An A1C level of 6.5% or higher indicates diabetes. This blood test measures the percentage of your hemoglobin (the oxygen-carrying protein in red blood cells) that has glucose attached to it, giving a picture of your average blood sugar over the past two to three months. Unlike a single blood sugar reading that captures one moment in time, A1C reveals a longer pattern, which is why it’s one of the primary tools for diagnosing both diabetes and prediabetes.
The Three A1C Ranges
A1C results fall into three categories. A result below 5.7% is considered normal. Between 5.7% and 6.4% falls in the prediabetes range, meaning blood sugar is elevated but not yet high enough for a diabetes diagnosis. At 6.5% or above, the result indicates diabetes. Doctors typically confirm the diagnosis by repeating the test or combining it with another blood sugar test, since no single result is considered definitive on its own.
A result in the prediabetes range is worth paying attention to. It means glucose levels have been running higher than normal for weeks or months, and without changes, progression to type 2 diabetes becomes more likely.
Why A1C Reflects Two to Three Months
Red blood cells live for about 120 days before your body replaces them. The entire time a red blood cell is circulating, glucose in your bloodstream gradually attaches to the hemoglobin inside it. The higher your blood sugar runs, the more glucose sticks. When a lab measures your A1C, it’s reading the accumulated sugar buildup across all your red blood cells, old and new.
Not all of that window is weighted equally, though. About half of the glycation reflected in your A1C result happened in the most recent 30 days. Another 40% comes from the 31 to 90 day window, and only about 10% reflects anything beyond 90 days. So while A1C is often described as a “three-month average,” your most recent month of blood sugar control has the biggest influence on the number.
How A1C Compares to Other Diabetes Tests
A1C isn’t the only way to diagnose diabetes. A fasting plasma glucose test (which requires not eating for at least eight hours) and an oral glucose tolerance test are also used. Each has strengths and limitations.
A large systematic review comparing these tests found that A1C and fasting glucose perform similarly when used alone, each catching about half of diabetes cases when measured against the oral glucose tolerance test as a reference standard. Fasting glucose is slightly better at confirming diabetes when the result is positive (higher specificity at 98% versus 96% for A1C). But combining the two tests, using either an A1C of 6.5% or higher or a fasting glucose of 126 mg/dL or higher, boosts sensitivity to 64%, making it the best approach for catching cases that might otherwise be missed.
One practical advantage of the A1C test: you don’t need to fast. You can eat and drink normally before the blood draw, which makes scheduling easier and eliminates the discomfort of skipping meals.
When A1C Results Can Be Misleading
Several conditions can push your A1C result artificially higher or lower than your actual blood sugar levels would suggest. The CDC lists severe anemia, kidney failure, liver disease, blood loss or transfusions, and early or late pregnancy as factors that can distort results. Certain medications, including opioids and some HIV drugs, can also interfere.
Blood disorders deserve special mention. Sickle cell anemia and thalassemia change the structure or lifespan of red blood cells, which directly affects how much glucose accumulates on hemoglobin. A falsely high result could lead to unnecessary increases in diabetes medication, potentially causing dangerously low blood sugar. A falsely low result is equally problematic: it might make poorly controlled diabetes look fine, allowing high blood sugar to quietly damage eyes, nerves, and kidneys.
Hemoglobin Variants and Ancestry
The most common form of hemoglobin is called hemoglobin A, and standard A1C tests are calibrated to measure it. But millions of people carry hemoglobin variants, alternative forms inherited from their parents, that can throw off certain A1C testing methods.
The variants most likely to affect results include hemoglobin S (common among African Americans and people with ancestry from sub-Saharan Africa, India, and the Mediterranean), hemoglobin C (found in people of West African descent), and hemoglobin E (prevalent in Southeast Asian populations, especially Cambodian, Laotian, and Thai communities). Hemoglobin D, though less common in the United States, appears in people with Chinese, Indian, Turkish, and Brazilian ancestry.
If you have a hemoglobin variant, not all A1C testing methods will give you an accurate result. Some newer lab assays can handle common variants without issue, but it’s important that your healthcare provider knows about any variant so they can choose the right test or rely on alternative blood sugar measurements instead.
What Your A1C Number Actually Means Day to Day
Each percentage point on the A1C scale corresponds to a rough average blood sugar level. An A1C of 5.7% translates to an average blood glucose of about 117 mg/dL. At 6.5%, the diagnostic cutoff for diabetes, the average sits around 140 mg/dL. An A1C of 7%, a common treatment target for people already diagnosed, reflects an average of approximately 154 mg/dL.
These are averages, which means they smooth over the highs and lows. Two people could have the same A1C but very different daily experiences. One might have relatively steady blood sugar, while the other swings between spikes after meals and drops between them. That’s why A1C is useful for the big picture but doesn’t replace daily glucose monitoring for people who are actively managing diabetes.
How Often to Get Tested
If your A1C is in the normal range and you have no risk factors, testing every three years is generally sufficient. For prediabetes, annual testing helps track whether blood sugar is trending upward or responding to lifestyle changes like increased physical activity and dietary shifts. People with diagnosed diabetes typically get tested two to four times per year, depending on how stable their blood sugar control is and whether their treatment plan has recently changed.
Because A1C is weighted toward the most recent month, retesting sooner than three months after a treatment change won’t give a full picture of whether the adjustment is working. Waiting at least 12 weeks between tests gives your red blood cells enough turnover to reflect the new pattern.