Fasting Blood Glucose Test or A1C? Which Predicts Risk of T2D Better?

Written by Kathleen Doheny

If you're breathing a sigh of relief because your last fasting blood glucose test came back with normal results, don’t relax too much.

In a study of participants enrolled in workplace wellness programs, researchers found that those with a normal blood glucose result at the start of the study but an elevated result on another test, the hemoglobin A1C, were at increased risk for developing diabetes over the next four years.

"What we show is, it helps to do both [to predict risk]," says Dov Shiffman, PhD, a scientist at Quest Diagnostics, who funded the study along with the National Institutes of Health.

Dr. Shiffman and his team analyzed more than 21,000 men and women, all with normal fasting blood glucose levels at the start, defined as under 100 milligrams/deciliter. But over four years, 513 developed diabetes. And the A1C test results at the start of the study predicted who would be most likely to progress.1 A normal A1C is defined as under 5.7%. It looks at average blood sugars over the previous two or three months.

Healthy Blood Glucose Test Not a Safeguard

The researchers found that many who appeared healthy based on a normal blood glucose test progressed to diabetes if they had an elevated A1C. Based on the study results, "if you do not have diabetes and you have a fasting blood glucose below 100 but an A1C greater than 5.9, your risk of getting diabetes [over the next four years] is 8 times greater than if your A1C is less than 5.7," Dr. Shiffman says.

In those with A1Cs of 5.7 to 5.9 at the study start, the risk of getting diabetes over the next few years was about 2.7 times the risk of those with A1Cs below 5.7. 

The results are not meant to discourage use of the fasting blood glucose test, says James Meigs, MD, MPH, practicing primary care general internist at Massachusetts General Hospital’s Internal Medicine Associates.

He was a coauthor of the study, published in Diabetes Care. Both tests will continue to be used to diagnose and treat diabetes, he says. However, the bottom line from this study is that ''even healthy working individuals with normal fasting glucose may benefit from being screened with A1C to detect increased diabetes risk."

Under the 2018 American Diabetes Association's Standards of Care, a fasting blood glucose test, oral glucose tolerance test or an A1C are all considered appropriate to screen for type 2 diabetes.2

More on the Study

The study was not a direct comparison of the two tests, Dr. Shiffman says. "Every year the participants had an A1C and a fasting blood glucose," he says. "We didn't look at whether the fasting glucose went up," he says. The aim of the study was to find out if those with normal fasting blood glucose at the start would progress to diabetes within four years and if their A1C results at the study start would help identify those people. The A1C at the start of the study was, in fact, a strong predictor, they found.

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The researchers adjusted for other factors that boost diabetes risk, such as obesity, age, and cholesterol levels, and the link held. The study has limitations, as all do, the researchers say. For instance, participants could self-report that their doctor had told them they have diabetes. The people studied were mainly Caucasians, Dr. Meigs says, ''so the value of A1C in [predicting diabetes in] non-Caucasian individuals warrants additional investigation."

Expert Perspective

The study results are not a surprise, says Elena Christofides, MD, FACE, a Columbus, Ohio, endocrinologist who was not involved in the study. "The A1C represents an average blood sugar," she says. "If you are running a higher than average blood sugar at any time of day, then your A1C will likely be abnormal and will predict disease development."

"Patients get hung up on the fact that one number or the other is normal and think that this is magical protection," she says. However, blood glucose is a ''roller coaster curve of numbers throughout the day." Just because it is normal at one point tells you nothing about whether you were on the upward or downward part of the roller coaster, she tells OnTrackDiabetes.

Her advice: "Patients should pay attention to both of these numbers all of the time. They should also stop obsessing about the actual number and reflect more on what it means in their overall health. If their A1C is rising or their fasting blood glucose is rising, they are in trouble," Dr. Chrisofides says. She advises against waiting until an arbitrary threshold is crossed—it may be too late to prevent the diagnosis.

Action Plan: Lower Your Risk

If you want to lower your risk of getting either prediabetes or developing diabetes, the American Diabetes Association suggests you:

Dr. Meigs is an advisor to Quest Diagnostics. Dr. Christofides has no relevant disclosures

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