A1c vs Time-in-Range

By Nilou, Member GuideOctober 2, 2019

What is A1c? What is time-in-range? How do they relate? Why is the diabetes community moving more and more towards time-in-range?

What is A1c?

In simple terms, Hemoglobin A1c (HgbA1c, HbA1c or A1c) is the average blood glucose level over the past 2-3 months. An A1c test measures the amount of glucose that attaches to red blood cells. Since these red blood cells survive for 2–3 months, A1c can reflect someone’s average glucose levels over that same time period.

What is time-in-range?

Time-in-range is the percentage of time you spend in a target blood glucose range for a given time period. Time-in-range has only become possible with the magic of a continuous glucose monitor (CGM). CGM allows people with diabetes to measure their blood glucose levels all day, 24 hours a day without pricking their finger every minute. CGM is able to give insight to how much time is spent in the standard range of 70–180 mg/dL, time in high (standard greater than 180 mg/dL) or time on low (standard less than 70 mg/dL).

Improving your time-in-range can improve short- and long-term health and avoid diabetes-related complications. Using our tool, we can also look at time in your personal target range if you have one (some examples: 70–140 mg/dL, 80–160 mg/dL, etc.), in addition to the standard range.

caption="Consensus on Time in Range. Diabetes Care 2019 Jun; dci190028."
Consensus on Time in Range. Diabetes Care 2019 Jun; dci190028.

What are the limitations of A1c? When is A1c unreliable?

One limit of A1c is that it cannot tell you anything about fluctuations in your blood glucose values throughout the day and night. If your A1c is high, all you know is that your average blood glucose for the last 3 months has been on the higher side. But, you still can have serious, unrecognized low blood glucose values. On the other side, if your A1c is low, you may be spending a lot of time in the low range without knowing what time of day and what the possible causes are for them. This can mislead some people into thinking they have great control and are “at goal” with an A1c below 7%.

In fact, two people can have an A1c of 7%, but one actually has very stable BGs, whereas another has a lot of variation, including lows.

caption="Same A1c, but completely different time-in-range. This variation in BG values cannot be captured by an A1c. This is only possible with a CGM."
Same A1c, but completely different time-in-range. This variation in BG values cannot be captured by an A1c. This is only possible with a CGM.

Why is the diabetes community moving more and more towards time in range?

For people with diabetes, it’s become much easier to access time-in-range data with technology like a CGM. Time in range is readily available and we can access it almost instantly. Getting an A1c requires the patient to go to a lab and have their blood drawn.

Another benefit of time-in-range is the ability to select time periods and compare them. Let’s say you make a new change in your diabetes management on June 10th. If you want to see how this new change has affected you, you can just select to see your time in range for before and after June 10th. This is a huge benefit that CGM and time-in-range have provided for people with diabetes. You no longer have to wait 3 months to see the effects of a new change in your diabetes management.

A1c has been so dominant in this sphere because it’s been proven to correlate to macrovascular complications (coronary artery disease, peripheral arterial disease, and stroke) and microvascular complications (diabetic nephropathy, neuropathy, and retinopathy). Time in range could provide the same value as A1c — they have been shown to have a strong relationship (see image below). Therefore, time in range doesn’t replace A1c, but for people using a CGM, it could add these advantages of being more available, accessible, and customizable.

caption="Relationship between Time-in-Range and HgbA1c."
Relationship between Time-in-Range and HgbA1c.

Thanks to Calvin Wu, MD, Henrik Berggren, and Björn Hansell, MD.

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