Ask Dana: How accurate are continuous glucose monitors?


Dear Dana,

How should I think about “time in range” from my CGM when I’m also considering all of the other diabetes metrics that I keep track of? I always review this with my health care team but what is a healthy expectation and range to strive for in light of all other numbers I have to think about?

– Devon McNamara
Boston, Massachusetts


Dear Devon,

Your question touches on a crucial aspect of diabetes management that is gaining recognition for its importance: the “Time in Range” (TIR) percentage. 

Understanding how to integrate TIR into the fabric of diabetes management can be extremely helpful in gaining optimal glucose control and improving your quality of life.

TIR refers to the percentage of time your blood glucose levels are within a target range, typically this is set at 70-180 mg/dL (3.9-10.0 mmol/L) for most adults with diabetes. This TIR offers a glimpse into your glucose management that traditional measures like HbA1c cannot provide. HbA1c gives a 3-month retrospective measure of your average glucose levels but TIR can highlight patterns and give immediate feedback for lifestyle and treatment adjustments.

While many healthcare providers still use HbA1c as a measure of diabetes management, TIR provides a much more detailed view of daily glucose variation that may impact your well-being and risk for hypoglycemia or hyperglycemia.  TIR is fast becoming a more insightful way to look at diabetes management and a more personalized way to think about blood sugar variation rather than just an average in HbA1c.

As such, the International Consensus on Time in Range recommends a TIR (70-180 mg/dL) of at least 70% for most adults with type 1 diabetes.

The Consensus also recommended a goal of less than 20% time above range (>180 mg/dl) coupled with less than 5% of those pesky severe highs considered to be over 250 mg/dl.  It should be noted in these metrics that hypoglycemia under 70 mg/dl should be as minimal as possible, specifically less than 3-4%.

recommended time in range

Glycemic variability is another metric accessible through CGM.  This variability can be measured with the standard deviation (SD) or Coefficient of Variation (CV) shown in CGM reports. While considering glucose variability, it is important to remember that everyone living with diabetes will have some variability with their blood sugars.  A healthy goal for the CV is less than one-third of your glucose average, meaning a CV less than 33%. A healthy TIR with a lower amount of variability not only improves quality of life but also helps to reduce diabetes burden.

But more important than the standard recommendations, there is not a definitive rule that dictates a ”one size fits all” in terms of TIR.

The type and duration of your diabetes and the presence of any diabetes complications must be considered when discussing your goals with your healthcare team.  Additionally, you should consider your risk of hypoglycemia and the safety of this risk when considering a goal that might be too aggressive for your overall health goals.

Also consider that the complementing alarms that CGM systems offer can help you reach these goals more frequently if set appropriately.  These alarms are meant to help guide people to take steps to manage their diabetes – not to burden them with noise!  Try to set these alarms to reduce the worry about lows and highs but to also decrease the need to constantly look at your CGM screen to stay within your TIR goals.

Remember, the best approach to managing your diabetes management is one that is personalized and matches your unique needs and lifestyle. Have your healthcare team help you customize your goals.

If you need more individualized guidance, connect with our team here at IDS!