A1C is a blood test that measures your average blood sugar over the past two to three months. Instead of capturing a single moment like a finger-stick glucose reading, it reflects the bigger picture of how your body has been handling sugar over time. Doctors use it both to diagnose diabetes and prediabetes and to monitor how well treatment is working.
How A1C Works
Sugar in your bloodstream naturally attaches to hemoglobin, the protein inside red blood cells that carries oxygen. Once sugar binds to hemoglobin, it stays there for the life of that cell. The A1C test measures what percentage of your hemoglobin has sugar attached to it. A higher percentage means more sugar has been circulating in your blood.
Red blood cells live about 115 days on average, though the range spans roughly 70 to 140 days. Because older cells have had more time to accumulate sugar, your A1C result reflects a weighted average that leans toward the most recent four to six weeks while still capturing the full two- to three-month window. This is why your doctor may recheck it every few months if your treatment plan changes.
What the Numbers Mean
The American Diabetes Association defines three ranges based on your A1C percentage:
- Below 5.7%: Normal blood sugar regulation
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is elevated but not yet in the diabetes range
- 6.5% or higher: Diabetes
A result in the prediabetes range is a warning, not a diagnosis of inevitability. It signals that your body is starting to struggle with blood sugar control, and lifestyle changes at this stage can meaningfully reduce the risk of progressing to type 2 diabetes.
Converting A1C to Everyday Blood Sugar
Percentages can feel abstract, so there’s a formula that translates A1C into an estimated average glucose (eAG) in the same units you’d see on a home glucose meter. The conversion is: eAG in mg/dL equals 28.7 times your A1C minus 46.7. In practical terms:
- An A1C of 5.7% corresponds to an average blood sugar around 117 mg/dL
- An A1C of 6.5% corresponds to roughly 140 mg/dL
- An A1C of 7.0% corresponds to about 154 mg/dL
- An A1C of 8.0% corresponds to about 183 mg/dL
- An A1C of 9.0% corresponds to about 212 mg/dL
These are averages, so they don’t reveal how much your blood sugar swings throughout the day. Two people with the same A1C could have very different daily patterns, one staying relatively steady and another bouncing between highs and lows.
How A1C Differs From a Glucose Test
A fasting glucose test captures your blood sugar at a single point in time. It requires you to skip food for at least eight hours beforehand, and the result can shift based on stress, illness, recent exercise, or even how long the blood sample sits before being processed (glucose in a test tube drops 5 to 7% per hour). Daily glucose readings also fluctuate throughout the day on their own.
A1C sidesteps most of those issues. You don’t need to fast, the result doesn’t change based on what you ate that morning, and short-term factors like a stressful week or a recent cold have minimal effect. This makes it a more stable and convenient measurement. Your doctor can draw the blood at any appointment, regardless of when you last ate. That said, if your doctor orders other tests alongside A1C (cholesterol panels, for example), those may still require fasting.
A1C Goals Vary by Person
For most adults with diabetes, the general target is below 7%. But this isn’t one-size-fits-all. Your age, overall health, risk of low blood sugar episodes, and how long you’ve had diabetes all factor in.
For healthy older adults, a target below 7.5% is often appropriate. For older adults managing several chronic conditions, the goal loosens to around 8% or lower, because pushing blood sugar too aggressively low can cause dangerous drops that lead to falls, confusion, or hospital visits. For people in poor overall health or with significant cognitive decline, an A1C below 8.5% (an average blood sugar of about 200 mg/dL) may be sufficient, with the priority shifting toward quality of life and avoiding emergencies rather than hitting a strict number.
Younger, otherwise healthy adults with a new diabetes diagnosis typically aim for tighter control because keeping blood sugar closer to normal over decades reduces the risk of complications affecting the eyes, kidneys, and nerves.
How Often You Need the Test
If your blood sugar is well controlled and your treatment plan hasn’t changed, testing every six months is standard. If you’re adjusting medications, starting a new treatment, or your numbers aren’t at goal, your doctor will likely check it every three months. Since A1C reflects a two- to three-month window, testing more frequently than that wouldn’t capture meaningful new information.
When A1C Can Be Misleading
Because the test depends on hemoglobin inside red blood cells, anything that changes your red blood cells can skew the result. Conditions that shorten the lifespan of red blood cells, such as sickle cell disease, certain anemias, significant kidney disease, or liver failure, can produce an A1C that’s artificially low. The red blood cells don’t survive long enough to accumulate the sugar they normally would, so the percentage looks better than your actual blood sugar control.
Hemoglobin variants, which are more common in people of African, Southeast Asian, or Mediterranean descent, can also interfere with the test depending on the laboratory method used. Some variants push the result falsely high, others falsely low. If your A1C doesn’t seem to match your daily glucose readings or your symptoms, your doctor may use an alternative test or a different lab method to get a more accurate picture.
Recent blood transfusions or heavy blood loss can also throw off results temporarily, since the new or regenerating red blood cells haven’t had a normal exposure period to your blood sugar levels.