Why CGM?

May 16, 2020
Editor: David L. Joffe, BSPharm, CDE, FACA

Author: George McConnell, PharmD. Candidate, LECOM School of Pharmacy 

Why CGM use is increasing, and blood glucose monitors – a breakthrough that followed urine tests and lasted over 50 years – may soon go into the museums, showing how patients with diabetes used to check their blood sugars.  

There has been significant innovation and advancement in diabetic drug therapies and devices for diabetes management in the past few decades. However, diabetes still places a substantial economic and quality of life burden on patients with diabetes. Diabetes is still the leading cause of kidney failure, lower-limb amputations, myocardial infarctions, and myriad other conditions such as higher rates of depression, anxiety, and stress associated with the diagnosis of diabetes. 


There are numerous advancements and refinements of CGM technology since it first entered the US market in 2000. Monitors now are much smaller, have improved accuracy, and can be worn for more extended periods. Studies have shown that diabetes is managed better in patients who consistently use CGM technology, and coupled with continued improvements in this technology, patient outcomes are very likely to keep improving. 

Hemoglobin A1c (HbA1c) remains the gold standard for the assessment of glycemic management for patients with diabetes. The landmark Diabetes Control and Complications Trial demonstrated a strong association between HbA1c levels and the risk of chronic diabetic microvascular complications. Many organizations currently recommend a target HbA1c of <7.0% for adults, although quite a few organizations will recommend a stricter goal of <6.5%. 

There are, however, numerous limitations of HbA1c, which is used primarily for assessing glycemic control and for predicting the risk of developing long-term complications. First of all, HbA1c is an average value, which means an individual could have wildly fluctuating glycemic values, have a poor quality of life, and yet their average HbA1c will continue to seem at goal. Regrettably, the HbA1c does not provide information about how much time a patient is within the target range, nor the glycemic variability, or the patients daily patterns of blood glucose levels. Another shortcoming of HbA1c is that it cannot reflect the impact of therapy on daily life, the quality of life, stress, and worry. 

Another caveat to depending on HbA1c so firmly is that there can be significant interpatient variability in the relationship between HbA1c values and mean glucose concentration. Recent studies have shown that the mean glucose concentration for a given HbA1c differs by race. For example, black patients with diabetes have a significantly lower mean glucose concentration for any given HbA1c value compared to white patients with diabetes. Another study found that an HbA1c value of 7% can correlate to an average blood glucose concentration range between 123 to 185mg/dL, but for an HbA1c value of 11% the range is even wider at 217 to 314mg/dL. Studies have shown that even patients with identical HbA1c levels and similar mean glucose concentrations can have significant variability in their patterns of glycemic control. 

Other factors that can cause incongruencies between the HbA1c and glycemia include hemoglobinopathies, hemolytic anemia, and chronic renal failure. However, even patients who do not have an established red blood cell disorder can have a broader range of mean glucose concentrations for identical HbA1c values, which is most likely due to variations in the lifespan of red blood cells. 

HbA1c also does not reflect the amount of time a patient spends in hypoglycemia. This is significant because hypoglycemia has been associated with numerous other health complications such as an increased risk of subsequent severe clinical hypoglycemia events, impaired hypoglycemia awareness (which is associated with an increased risk of severe clinical hypoglycemic events), cognitive function impairment, increased cognitive arrhythmias, increased car accidents, adverse effects on quality of life, and reduced work productivity.  

Even after explaining the variety of deficiencies of using HbA1c as a metric, it remains a useful and validated method for assessing the success of therapy. However, the additions of other metrics can help complete the overall picture of a patients diabetes and fill in the gaps that HbA1c does not reflect. We can gauge therapeutic success more comprehensively with the use of CGM, which provides an accurate and reliable assessment of other glycemic controls, which, when used with HbA1c, gives a complete picture of a patients glycemic patterns. 

As mentioned before, HbA1c does not reflect the time a patient spends in a hypoglycemic state, and unfortunately, glucose self-monitoring cannot predict or alert patients of imminent hypoglycemia. These limitations can be addressed through the use of CGM. 

CGM assesses a patients blood glucose every few minutes, with 280 or more blood glucose readings taken automatically daily. This data can be used to show real-time trends in hypoglycemia, hyperglycemia, and glucose variability. Patients and healthcare providers can evaluate the time they spend in clinically significant hypoglycemic or hyperglycemic states, which are significant revelations that can be used to drastically improve a patients glycemic control and quality of life. This data can be used to tailor the therapy individually, and greater use of CGM can aid patients with T1DM, T2DM, and pregnant patients for better and safer healthcare outcomes. 

We are only beginning to understand the implications and benefits that the data and opportunities of CGM can provide to help patients achieve a personalized and more effective glycemic management plan. 

Practice Pearls: 

  • HbA1c remains the gold standard for the assessment of glycemic management in patients with diabetes. 
  • There are numerous limitations of HbA1c since it does not reflect the time a patient spends in clinically significant hypoglycemic or hyperglycemic states. 
  • CGM can fill in the gaps for a better understanding of a patient’s glycemic patterns and allow healthcare providers to tailor effective individual therapies. 


Šoupal, Jan, et al. “Glycemic Outcomes in Adults With T1D Are Impacted More by Continuous Glucose Monitoring Than by Insulin Delivery Method: 3 Years of Follow-Up From the COMISAIR Study.” Diabetes Care, American Diabetes Association, 1 Jan. 2020, care.diabetesjournals.org/content/43/1/37. 


George McConnell, PharmD. Candidate, LECOM School of Pharmacy