Today, I review, link to, and excerpt from CoreIM‘s Continuous Glucose Monitor (CGM): 5 Pearls Segment.
All that follows is from the above resource.
Posted: June 2, 2025
By: Dr. Rebecca Easly-Merski, Dr. Thomas Martens, Dr. Kristen Flint, Dr. Zach Taxin and Dr. Shreya P. Trivedi
Graphic: Dr. Jesse Powell
Peer Review: Dr. Jonathan Li, Dr. Michael Weintraub
Time Stamps
- What are clinically relevant limitations of an A1c vs. a CGM
- 10:49 What do the different parts of the ambulatory glucose profile tell you?
- 16:24 What are common pitfalls, medications, lifestyle patterns that are important to ask your patient about when seeing them for a visit to manage blood sugar?
- 21:30 How can you titrate insulin based on when you see the hypoglycemia on the ambulatory glucose profile
- 27:45 When there is too much hyperglycemia, how do you titrate insulin when adding on GLP-1 agonists?
Sponsor: The New England Journal of Medicine Fellowship Program. Learn more about the one-year, full-time paid opportunity here and the application process.
Show Notes
Pearl #1: CGM is a reliable measure of glucose trends that can give an indication of recent glycemic control. Hemoglobin a1c is better used for population measures and serum blood glucose remains the gold standard.
How does CGM compare to other measures of blood glucose control including point of care blood sugar and hemoglobin a1c?
- Hemoglobin A1c: average glycemic control over the past three months
- Diagnostic limitations:
- Misses variability over shorter periods of time
- Does not show hypoglycemia as clearly as CGM can
- Best used as a measure of population measures
- A1c is limited has its limitations in different contexts: “Hemoglobin A1c and Glucose Measurements”
- Increased red blood cell turnover -> Falsely lower the hemoglobin A1c
- hemolysis
- blood loss
- iron and vitamin B12 deficiency
- Decrease red blood cell turnover -> Falsely increase A1c (RBCs hve more time to become glycated)
- nutritional deficiencies
- hematologic malignancies
- CGM: reliable measure of recent glucose trends
- How precise are CGMs?
- Serum blood glucose remains the gold standard to which all measures of blood glucose are compared
-
- Continuous glucose monitors (CGMs) has a MARD (mean absolute relative difference) is around 8-12% compared to serum blood glucose
- Point of care glucometers have a MARD of 5-6%.
- Inaccuracies in CGM matter the most when the CGM is reading low
- CGM measures glucose in the interstitial fluid
- This lags behind blood glucose readings by 5-15 minutes
- Educate patients to check point of care glucose monitors if they are having symptoms of hypoglycemia
- What may cause inaccuracies in the CGM reading?
- Warm up period
- CGMs have a warm up period in which accuracy can be lower in the first 24 hours of sensor placement
- Pressure on the sensor (compression artifact)
- Certain medications
Pearl #2: CGMs can provide real time data as well as ambulatory glucose profiles to help manage patient data in real time.
When you have a patient with a CGM device, how do you access the data? What does each section of the ambulatory glucose profile tell you?
- The Ambulatory Glucose Profile (AGP) is downloadable and accessible through accounts most clinics have.
- Granular day to day information
- Larger trends over the last two weeks
- The AGP has three sections:
- Time in Range
- Ambulatory Glucose Profile
- Daily Glucose Profile
- Time in Range (Top Section): Shows the percentage of time a patient is in:
- range (70-180 mg/dL – green)
- above range (greater than 180- yellow)
- below range (<70 – red).
- Goal: time in range should be >70%, which loosely correlates with an A1c around 7%.
- Goal: time below range (<70 mg/dL) should be <4%
- Minimize the risk of significant iatrogenic hypoglycemia
- Patients on insulin or sulfonylureas patients are at higher risk of hypoglycemic episodes.
- This section also contains the Glucose Management Index (GMI) which estimates the A1c based on the last 14 days, and can be useful in counseling patients on more recent changes.
- Ambulatory Glucose Profile (Middle Section): Contains the modal day view or ambulatory glucose profile
- Glucose trends from the last 14 days onto a single 24-hour timeline
-
- median
- 95th percentile
- 5th percentile
- Helps identify consistent patterns of hypo- or hyperglycemia at specific times of day.
- Daily Glucose Profile (Bottom Section): Displays the daily glucose profiles over the last 14 days, which allows for more granular views of glucose data.
- This helps with identification of patterns related to weekends, sickness, or other life events.
Pearl #3: Before we get to titration, put your internist hat on and ask detective Q’s on why you may see the patterns that you do and what may be adjusted before med changes.
What are common pitfalls, medications, lifestyle patterns that are important to ask your patient about when seeing them for a visit to manage blood sugar?
- Walk with your patient through their insulin administration steps:
- How do you administer your insulin?
- How do you store their insulin and and when is the expiration date?
- What insulin is are you taking and when?
- Are you rotating insulin injection sites?
- Are you injecting your meal time insulin (if applicable) 15 minutes prior to a meal?
- Look for evidence of lipohypertrophy on physical exam
- Diet and physical activity can also have major impacts on glycemic control.
- Is there any acute illness that could be causing a rise in glucose?
- Ask about what is happening during times of hypoglycemia or hyperglycemia.
- Alcohol intake can significantly modify glycemic control
- Increases insulin sensitivity
- Inhibits the liver’s ability to counter-regulate hypoglycemis
- Placing patients at higher risk of hypoglycemic events.
- Pairing CGM data review with counseling helps patients understand how their body responds to different factors, making the CGM a learning tool.
- Real-time feedback from CGM can be a powerful motivator for patients to make positive lifestyle changes!
Pearl #4: Titration of insulin with hypoglycemia
How do you use CGMs to avoid hypoglycemia?
- Hypoglycemia is our main safety parameter and is important to minimize when patients are on insulin or sulfonylureas,
- Goal to keep hypoglycemia less than 3-4% of the time!
- CGMs have safety features with alarms that cannot be turned off to alert patients to hypoglycemia.
- Alarms are set at ~54 mg/dL
- Patients with frequent hypoglycemia are at risk of hypoglycemic unawareness resulting in lack of symptoms when blood glucose is downtrending
- STOP the sulfonylureas if you see hypoglycemia on the patients CGM!
- If a patient is on basal and/or prandial insulin -> look at the ambulatory glucose profile to identify when the hypoglycemia is occurring
- If at night ->consider decreasing the basal insulin as it may be too high
- A common pitfall in diabetes management is treating post-prandial highs by increasing the basal insulin dose
- Patients who are on doses of greater than 0.5 units/kg/day are at the higher risk of over-basalization
- If during a specific time of the day (such as the afternoon or mid-day) -> adjust by reducing prandial insulin by 10%
- If BOTH at night and during the day -> decrease the total daily insulin dose, reducing both basal and mealtime doses
- Don’t forget to ask the patient about what is going on during these hypoglycemic episodes– keep an eye out for any alcohol use, concern for compression artifact while sleeping or other interference.
- There is a lot of value to frequent patient check-ins when titrating insulin and virtual visits/ digital communication can be a big win for making changes.
- Hypoglycemia risk factors include hepatic dysfunction, renal dysfunction, longer duration of diabetes, and older age.
Pearl #5: Titrating insulin using CGMs when starting patients on GLP-1 agonists.
How much should you decrease insulin when starting patients on GLP-1 agonists? What trends can you expect to see when looking at the glucose profile? How do you counsel patients?
- In general, the goal is for patients to have a Time in Range (TIR) of around 70%.
- When patients have a TIR of <70%, which suggests worsened glycemic control further action is indicated
- On the app this means that a patient has more time in the yellow range and less time in the green range
- The first step to treat hyperglycemia is actually to initiate a GLP-1 agonist or dual GIP/ GLP-1 receptor agonist as the first line injectable.
- GLP-1 agonists have other benefits
-
- treat obesity
- treat OSA,
- treat MAFLD
- cardiovascular protection
- Low risk of hypoglycemia with GLP-1 agonist
- Less frequent dose titration with GLP-1 agonist
- Initiating a patient on GLP-1 agonist therapy who is already on insulin therapy also comes with risks!
- Patients become
- How to start a GLP-1 in patients on insulin?
- If A1c of either or less and a time in range of 50% or more (good glycemic control)
- reduce the total insulin dose by 20%
- If A1c over 8%, and the TIR is less than 50% (poor glycemic control)
- no need to adjust the insulin dose
- Just add the GLP-1!
- The GLP-1 agonist dose will be titrated upward over time while insulin requirements decrease over time.
- GLP-1 agonists tend to dampen postprandial glucose spikes, potentially reducing or eliminating the need for mealtime insulin in some patients on basal-bolus regimens. This can simplify the treatment regimen.
- What is the patient cannot start a GLP-1 due to insurance or side effects?
- Consider where they could benefit from additional insulin coverage!
- If they are on basal insulin only:
- consider adding prandial insulin
- this is often the go-to approach when a patient has too much time below range but also not enough time in range.
- If they are on basal–bolus insulin:
- consider increasing the total daily insulin dose by 10% divided between basal and bolus dosing
- If they are not on any injectable therapy:
- consider starting at 0.1 units/kg per day, and increase by 10% every 2 weeks until TIR is >70%
- After any titration of insulin or addition/up-titration of a GLP-1 agonist, it is important to revisit the AGP after about two weeks to assess the impact and make further adjustments as needed.
- Remember that CGM provides actionable trends, even if the absolute precision has some variability and can be important to alerting patients of hypoglycemia!
