Continuous Glucose Monitors (CGMs): 5 Pearls Segment

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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 is average glycemic control over the past three months. It has diagnostic limitations as it misses variability over shorter periods of time and does not show hypoglycemia as clearly as CGM can. 
  • Hemoglobin A1c is best used as a measure of population measures. 
• A1c is limited has its limitations in different contexts
• Spaced repetition of the Mind the Gap episode “Hemoglobin A1c and Glucose Measurements” 
  • A1c is limited by patient states that increase blood turnover like hemolysis, blood loss, iron and vitamin B12 deficiency all of which can falsely lower the hemoglobin a1c
  • A1c is also limited by patient states that decrease red blood cell turnover such as nutritional deficiencies and other hematologic malignancies can increase hemoglobin A1c as red blood cells have more time to become glycated. 
• CGM is a reliable measure of glucose trends and can indicate recent glycemic control including the impact of recent changes in lifestyle or medication changes as well as in patients with severe renal dysfunction 
  • In severe renal dysfunction hemoglobin a1c is unreliable due to varying degrees of cell turnover and glucose shifts after dialysis. CGM has proven to be useful in managing these patients. 

• CGM can be more useful in identifying hypoglycemia and titrating therapies compared to A1c, even when A1c is at goal.
  • Caution must be used in patients with excellent CGM data and a hemoglobin a1c that is greater than goal, as overtreatment to goal a1c may result in hypoglycemia. 
  • A systematic review attempting to answer this question suggests that CGM could be more helpful with identifying hypoglycemia and achieving a lower hemoglobin a1c. 
• The precision of continuous glucose monitors 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%. 
  • Serum blood glucose remains the gold standard to which all measures of blood glucose are compared. 
  • Patients should be counseled that inaccuracies in CGM matter the most when the CGM is reading low. 
• CGM measures glucose in the interstitial fluid, which lags behind blood glucose readings by 5-15 minutes and is important to educate patients to check point of care glucose monitors if they are having symptoms of hypoglycemia. 
  • 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) can cause inaccurate readings.
  • Certain medications like high doses of vitamin C, hydroxyurea, and acetaminophen can affect CGM accuracy.

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. It is useful in looking at granular day to day information and larger trends over the last two weeks. 
• The AGP has three sections, the Time in Range, the Ambulatory Glucose Profile, and the Daily Glucose Profile. 
• Top Section: Shows the percentage of time a patient is in range (70-180 mg/dL - green), above range (greater than 180-  yellow), and below range (<70 - red). The Targets determined for time in range are based on a 2019 expert consensus opinion. 
  • The goal is to have time in range over 70%, which loosely correlates with an A1c around 7%.
    • The time in range goal for older adults is even more relaxed to avoid hypoglycemia with a goal of >50%
  • The goal for time below range (<70 mg/dL) should be less than 4% to minimize the risk of significant iatrogenic hypoglycemia. Patients on insulin or sulfonylureas patients are at higher risk of hypoglycemic episodes.
  • The top 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. 
• Middle Section: Contains the modal day view or ambulatory glucose profile which consolidates glucose trends from the last 14 days onto a single 24-hour timeline, showing the median, 95th percentile, and 5th percentile lines to help identify consistent patterns of hypo- or hyperglycemia at specific times of day.
• 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. 
  • Ask your patient to demonstrate to you how they administer their insulin. 
  • Ask your patient how they store their insulin and when the expiration date is. 
  • What insulin is your patient taking and when? 
  • Examine your patient for evidence of lipohypertrophy and ask if they are rotating insulin injection sites. 
  • Ask patients who are taking mealtime insulin if they are injecting 15 minutes prior to a meal. 
• 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 as alcohol increases insulin sensitivity and inhibits the liver’s ability to counter-regulate hypoglycemia, thus 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, with a 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. These alarms are set at around 54 mg/dL. 
• Patients with frequent hypoglycemia are at risk of hypoglycemic unawareness resulting in lack of symptoms when blood glucose is downtrending
• If you see hypoglycemia on your CGM the first step is to discontinue sulfonylureas if a patient is on one. 
• If a patient is on basal and/or prandial insulin it can be helpful to look at the ambulatory glucose profile to identify where the hypoglycemia is occurring. 
  • If hypoglycemia is occurring 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 hypoglycemia is occurring during a specific time of the day (such as the afternoon or mid-day) consider starting to adjust by reducing prandial insulin by 10%. 
  • Lows occurring throughout the day and night indicate a need to 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 AGP 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 including treating obesity, OSA, MAFLD, and cardiovascular protection. 
  • There low risk of hypoglycemia in patients on GLP-1 agonist therapy and less frequent dose titration. 
• Initiating a patient on GLP-1 agonist therapy who is already on insulin therapy also comes with risks as patients become more insulin sensitive, reduced glucagon production and have significant changes in appetite and intake. 
  • In general if a patient has an A1c of either or less and a time in range of 50% or more, indicating good glycemic control, reduce the total insulin dose by 20% when starting a GLP-1 agonist to avoid precipitating hypoglycemia. 
  • If a patient has an A1c over 8%, and the TIR is less than 50%, indicating poor glycemic control there is no need to adjust the insulin dose and a GLP-1 agonist should be added. 
    • 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.
• If a patient is unable to be started on GLP-1 agonist therapy for reasons such as insurance coverage or intolerance to side effects, it will be important to look at a patient’s TIR and 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. 

Contributors

Shreya Trivedi, MD, ACP Member – Host, Editor, Producer
Rebecca Easly-Merski, MD – Host, Editor, MOC Questions
Thomas Martens, MD, FACP* – Guest
Kristen Flint, MD - Guest
Zach Taxin, MD - Guest

Reviewers

Michael Weintraub, MD
Jonathan Li, MD

Thomas Martens, MD, FACP* -  Abbott Diabetes Care, Dexcom, Insulet Corp., Medtronic, Novo Nordisk, Senseonics, Tandem Diabetes Care, Inc (Grant) Dexcom, Eli Lilly, Sanofi, (Consultant)

Those named above, unless otherwise indicated, have no relevant financial relationships to disclose with ineligible companies whose primary business is producing, marketing, selling, re-selling, or distributing healthcare products used by or on patients.  All relevant relationships have been mitigated.

Release Date:  June 2, 2025

Expiration Date: June 1, 2028

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