?
• Chapter 6: What are the different CGMS currently available in Hong Kong?
• Chapter 7: Real-time CGMS information - trend arrows
• Chapter 8: What are alerts?
• Chapter 9: What are our targets and how do we measure them?
• Chapter 10: How do I interpret a CGMS report?
• Chapter 11: Tips for applying CGMS and skin care
• Chapter 12: How does CGMS help in managing glucose levels during exercise?
• Chapter 13: Can I travel with CGMS?
• Chapter 14: What are the precautions for using CGMS?
• Chapter 15: Real-life examples

Chapter 1: What is Continuous Glucose Monitoring System (CGMS)?

• CGMS is a device inserted under the skin to measure interstitial glucose level continuously
• Sensor readings may be different from blood glucose readings because it measures glucose concentration in interstitial fluid
• There is a lag time of 5-10 minutes between CGMS and blood glucose
• Unlike traditional blood glucose meters, CGMS gives additional information on your glucose trends based on your previous glucose level
• Some models also have alarms to alert you to high or low glucose levels

Chapter 2: What are the components of CGMS?

CGMS consists of three components – sensor, transmitter and receiver

Sensor

• Contains a small filament that sits beneath the skin and measures interstitial fluid glucose levels
• After placement, there is a period of warm-up time which varies with each model. During warm-up time, finger-prick tests should be used for treatment decisions
• Depending on the model, the sensor can be worn for 7-14 days

Transmitter

• Either separate or built-in with the sensor
• Directly contacts the sensor and sends glucose information wirelessly to the receiver / smartphone, either automatically or when you scan the sensor

Receiver/smartphone

• Receives glucose information from the transmitter
• Depending on CGMS model, it can either be a small device about the size of a mobile phone or an app on your smartphone

Chapter 3: What are the advantages and disadvantages of using CGMS?

Advantages

• Checking blood glucose levels is a crucial part in diabetes management which requires routine finger-prick. CGMS captures hundreds of sensor glucose readings a day and lessens the number of finger-prick blood glucose monitoring that has to be done
• It provides information on how your glucose levels fluctuate with insulin doses, diet, exercise, sickness, etc.
• Alarm function and trend arrows help with better adjustment of insulin dosages and prevent low blood glucose
• Glucose data can be shared with caretakers/parents and the medical team
• Glucose metrics including time-in-range, glucose variability and trend graphs help medical team to make management decision more accurately
• Research studies have demonstrated that use of CGMS helps to improve glucose control and reduce events of low blood glucose in people with diabetes

Disadvantages

• Alarm fatigue and information overload
• Skin irritation
• Having a device attached to the body
• Cost
• Medical wastes

Chapter 4: Who should use a CGMS?

People with either Type 1 or Type 2 diabetes may benefit from wearing CGMS.

But you should be...

• Willing to wear a sensor most of the time
• Ready to respond to CGMS alarms, do finger-prick tests when needed and replace the sensor regularly

Chapter 5: Do I still need to do finger-prick tests?

Finger-prick tests are still necessary in these occasions

• Some models need to be calibrated by finger-prick regularly to maintain accurate readings
• When your sensor glucose readings are low (< 4 mmol/L) or high (> 15 mmol/L), especially when you need to give a correction dose of insulin
• When you have symptoms of low or high blood glucose
• When your sensor glucose reading appears to be 'unusual' or not available, for example when your sensor is warming up

Chapter 6: What are the different CGMS currently available in Hong Kong?

Abbott FreeStyle Libre

• Intermittently scanned CGMS (available only when sensor is scanned)
• No calibration
• Sensor can be worn for 14 days
• No Transmitter
• 1 hour warm-up period
• Approved age: age 4 or up
• Recommended sites: arm
• Can share data
• Use Reader / Smartphone as receiver
• Frequency of glucose measurement: every minute (a calculated value of previous 15 minutes)
• Data loss if the CGMS has not been scanned for more than 8 hours
• No predictive low glucose alert
• Can use with LibreView
• Data can be interfered by Vitamin C, aspirin

Abbott FreeStyle Libre 3 (Expected to be launched in Hong Kong soon)

• Real-time CGMS
• No calibration
• Sensor can be worn for 14 days
• Disposable Transmitter
• 1 hour warm-up period
• Approved age: age 4 or up
• Recommended sites: arm
• Can share data
• Use Reader / Smartphone as receiver
• Frequency of glucose measurement: every minute
• Data loss when transmitter and receiver / smartphone are > 6 meters apart
• With predictive low glucose alert
• Can use with LibreView
• Data can be interfered by Vitamin C, aspirin

Dexcom G6

• Real-time CGMS
• No calibration
• Sensor can be worn for 10 days
• Transmitter can be used for 3 months
• 2 hour warm-up period
• Approved age: age 2 or up
• Recommended sites: arm / abdomen / buttock (for aged 2-17 only)
• Can share data
• Use Reader / Smartphone as receiver
• Frequency of glucose measurement: every 5 minutes
• Data loss when transmitter and receiver / smartphone are > 6 meters apart
• With predictive low glucose alert
• Can use with Dexcom Clarity
• Data can be interfered by Hydroxyurea, Acetaminophen

Dexcom G7

• Real-time CGMS
• No calibration
• Sensor can be worn for 10 days
• Disposable Transmitter
• 30 minutes warm-up period
• Approved age: age 2 or up
• Recommended sites: arm / abdomen / buttock (for aged 2-6 only)
• Can share data
• Use Reader / Smartphone as receiver
• Frequency of glucose measurement: every 5 minutes
• Data loss when transmitter and receiver / smartphone are > 6 meters apart
• With predictive low glucose alert
• Can use with Dexcom Clarity
• Data can be interfered by Hydroxyurea

Medtronic Guardian Sensor 3

• Real-time CGMS
• Calibration is required twice per day
• Sensor can be worn for 7 days
• Transmitter can be used for 1 year
• 2 hours warm-up period
• Approved age: age 7 or up
• Recommended sites: arm / abdomen
• Can share data
• Use Smartphone / insulin pump as receiver
• Frequency of glucose measurement: every 5 minutes
• Data loss when transmitter and receiver / smartphone are > 6 meters apart
• With predictive low glucose alert
• Can use with carelink
• Data can be interfered by Hydroxyurea

Medtronic Guardian 4 Sensor

• Real-time CGMS
• No calibration
• Sensor can be worn for 7 days
• Transmitter can be used for 1 year
• 2 hours warm-up period
• Approved age: age 7 or up
• Recommended sites: arm / abdomen (for aged 18 or above) / buttock (for aged 7-17 only)
• Can share data
• Use Smartphone / insulin pump as receiver
• Frequency of glucose measurement: every 5 minutes
• Data loss when transmitter and receiver / smartphone are > 6 meters apart
• With predictive low glucose alert
• Can use with carelink
• Data can be interfered by Hydroxyurea

Chapter 7: Real-time CGMS information – trend arrows

• Indicate rate of change of glucose levels
• Tell you how rapid glucose levels are rising or falling
• Allow you to take action before blood glucose level reaches alarming levels

Below are the trend arrows of different CGMS models showing the estimated changes of glucose levels within 30 minutes.

Freestyle Libre

• One up arrow: Increase > 3.00 mmol/L
• One up right arrow: Increase 1.80 to 3.00 mmol/L
• One right arrow: Change less than 1.80 mmol/L
• One down right arrow: Decrease 1.80 to 3.00 mmol/L
• One down arrow: Decrease > 3.00 mmol/L

Dexcom

• Two up arrows: Increase > 5.00 mmol/L
• One up arrow: Increase 3.40 - 5.00 mmol/L
• One up right arrow: Increase 1.60 - 3.40 mmol/L
• One right arrow: Change less than 1.60 mmol/L
• One down right arrow: Decrease 1.60 - 3.40 mmol/L
• One down arrow: Decrease 3.40 - 5.00 mmol/L
• Two down arrows: Decrease > 5.00 mmol/L

Medtronic

• Three up arrows: Increase > 5.00 mmol/L
• Two up arrows: Increase 3.33 - 5.00 mmol/L
• One up arrow: Increase 1.67 - 3.33 mmol/L
• One right arrow: Change less than 1.67 mmol/L
• One down arrow: Decrease 1.67 - 3.33 mmol/L
• Two down arrows: Decrease 3.33 - 5.00 mmol/L
• Three down arrows: Decrease > 5.00 mmol/L

Chapter 8: What are alerts?

Some CGMS models have audible beep and/or vibration for different alerts. It is important to pay attention to these alerts and take action.

There are several types of alerts:

• Threshold alerts:
  • Alarms for high or low glucose when a preset threshold glucose is reached
  • Urgent low value (except Freestyle Libre): device preset value is 3.1 mmol/L

• Predictive alerts:
  • Alarms when glucose level is predicted to go low or high in the near future. CGMS utilizes data from past few glucose levels to predict this rise or fall of glucose level

• Rate-of-change alerts:
  • Alarms for rapid fall or rise in glucose levels

• Signal loss alerts

*** User can preset alerts as required except urgent low alert

Your medical team may discuss with you on what glucose thresholds to use.

Chapter 9: What are our targets and how do we measure them?

People with diabetes routinely have blood test called haemoglobin A1c (HbA1c) every few month. Our target is to optimize blood glucose control with HbA1c less than or equal to 7%

However, there are some limitations of HbA1c

• HbA1c only indicates the average blood glucose levels in the past 3 months. It does not detect daily high or low glucose, or rapid changes in glucose levels
• HbA1c does not provide enough data to adjust insulin dosage
• HbA1c is unreliable in patients with anaemia, haemoglobinopathies, iron deficiency and pregnancy

Chapter 10: How do I interpret CGMS Reports?

All CGMS provide standardized reports called Ambulatory Glucose Profile (AGP)

1. Sensor usage / % of time CGMS is active
   • Ideally at least 70% of the time
2. Glucose Management Indicator (GMI)
   • Estimated HbA1c level from CGMS glucose data in at least past 2 weeks
3. Glucose Variability
   • Measures how far apart glucose values are. Stable glucose levels are defined as glucose variability < 36%
4. Time In Range (TIR)
   • Rather than providing an average value of HbA1c, CGMS offers information on the percentage of time that glucose levels are within the target range
     • TIR is set between 3.9 to 10 mmol/L. The recommended TIR is 70% or higher (i.e. ≥ 16.8 hours a day) as this corresponds to HbA1c of ≤ 7%
     • The target percentage of time for other ranges are as below

       Glucose Range	Targets % of Readings
       3.9 - 10.0 mmol/L	> 70%
       < 3.9 mmol/L	< 4%
       < 3.0 mmol/L	< 1%
       > 10.0 mmol/L	< 25%
       > 13.9 mmol/L	< 5%

     • This can vary in special circumstances e.g. in young toddlers, where medical team may suggest a higher target to avoid low blood glucose
5. Overall Trend
   • Data loss could be due to connectivity issues and lack of regular scanning in some models
6. Glucose Profile
   • Daily graphs are combined to make a picture

Most CGMS apps allow users to enter details, for example, insulin doses, food intake, activities and special notes for interpretation. All these information will be integrated to the CGMS report, allowing the family and medical team to review and make adjustment accordingly.

Chapter 11: Tips for applying CGMS and skin care

When applying CGMS sensor

• CGMS should be inserted to subcutaneous fat beneath the skin, i.e. arm, abdomen or buttock area. For the recommended sites and approved age for different models, please refer to Chapter 6
• Avoid scars, body creases and/or pant lines / belt lines
• Avoid areas that you usually sleep on: compression on the CGMS sensor may result in falsely low readings
• Avoid swollen tissues
• Clean the skin with soap and water to remove lotion and oil
• Make sure the skin is completely dry before inserting the CGMS sensor
• Extra tape or bandage can be applied over the sensor for better security
• Rotate sites when changing to a new sensor

Skin irritation or itchiness is one of the most frequent problems among CGMS users

• Skin prep (or skin barrier) can be applied to the skin prior to insertion of sensor
• During removal, baby oil or similar products can help peel the tape surrounding the sensor
• Inform the medical team if redness or swelling persists in areas of CGMS insertion

Bleeding after insertion

• If the bleeding does not stop, remove the sensor

Chapter 12: How does CGMS help in managing glucose levels during exercise?

Blood glucose levels may fluctuate more during exercise, depending on the intensity and duration of exercise.

Blood glucose levels also fluctuate with the types of exercise, in general:

• Aerobic exercise (e.g. walking, jogging, and cycling): drop in blood glucose
• Anaerobic exercise (e.g. sprinting and power-lifting): rise in blood glucose

Children are also at risk of low blood glucose in the post-exercise period. Close monitoring of blood glucose levels is necessary.

CGMS can help in monitoring the glucose levels during and after exercise. Trend arrows also help to prevent low blood glucose, including night time. (For details, please refer to the table on Chapter 7)

CGMS can be used during water sport as sensors and transmitters are waterproof.

Immersion depths and time vary with different CGMS models:

• Freestyle Libre: Do not immerse deeper than 1 meter for up to 30 minutes
• Dexcom: Do not immerse deeper than 2.4 meters for up to 24 hours
• Medtronic: Do not immerse deeper than 2.4 meters for up to 30 minutes

Waterproof tapes for CGMS can be applied to the sensor for extra protection.

Glucose response varies with individuals during exercise.

In general, varies are based on exercise habits and risk of hypoglycaemia, please contact your medical team for details.

Exercise can lower your blood glucose for up to 24 hours after your workout by making your body more sensitive to insulin. Therefore, it is important to closely monitor your blood glucose after exercise.

If exercise is performed in the late afternoon/evening, consider taking extra carbohydrates before bed to prevent nocturnal hypoglycaemia.

Chapter 13: Can I travel with CGMS?

Always ask for a letter from your medical team stating your condition and the necessary glucose-monitoring equipment you need to bring on board. CGMS can be worn during flights.

However, there are a few extra things to take note of when travelling with CGMS

• Whole body scanner could be a form of X-ray. Remove the sensor and transmitter prior to the scan or request an alternative screening process
• During hand/carry-on luggage examination, request the security personnel to visually inspect the equipment, do not send your devices through the X-ray machine
• Set your smartphone app to airplane mode and keep the Bluetooth or NFC on. If you are using a receiver, leave your receiver turned on as well
• Upon arrival, adjust the time of the receiver or your smartphone to the corresponding time zone
• Do remember to bring enough insulin pens and pen needles, extra sensors with transmitter, glucagon, finger-prick test strips and your blood glucose meter in your hand carry luggage

Chapter 14: What are the precautions for using CGMS?

Sick days

• More frequent monitoring of glucose levels is necessary when a child is sick
• Always do confirmatory finger-prick tests when sensor glucose readings are at extreme ranges
• Certain medication (e.g. paracetamol) may falsely raise your sensor glucose readings

Medical procedure

• Do not wear a CGMS when undergoing X-ray, CT scan, MRI and other imaging examinations
• Always notify your doctor when you have to undergo an operation as CGMS needs to be removed if some haemostatic devices are being used

Accuracy

• Falsely low readings in the first 24 hours
• When there is a rapid change in blood glucose, the difference between blood glucose value and sensor glucose value becomes greater

Chapter 15: Real-life examples

For illustration only, all dosage changes should be advised by medical team

Example 1: Falling nocturnal blood glucose with low blood glucose in the morning

• Ying Ying's blood glucose dropped overnight and resulted in low blood glucose early in the morning
• Solution: Reduce 1 unit of long acting insulin
• Doctors would advise the insulin dosage according to each child's condition.

Example 2: Normal pre-dinner blood glucose but post-lunch blood glucose excursion

• Ceci's pre-lunch finger-prick test was normal but it rose to high level at 16 mmol/L after 2 hours, and dropped before dinner
• Solution: Increase 1 unit pre-meal fast-acting insulin
• Doctors would advise the insulin dosage according to each child's condition.

Example 3: High blood glucose after breakfast, but low blood glucose before lunch

• Long Long's blood glucose was high 1 hour after breakfast, but it dropped to borderline low level before lunch (sometimes even lower than 4 mmol/L)
• Solution
  • Give fast-acting insulin 15 minutes before breakfast
  • Add food with higher protein or fibres content at breakfast
  • Take snacks between breakfast and lunch
• Doctors would advise the medical plan according to each child's condition.

Jointly produced by

Endocrine and Diabetes Team, Hong Kong Children's Hospital
Youth Diabetes Action

Written by

Endocrine and Diabetes Team, Hong Kong Children's Hospital
Dr. Sarah Poon
Dr. Gloria Pang
Dr. Joanna Tung
Ms. Iris Poon
Ms. Candy Chan

Acknowledgment

Patients and families of Endocrine and Diabetes Team, Hong Kong Children's Hospital

Warm reminder: Technology keeps improving with time and there might be further modifications in the future

November 2023

References:

1) Clinical Targets for Continuous Glucose Monitoring Data Interpretation： Recommendations From the International Consensus on Time in Range. Diabetes Care. 2019 Aug;42(8)：1593-1603

2) Glucose management for exercise using continuous glucose monitoring (CGM) and intermittently scanned CGM (isCGM) systems in type 1 diabetes： position statement of the European Association for the Study of Diabetes (EASD) and of the International Society for Pediatric and Adolescent Diabetes (ISPAD) endorsed by JDRF and supported by the American Diabetes Association (ADA). Pediatric Diabetes. 2020 Dec;21(8)：1375-1393