An A1c level of 6.5% or higher is considered diabetic. An A1c between 5.7% and 6.4% falls in the prediabetes range, and anything below 5.7% is normal. These thresholds apply to both adults and adolescents being screened for type 2 diabetes.
What the A1c Test Measures
The A1c test works by measuring how much sugar has attached to hemoglobin, the oxygen-carrying protein inside your red blood cells. When blood sugar is high, more glucose sticks to hemoglobin. Since red blood cells live about three months before your body replaces them, the test captures a rolling average of your blood sugar over the past two to three months rather than a single snapshot.
That long window is what makes A1c useful for diagnosis. A fasting glucose test tells you what your blood sugar is doing right now, which can swing based on what you ate last night, how well you slept, or whether you’re fighting off a cold. A1c smooths all of that out. It also doesn’t require fasting, so you can have it drawn at any time of day without preparation.
The Three A1c Ranges
The diagnostic categories break down like this:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
To put those percentages in terms of daily blood sugar: an A1c of 6.0% corresponds to an estimated average glucose of about 126 mg/dL. At 6.5%, the diagnostic cutoff for diabetes, the average is roughly 140 mg/dL. Someone with an A1c of 8% has been averaging around 183 mg/dL, and at 10% that average climbs to about 240 mg/dL. The conversion formula is straightforward: multiply the A1c by 28.7, then subtract 46.7.
How a Diagnosis Gets Confirmed
A single A1c result of 6.5% or higher doesn’t automatically finalize a diabetes diagnosis. Standard practice calls for repeat testing on a separate day to confirm the result. The exception is when someone already has classic symptoms of high blood sugar, like frequent urination, excessive thirst, and unexplained weight loss, along with a random blood sugar reading of 200 mg/dL or above. In that scenario, one test is typically enough.
If a repeat A1c doesn’t match the clinical picture, or if the results seem inconsistent, your doctor may switch to a different test entirely. The two main alternatives are a fasting plasma glucose test (diabetes is diagnosed at 126 mg/dL or higher) and an oral glucose tolerance test, where you drink a sugary solution and have your blood drawn two hours later (diabetes is diagnosed at 200 mg/dL or higher). All three tests are considered equally valid for diagnosis.
What Prediabetes Means in Practice
Landing in the 5.7% to 6.4% range doesn’t mean you’ll inevitably develop diabetes, but it does signal real risk. In a large study of nearly 18,000 adults tracked over about five years, roughly 15% of people with A1c-defined prediabetes progressed to diabetes during that period. That works out to about 3% per year, which is significant but also means the majority of people in this range did not progress within that timeframe.
Prediabetes is the stage where lifestyle changes have the most measurable impact. Losing 5% to 7% of body weight and getting regular physical activity have consistently been shown to reduce or delay progression. If your A1c comes back at, say, 6.1%, you’re not in a holding pattern waiting for diabetes. You’re at a decision point where changes can push that number back down.
When the A1c Test Can Be Inaccurate
Because the test depends on hemoglobin inside red blood cells, anything that changes those cells can skew the result. Conditions that shorten red blood cell lifespan, like sickle cell disease or other hemoglobin variants, can produce falsely low A1c readings. So can significant blood loss or blood transfusions. Iron deficiency anemia, on the other hand, can push A1c artificially higher because red blood cells stick around longer than normal.
Pregnancy also affects the test’s reliability. Blood volume increases substantially during pregnancy, and red blood cell turnover changes, making A1c less dependable for diagnosing gestational diabetes. Kidney disease in advanced stages can interfere as well. If any of these conditions apply to you, your doctor will likely rely on direct blood sugar measurements rather than A1c for diagnosis.
A1c for Children and Adolescents
The same 6.5% threshold applies when screening children and adolescents for type 2 diabetes. The American Diabetes Association does note that the supporting data is more limited in younger populations than in adults, but it still recommends A1c as a diagnostic tool for pediatric type 2 diabetes. The main exceptions are children with cystic fibrosis, those with hemoglobin disorders that interfere with the assay, and cases where symptoms suggest the rapid onset of type 1 diabetes, where other tests are preferred.
How Often to Get Tested
If your A1c is normal and you have no risk factors, routine screening generally starts at age 35. If you have risk factors like a family history of diabetes, a BMI in the overweight or obese range, or a history of gestational diabetes, screening may start earlier. For people in the prediabetes range, retesting every one to two years helps track whether your numbers are stable, improving, or creeping upward. Once someone has a confirmed diabetes diagnosis, A1c testing typically happens two to four times a year to monitor how well blood sugar is being managed over time.