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 reflects one’s average glucose levels over that same time period.
What is Time-in-Range (TiR)?
TiR is the percentage of time you spend in a target blood glucose range for a given time period.
TiR has only become possible with the magic of a continuous glucose monitor (CGM).
CGM allows people to measure their blood glucose levels all day, 24 hours a day without pricking their finger regularly.
CGM is able to give insight into how much time is spent in the standard range of 70–180mg/dL, time in high (greater than 180mg/dL) or time in low (less than 70mg/dL).
Improving your TiR can improve short and long-term health while avoiding diabetes-related complications.
Limitations of A1c & When is it 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.
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 TiR?
For people with any form of 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, while getting an A1c requires one 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 glucose 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. You no longer have to wait three months to see the effects of a new change.
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 is a better indicator of actual glucose patterns/control. It doesn’t replace A1c, but for people using a CGM, it adds the advantages of being more available, accessible, and customizable.