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.
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.