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Time in range, a new metric for people with diabetes, would have made no sense to those living with this disease 75 years ago because they rarely, if ever, knew what their blood sugar level was at a particular moment.

In the 1940s, measuring the amount of glucose in the bloodstream — essential knowledge for people with diabetes — was a laborious and indirect process. People had to add eight drops of urine to a teaspoon of Benedict’s solution, boil the mixture for at least five minutes, and then compare the color to a chart. That’s not the kind of thing that can be easily done, and it wasn’t very accurate.

By the 1960s, it was possible to urinate on a test strip to get an indirect measure of blood sugar. Then came test strips that measured glucose in a drop of blood. Such finger-stick testing doesn’t take much time, but it isn’t conducive to driving, business meetings, eating a meal, exercising, or sleeping. And since these tests show the glucose level only at the moment of the finger stick, they offer only a few snapshots of an individual’s blood sugar over the course of a day.


A different blood test introduced in the 1980s, called hemoglobin A1c (HbA1c), estimates an individual’s average blood sugar level over a three-month period. In people without diabetes, an HbA1c reading is under 5.7%. This test can help tell people living with diabetes and their doctors whether a treatment plan is working, but it isn’t perfect.

Two of my patients show one of the limitations of finger sticks and HbA1c. Each had an HbA1c of 6.3%, equivalent to an average glucose of 129 milligrams per deciliter (mg/dL), which is pretty good. During that three-month period, patient A had finger sticks that were generally in a healthy range close to 150 mg/dL, while patient B’s finger sticks ranged from 218 mg/dL (very high) to 40 mg/dL, which is low enough to cause seizures and necessitated several visits to the emergency department. Although the HbA1c reading indicated that both patients were optimally controlling their diabetes, in reality patient B wasn’t.


The big breakthrough in blood sugar testing came with the development of continuous glucose monitors (often just called CGMs), which were commercially introduced in 1999. I think of them as movies that provide much more information than the snapshots of finger sticks and HbA1c averages.

A CGM consists of a tiny sensor inserted just below the skin. It painlessly monitors glucose levels in the interstitial spaces between cells. The sensor wirelessly sends blood sugar readings every few minutes to a small, portable receiver or a compatible smartphone or tablet. Alarms and alerts indicate glucose levels above or below user-set thresholds, and can also be set for rapid rises or falls in blood sugar.

CGMs offer a nuanced look at blood sugar levels around the clock. Users can see how various foods and exercise affect their blood sugar. They can correct a high reading with a calibrated dose of insulin or a low one with a handful of raisins or drink of juice.

With nearly 300 blood sugar measurements a day, CGMs offer a new way to evaluate how well an individual is controlling his or her diabetes: time in range. This is expressed as a percentage of the time an individual’s blood sugar is within the target values. This metric, recently endorsed by the American Diabetes Association and by an international consensus committee, correlates nicely with control of diabetes and the implied development of complications such as vision loss, kidney problems, and low blood sugar excursions. Greater time in range has been linked to more stable glucose control, which should lead to fewer complications.

Time in range
Continuous glucose meters provide helpful feedback for people with diabetes. This analysis of a month of blood sugar measurements shows average readings at different times of the day along with the average blood sugar reading during this period, its corresponding average HbA1c, and the time in range.

As more of my patients use continuous glucose monitors, I find myself relying less on HbA1c and explaining more about time in range. CGMs make it possible to see how much time a patient spends with low blood sugar, which can go unnoticed, especially while sleeping, as well as the amount of time lived with high blood sugar. Looking at conglomerate data makes it easier to identify patterns, such as increased activity on weekend days, premenstrual patterns in women, changes in shifts at work, sick days, and the like.

As with all new metrics, it will take patients and physicians time to use it fully. I trust it will soon be standard of care for patients on intensive insulin regimens and a game changer with the advent of the artificial pancreas — a closed-loop system that links a CGM to an insulin pump.

Lorena Alarcon-Casas Wright, M.D., is a physician at the University of Washington Diabetes Institute in Seattle and the endocrine clinics at UW Roosevelt and Harborview Medical Center; an assistant professor of medicine and metabolism, endocrinology and nutrition at the University of Washington School of Medicine; and conducts clinical research in diabetes with the UW diabetes Research Group at the Department of Veterans Affairs, Puget Sound Healthcare System, and the UW Latino Center for Health.

  • I have type 2, diagnosed just a few years ago. My A1c averages between 5.9 and 6.2. I do random finger sticks which run about 110 to 115. A complication is that I have only 1 kidney having given one away. I am in very good health at 77.
    I would very much like to monitor my BG using the BGM. What are my options?

  • I have had type 1 since 1990, and I am now 72 years old. I so much want a CGM for better control, especially because I have severe low blood sugars too often. My A1C is in the 5.2 range, but of course I know that does nothing to show time in range. The problem is insurance does not cover CGM, so it is not available for me, even though it could save my life when I crash during sleep.

    • It’s very short-sighted for insurance companies not to pay for the CGMs. It would save them megabucks as many people would learn how to control their blood sugar and prevent the horrendous side effects of their disease. BTW, congratulations on keeping your A1C so low!

  • This machine change my life. i have had it for 1 years in norway and it has helped me to keep bloodsuger level when i drive my truck.

  • My CGM was a lifesaver. I was driving and the alarm went off. I pulled the receiver out of my pocket and my brother told me I was dropping rapidly. I didn’t feel it (yet), but I was already somewhat low. He pulled a juice box out of my purse, which I drank, and held onto the liquid glucose while watching the receiver. It dropped a bit more, leveled off, and began to climb slowly. By the time we got home it was normal. If not for this device, I would have continued to drop until I was symptomatic—shaky, sweaty, nasty. I would have had to pull over, test, and treat. With this, I treated BEFORE I was dangerously low.

    I have not had my Dexcom for two years because I retired and Medicare would not cover. Now it is covering the G6, and I am awaiting delivery (I got the word today). A CGM is an essential tool.

  • My sister has diabetes & when her caregiver leaves I am here to monster her blood sugar which constantly fluctuates.

  • So many problem in life first u n o not hold the power ful country and not gives right people and nation is tension it cause diabetes and most of country medicine business we should do so

  • @ John Cody, I want to wish you good health, good luck, & urge you to hang in there. I have a very distinct pic in my head from the 70s & 80s how it was for my grandmother to be type 1. She had no way at all to check her blood sugar, she was over 25 miles from her doctor out in the country; if she wanted any low sugar, or low salt options she had to cook them herself. The same Walmart that carries entire lines of sugar free foods (ask for a list at customer service) did not exist like we know it today. It was over 25 miles in the other direction. This is why I learned to cook. We now have so much more that both I & another family member are grateful for in dealing with diabetes today. Hang in there & find a pro to help you. Good luck!

    • Thank you! I know exactly how that is living in a rural Colorado mountain town, 29.9miles from Walmart. It’s also the closest pharmacy to me. Although i’m sure the struggle for your grandmother was much more than I can imagine. I don’t ind cooking for myself, problem is wing what to cook. My diet was for so long unrestricted and now… Seems I can’t have anything, sodium, potassium, protein, from what I understand my liquid intake will be limited as well. Besides all of that I have no teeth to chew with at all w/o dentures either. The cherry on top is the gastroparesis. I know we all have our problems, wish I could find some “pro” help, I feel so lost. My quality of life is a thing of the past it seems it does not exist at all anymore 🙁

  • Very helpful article as Im a diabetic 2 . when do yu think…drs will do this ?? Tk yu again for yur article and time..which we seniors dnt get much time of!! 😁

    • Durable Medical Equipment- To be covered by Medicaid requires a patients HbA1c to be >7.0% and a log of a minimum of 6 finger sticks manual tests per day for at least 3 months documented. It is only available to patients that have glucose controlled. I’m Type 1 was diagnosed when I was 17yrs old in 2004. To make a very long story short, naive+neglect+time=complications. I never developed a habit to test my glucose like I should have. I think a CGM will literally save my life at this point.

  • I have had type 1 for 56 years. I would be interested hearing more about this Time in range. I have a Dexcom.

    • Cheryl,
      Download the Dexcom Clarity app, which produces reports on the data uploaded from your CGM. As part of the “summary” it shows the time in range.
      T1D for 34 yrs; CGM is the best thing that’s happened!

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