An A1C test is a blood test that measures your average blood sugar level over the past two to three months. It’s one of the primary tools used to diagnose diabetes and prediabetes, and it’s the standard way doctors track how well blood sugar is being managed over time. Unlike a finger stick or fasting glucose test, the A1C doesn’t capture a single moment. It reflects the bigger picture.
How the A1C Test Works
The test measures how much glucose has attached to hemoglobin, the protein inside red blood cells that carries oxygen throughout your body. When blood sugar is higher, more glucose coats the hemoglobin. Since red blood cells live for about three months before your body replaces them, the amount of glucose stuck to hemoglobin at any given time reflects your average blood sugar over that entire lifespan. A higher percentage means more sugar has been circulating in your blood.
The result is reported as a percentage. Someone without diabetes typically has an A1C below 5.7%. A result between 5.7% and 6.4% falls into the prediabetes range. An A1C of 6.5% or higher on two separate tests indicates diabetes.
What Your A1C Number Translates To
A1C percentages can feel abstract, so doctors often convert them into an estimated average glucose (eAG), which looks like the numbers you’d see on a blood sugar meter. The conversion is straightforward: each percentage point corresponds to roughly 29 mg/dL of average blood sugar. Here’s how the most common A1C values translate:
- 5%: about 97 mg/dL average blood sugar
- 6%: about 126 mg/dL
- 7%: about 154 mg/dL
- 8%: about 183 mg/dL
- 9%: about 212 mg/dL
- 10%: about 240 mg/dL
These are averages with a range around them. An A1C of 7%, for example, corresponds to an average glucose somewhere between 123 and 185 mg/dL for most people. That spread matters because two people with the same A1C can have different day-to-day blood sugar patterns, one with steady levels and another swinging between highs and lows.
No Fasting Required
One of the biggest practical advantages of the A1C test is that you don’t need to fast beforehand. You can eat and drink normally, and the blood draw can happen at any time of day. A fasting glucose test, by contrast, requires you to skip food for at least eight hours, which means scheduling a morning appointment and showing up hungry. An oral glucose tolerance test is even more involved: you drink a sugary solution and then wait two hours for repeated blood draws.
The A1C also isn’t thrown off by short-term factors that can skew other glucose tests. A stressful day, a bout of exercise, or a meal the night before won’t change your result. That stability makes it especially useful for screening large numbers of people, including those who might not know they have diabetes. Because the test is so convenient, more people are likely to actually get screened compared to tests that require fasting or multiple visits.
How Often You’ll Get Tested
If you have diabetes and your blood sugar is well controlled, the American Diabetes Association recommends testing at least twice a year. If your treatment has recently changed, or your blood sugar isn’t where it needs to be, expect testing every three months or more. Your doctor uses the trend across multiple tests, not a single number, to judge whether your management plan is working.
For people without a diabetes diagnosis, A1C is commonly included in routine bloodwork during annual physicals, especially after age 45 or if you have risk factors like obesity, a family history of diabetes, or a history of gestational diabetes.
A1C Goals Vary by Person
A target of below 7% is the general guideline for most adults with diabetes, but the right goal depends on your age, overall health, and risk of low blood sugar episodes. Pushing for a very low A1C can sometimes cause dangerous drops in blood sugar, particularly in older adults or people on certain medications.
For healthy older adults, guidelines set the target at below 7.5%. For those with complex health conditions or who are more vulnerable to side effects from treatment, the target relaxes to below 8% or even 8.5%. The American College of Physicians recommends a general range of 7% to 8% for most patients. These relaxed targets reflect a practical tradeoff: aggressive blood sugar lowering carries its own risks, and for some people, avoiding those risks matters more than hitting a lower number.
Children and younger adults often have different targets as well, set in collaboration with their care team based on growth, activity levels, and how consistently they can manage their blood sugar day to day.
When the Test Can Be Inaccurate
The A1C test depends on normal hemoglobin and normal red blood cell turnover. Anything that changes either of those can push the result higher or lower than your true average blood sugar. Conditions that can distort the result include:
- Blood disorders: Sickle cell anemia and thalassemia alter the structure of hemoglobin, which can make the test unreliable.
- Severe anemia: When red blood cells are being destroyed or produced abnormally, the hemoglobin available for glucose to attach to changes.
- Kidney or liver disease: Both can affect red blood cell lifespan or hemoglobin levels.
- Recent blood loss or transfusions: These directly change the age and volume of red blood cells in circulation.
- Pregnancy: Both early and late pregnancy alter blood volume and red blood cell production, potentially skewing results.
- Certain medications: Some drugs, including opioids and certain HIV medications, can interfere with the test.
If any of these apply to you, your doctor may rely on alternative measures like fructosamine testing (which reflects a shorter two-to-three-week window) or continuous glucose monitoring to get a more accurate picture of your blood sugar control.
A1C Compared to Daily Glucose Monitoring
The A1C and daily glucose checks serve different purposes. A finger-stick glucose reading tells you what’s happening right now: is your blood sugar too high after a meal, or too low before bed? The A1C tells you how the last three months have gone overall. Neither replaces the other. If you’re managing diabetes, you’ll likely use both, with daily checks guiding your immediate decisions about food, activity, and medication, and the A1C showing whether those daily decisions are adding up to good control over time.
People who use continuous glucose monitors get an additional metric called time in range, which measures what percentage of the day your blood sugar stays within a target zone (typically 70 to 180 mg/dL). This captures something the A1C misses: variability. Two people with an A1C of 7% might have very different daily experiences. One stays between 120 and 180 most of the day, while the other swings from 60 to 300. The A1C alone can’t distinguish between those patterns, which is why doctors increasingly look at multiple measures together rather than relying on any single number.