An A1C test measures the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it. It reflects your average blood sugar over the past two to three months, giving a broader picture than a single finger-stick reading ever could. An A1C below 5.7% is normal, 5.7% to 6.4% signals prediabetes, and 6.5% or higher indicates diabetes.
How the A1C Test Works
Glucose in your bloodstream naturally sticks to hemoglobin, the oxygen-carrying protein inside red blood cells. The more sugar circulating in your blood, the more hemoglobin gets coated. Because red blood cells live for roughly three months before your body replaces them, measuring how much glucose is attached to hemoglobin tells you what your blood sugar has been doing over that entire window, not just at the moment the blood was drawn.
This is the key advantage over a standard blood sugar test. A fasting glucose reading is a snapshot of one moment. Your A1C is more like a three-month average, which makes it far more useful for spotting patterns and tracking whether blood sugar management is actually working over time.
What the Numbers Mean
The American Diabetes Association uses these cutoffs for diagnosis:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
Each percentage point translates to a specific estimated average glucose. An A1C of 5% corresponds to an average blood sugar of about 97 mg/dL. At 6%, that average rises to roughly 126 mg/dL. A result of 7% means your blood sugar has been averaging around 154 mg/dL, and at 8% you’re looking at about 183 mg/dL. The relationship is linear: each 1% increase in A1C adds roughly 29 mg/dL to your estimated average.
If your result comes back in the prediabetes range, it means your blood sugar is higher than normal but hasn’t crossed into diabetes territory. This is the window where lifestyle changes, like adjusting your diet and increasing physical activity, tend to have the most impact on preventing progression.
A1C Targets for People With Diabetes
For most non-pregnant adults with diabetes, the recommended target is an A1C below 7%, which translates to an average blood sugar under about 154 mg/dL. That said, this number is not one-size-fits-all. Your target may be tighter or looser depending on several factors: how long you’ve had diabetes, your age, other health conditions, whether you’ve had cardiovascular problems, and how prone you are to episodes of dangerously low blood sugar.
Younger, otherwise healthy people who were recently diagnosed often benefit from aiming lower, sometimes closer to 6.5%. On the other hand, a target of below 8% may be more appropriate for older adults or people with a limited life expectancy, where the risks of aggressive blood sugar lowering (especially frequent drops in blood sugar) outweigh the long-term benefits. If you’re experiencing frequent or severe low blood sugar episodes, that alone is a reason to adjust the goal upward.
How Often You’ll Be Tested
If your blood sugar is stable and you’re meeting your target, testing twice a year is the standard recommendation. If you’ve recently changed medications, started a new treatment plan, or your results are not at goal, your doctor will likely check it every three months. Since the test reflects a rolling three-month average, testing more frequently than that doesn’t add much useful information.
One practical perk: the A1C test does not require fasting. You can eat and drink normally beforehand. However, your doctor may bundle it with other blood work, like a cholesterol panel, that does require fasting, so it’s worth confirming ahead of time.
When A1C Results Can Be Misleading
The test relies on a predictable red blood cell lifespan of about three months. Anything that shortens or extends that lifespan can throw the result off, sometimes significantly.
Conditions that slow red blood cell turnover, like iron deficiency anemia or vitamin B-12 deficiency, allow glucose to accumulate on hemoglobin for longer than usual, pushing the A1C falsely high. Chronic kidney disease, chronic heavy alcohol consumption, and long-term use of certain medications including aspirin and opioids can have the same effect.
Conversely, conditions that speed up red blood cell turnover pull A1C results falsely low. Hemolytic anemia (where red blood cells break down faster than normal), significant blood loss, an enlarged spleen, and pregnancy all fall into this category. During pregnancy, A1C tends to read lower than expected through the second trimester, then may rise in the third.
Blood transfusions are a special case because they can push results in either direction, depending on the sugar content of the stored blood and the dilution effect of receiving someone else’s red blood cells.
People with certain hemoglobin variants, particularly those with sickle cell disease or hemoglobin C disease, face the biggest accuracy challenges. A1C testing is generally unreliable for anyone with a homozygous hemoglobin variant (meaning they carry two copies of the variant gene). It can still be used for those who carry just one copy, as long as the lab uses an appropriate testing method. If you know you carry a hemoglobin trait, mention it so an alternative blood sugar monitoring approach can be considered if needed.
A1C vs. Other Blood Sugar Tests
The A1C test is one of three ways to diagnose diabetes. The other two are fasting plasma glucose (a blood draw after at least eight hours without eating) and an oral glucose tolerance test (where your blood sugar is measured two hours after drinking a sugary solution). Each test captures something different: fasting glucose shows your baseline, the tolerance test shows how your body handles a sugar load, and A1C shows the long-term trend.
No single test is perfect. Sometimes they disagree. You might have a normal fasting glucose but an elevated A1C, or vice versa. When results are borderline or conflicting, your doctor will typically repeat the test or use a second method to confirm. An A1C diagnosis requires the result to be confirmed on a separate day unless you already have obvious symptoms of diabetes along with a blood sugar over 200 mg/dL.
For people already managing diabetes, A1C is the primary tool for evaluating long-term control, but it doesn’t replace day-to-day blood sugar monitoring. A1C can’t tell you about the spikes after meals or the dips overnight. Two people with the same A1C of 7% could have very different daily blood sugar patterns: one might be steady around 154 mg/dL, while the other swings between 80 and 250 mg/dL. Both matter for health outcomes, which is why A1C works best alongside regular glucose checks or continuous glucose monitoring rather than as a standalone measure.