An HbA1c (often written as A1C) is a blood test that measures your average blood sugar over the past two to three months. It works by checking how much glucose has attached to hemoglobin, the protein in red blood cells that carries oxygen. The result comes as a percentage: below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or above means diabetes.
How the Test Works
When sugar circulates in your bloodstream, some of it naturally sticks to hemoglobin. The higher your blood sugar has been, the more sugar-coated hemoglobin you’ll have. Since red blood cells live about 100 days on average, the test captures a rolling snapshot of your blood sugar levels over roughly that period. The hemoglobin needs about 70 days at a given blood sugar level to reach a stable degree of sugar coating, which is why the test reflects a two-to-three-month window rather than what happened yesterday or last week.
This makes the HbA1c fundamentally different from a finger-prick glucose reading or a fasting blood sugar test, both of which only tell you what your blood sugar is doing at a single moment. A person could have a normal fasting glucose in the morning but spend hours after meals with dangerously high levels. The A1c catches that pattern.
What the Numbers Mean
The CDC uses these thresholds:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
For people already diagnosed with type 2 diabetes, the American Diabetes Association recommends keeping A1c below 7%. You can translate these percentages into daily blood sugar numbers using a simple formula: multiply the A1c by 28.7, then subtract 46.7. An A1c of 7%, for example, works out to an estimated average blood sugar of about 154 mg/dL. An A1c of 6% translates to roughly 126 mg/dL.
What to Expect During the Test
One of the biggest practical advantages of the A1c test is that you don’t need to fast. Blood can be drawn at any time of day, regardless of when you last ate. This is different from a fasting plasma glucose test, which requires at least eight hours without food. Your doctor may order both tests, but if you’re only getting an A1c, there’s no special preparation needed.
The test itself is a standard blood draw. Results typically come back within a day or two. If your blood sugar is well controlled, you’ll likely be tested twice a year. If your levels aren’t at target or your treatment plan has recently changed, expect testing every three months.
When the Results Can Be Misleading
The A1c test relies on red blood cells living their normal lifespan. Anything that shortens that lifespan can push results artificially low, because younger red blood cells haven’t had as much time to accumulate sugar on their hemoglobin. This happens with conditions like hemolytic anemia (where red blood cells break down faster than normal), significant blood loss, and pregnancy. People with type 2 diabetes also tend to have slightly shorter red blood cell lifespans, averaging about 95 days compared to 100 in healthy adults, which can make their A1c read a bit lower than their actual blood sugar control would suggest.
The opposite problem, falsely high readings, occurs when red blood cells stick around longer than usual. Iron deficiency anemia is a common culprit: older red blood cells accumulate more sugar, inflating the A1c. Vitamin B12 and folate deficiencies cause the same effect. Late pregnancy in non-diabetic women can also push A1c higher due to iron deficiency.
Certain genetic hemoglobin variants, such as sickle cell trait, can interfere with the test’s accuracy in either direction depending on the lab method used. Kidney failure creates chemically altered hemoglobin that some testing methods misread. In people who develop type 1 diabetes very rapidly, A1c may appear normal or only slightly elevated even though blood sugar has spiked dramatically, simply because there hasn’t been enough time for the sugar to accumulate on hemoglobin.
Alternatives When A1c Isn’t Reliable
When any of these interfering conditions are present, doctors can turn to other tests that measure sugar attachment to different proteins. Fructosamine and glycated albumin both reflect blood sugar control over a shorter window, roughly two to three weeks, because the proteins they measure turn over faster than red blood cells. These shorter-term markers are also better at capturing blood sugar spikes after meals, which the A1c tends to smooth over.
Another option, called 1,5-anhydroglucitol, works differently. It’s a naturally occurring sugar-like compound in blood that drops when glucose levels spike. It’s particularly useful for detecting day-to-day blood sugar swings that the A1c misses entirely. None of these tests have replaced A1c as the standard, but they fill in the gaps when red blood cell conditions make the A1c unreliable, or when a doctor needs a more granular picture of glucose patterns throughout the day.