Q&A 015 · A1C vs Fasting Glucose

⏱ 9 min read

Dr. Hayes Q&A · No. 015

By Eleanor Hayes, M.D. · Hospitalist

The Question

“My fasting glucose came back at 104, which I was told is basically fine. The A1C from the same blood draw came back at 6.1, and that one gets called prediabetes. Aren’t they both blood sugar tests? How can one say I’m okay and the other say I’m not? Which one am I supposed to believe?”

— Readers keep asking

The Short Answer

Start with what the two tests are, because they are not the same test in two formats. A1C vs fasting glucose is a comparison between two time scales. Fasting glucose is a photograph of one moment, the morning you gave blood. A1C is a film of the previous two to three months. They are measuring the same thing at different shutter speeds, so when they disagree it does not mean one of them is broken. It means they were looking at different stretches of time. And there is a second point that matters more than the first. Nobody told you the whole truth about “basically fine.” A fasting glucose of 104 is not normal. It is inside the prediabetes range, just at the low end, and the 6.1 is inside the same range, at the high end. The two tests agree on the category and disagree only on where in it you sit. As for why the A1C sits higher, the table you sent holds a likely reason, and it is the iron tablets. I will explain.

Why A1C vs Fasting Glucose Don’t Tell the Same Story

The two rulers. The American Diabetes Association sets the lines. Fasting glucose from 100 to 125 mg/dL is prediabetes, and 126 or above is diabetes. An A1C of 5.7 to 6.4 percent is prediabetes, and 6.5 or above is diabetes. Those are the current diagnostic criteria, and either test is considered appropriate for screening. So your 104 and your 6.1 are both prediabetes. One is three points past the lower line. The other is three tenths short of the upper line. Same room, different corners.

What A1C actually counts. A1C is the percentage of hemoglobin in your red blood cells that has sugar stuck to it. The longer a red cell circulates in sugary blood, the more of it gets coated. Red cells live roughly 120 days, which is where the “three-month average” comes from. But it is not an even average. The most recent month accounts for about half of the result, and the two months before it split the rest. Look at the last line of your table. Two weeks of travel and irregular meals, right before the draw. Those two weeks land on the A1C with the heaviest weight of any in the window.

What fasting glucose actually counts. Fasting glucose is the result of the eight to twelve hours before the needle. Eat light the night before, sleep well, and the photograph comes out flattering. The same person on a different morning gets a different number, which is why a single fasting value is never used alone for diagnosis. It is repeated on another day, or checked against a second test like A1C.

The detail that changes the reading. Now the iron tablets. When the body is short on iron, red blood cells tend to live longer than usual. A red cell that lives longer spends more days exposed to glucose and collects more of it, so A1C drifts upward without any change in blood sugar. A systematic review in Diabetologia found that iron deficiency, with or without anemia, spuriously raises A1C with no matching rise in glucose. And it works in reverse. When iron is replaced and fresh red cells come online, A1C falls. A meta-analysis of ten studies found that iron replacement lowered A1C in nine of them, again independent of blood sugar. So your 6.1 may be two numbers blended into one: a real rise in glucose, plus a residue left by the iron deficiency that followed the bleeding. How much of each is not readable from this one frame. An A1C drawn three months after your hemoglobin has fully recovered is the number closer to the truth.

Other things that bend A1C. Iron is the most common distortion but not the only one. Chronic kidney disease, a recent transfusion, hemoglobin variants such as sickle cell trait, and heavy drinking that changes the red cells all push A1C away from what the glucose is doing. A1C is a convenient test, but it is only honest when the red blood cells are normal. The kidney side of that is in Q&A 013, and the hemoglobin side comes in the next article, Q&A 016.

Which one to trust. The answer is both, and then one more. The ADA does not diagnose from a single test. It requires either the same test repeated on a different day, or two different tests pointing the same direction. When two tests disagree and one of them is above the diabetes line, that one gets repeated. In your case, neither test crossed the diabetes line, and both are in the prediabetes range. So the current verdict is: not diabetes, and the direction is upward. Direction matters more than position. Your father had type 2 diabetes, you weigh 91 kg, and your fasting glucose is above 100. Those three facts are risk factors on their own, regardless of whether the iron inflated the 6.1. Even if the true number is 5.8, that is not a safer number than 5.7.

The line that still holds. There is one more test worth knowing about, and the reason is unusual. The ADA notes that the evidence for preventing diabetes, the trials that show lifestyle change works, comes mostly from people whose prediabetes was defined by a two-hour oral glucose tolerance test, not from people flagged by fasting glucose or A1C alone. That test watches how your body handles a measured dose of sugar over two hours, which is the part of the day that fasting glucose never sees. When two tests disagree, it is the tiebreaker, and it is the one that best predicts who benefits from acting now.

What I See in the Hospital

Two kinds of A1C show up on my admission labs. The first is the 11 or 12 that arrives with a patient who says he had no idea he was diabetic. Five years earlier, somewhere in his chart, there is a 6.0 with a note to recheck, and then no checks.

The second kind belongs to the man who trusted his fasting glucose. Fasting normal, year after year, while his after-meal sugars ran past 200 for hours every day. Fasting glucose is what insulin managed to hold overnight. After-meal glucose is what it could not hold. A normal morning does not mean a normal day, and A1C is the test that adds up the afternoons. That is why the two tests are partners, not rivals.

And there is a third use, the one I reach for most. When someone arrives sick, their glucose jumps, 170, 180, sometimes higher, from stress hormones alone. That is not diabetes. It is the body under fire. On that day the fasting glucose is useless as a photograph because the subject is shaking. So I order an A1C, because the film from the months before the admission tells me what this person’s blood sugar was like on an ordinary day. In the first article of this category I wrote that a single value is a photograph and a trend is the film. When the photograph is blurred, you go to the film. (Stress glucose gets its own article in Q&A 040.)

Plain-Language Reference

What to Actually Do

· Finish the iron course, confirm your hemoglobin has recovered, and then wait three months before repeating the A1C. That number is your real baseline. An A1C drawn while the red cells are still rebuilding is a photograph of a moving subject.
· If the two tests keep disagreeing, ask for an oral glucose tolerance test. It looks directly at the after-meal hours, and it is the test behind most of the evidence on preventing diabetes.
· Act on the risk factors now, without waiting for the tiebreaker. Family history, 91 kg, and a fasting glucose over 100 are enough. In the Diabetes Prevention Program, people who lost 5 to 7 percent of their body weight cut their progression to diabetes by 58 percent. At 91 kg, that is 4.5 to 6.5 kg. Q&A 031 will cover what reversing prediabetes actually involves.
· Read the A1C next to the hemoglobin and the creatinine on the same report. If either of those is off, the A1C is not telling the whole truth, and the answer is usually on the same page.

This article is for general information only and does not constitute medical advice, and reading it creates no provider-patient relationship. How an A1C of 6.1 or a fasting glucose of 104 should be interpreted depends on individual factors this article cannot assess, including red blood cell conditions and the specific assay used, and nothing here replaces the judgment of your physician. If high blood sugar comes with intense thirst, frequent urination, unexplained weight loss, or blurred vision, seek medical evaluation promptly rather than waiting for a scheduled recheck.

Take care of yourself. Nobody else will do it for you.
— Eleanor Hayes, M.D.