A1C — also called hemoglobin A1C, HbA1c, or glycated hemoglobin — is a blood test that measures your average blood sugar level over the past two to three months. According to the National Institute of Diabetes and Digestive and Kidney Diseases, it is one of the primary tools doctors use to diagnose prediabetes and diabetes and to monitor blood sugar management.
How the A1C Test Works
To understand A1C, you need to know about hemoglobin — the protein inside your red blood cells that carries oxygen throughout your body. When glucose (sugar) is present in your bloodstream, some of it naturally attaches to hemoglobin. This glucose-coated hemoglobin is called glycated hemoglobin, or HbA1c.
The higher your blood sugar has been over the past few months, the more glucose attaches to your hemoglobin. Since red blood cells live for about 120 days (roughly 2-3 months), the A1C test captures a rolling average of your blood sugar during that period.
The result is reported as a percentage. For example, an A1C of 5.5% means that 5.5% of the hemoglobin in your blood has glucose attached to it.
What A1C Numbers Mean
| A1C Result | Classification | Estimated Average Blood Sugar |
|---|---|---|
| Below 5.7% | Normal | Below 117 mg/dL |
| 5.7% – 6.4% | Prediabetes | 117 – 137 mg/dL |
| 6.5% or higher | Diabetes | 140 mg/dL or higher |
Each 1% change in A1C corresponds to roughly a 29 mg/dL change in average blood sugar. So an A1C of 6.0% reflects an estimated average glucose of about 126 mg/dL, while an A1C of 7.0% corresponds to about 154 mg/dL.
For a deeper look at specific A1C values, visit our A1C levels guide, which covers what each number means and when to take action.
A1C vs. Blood Glucose: What Is the Difference?
Both tests measure blood sugar, but they capture different information:
| Feature | A1C Test | Fasting Blood Glucose |
|---|---|---|
| What it measures | Average blood sugar over 2-3 months | Blood sugar at a single moment |
| Fasting required? | No | Yes (8-12 hours) |
| Affected by recent meals? | No | Yes |
| Normal range | Below 5.7% | Below 100 mg/dL |
| Prediabetes range | 5.7% – 6.4% | 100 – 125 mg/dL |
| Diabetes threshold | 6.5% or higher | 126 mg/dL or higher |
Think of fasting glucose as a snapshot — it tells you what your blood sugar is right now. A1C is more like a movie — it shows the trend over time. A single fasting glucose reading can be influenced by stress, illness, or what you ate the night before. A1C is more stable and less susceptible to day-to-day fluctuations, which is why many doctors prefer it for diagnosis and monitoring.
However, neither test is perfect on its own. Some doctors use both to get a complete picture. A normal fasting glucose with a slightly elevated A1C, for example, might suggest that your blood sugar spikes after meals even if it drops to normal overnight.
Who Should Get an A1C Test
The American Diabetes Association recommends A1C screening for:
- All adults age 35 and older, regardless of weight or risk factors, repeated at least every 3 years
- Adults of any age who are overweight (BMI 25+) and have one or more additional risk factors, such as family history, physical inactivity, high blood pressure, or belonging to a high-risk ethnic group
- Women who had gestational diabetes, tested every 1-3 years after delivery
- Anyone with a previous prediabetes result, tested every 3-6 months
If you have never had your A1C checked and you are over 35, ask your doctor to include it at your next checkup. It requires only a simple blood draw — no fasting, no special preparation.
How Often to Retest
The frequency of retesting depends on your result:
- Normal A1C (below 5.7%): Retest every 3 years, or sooner if risk factors develop
- Prediabetes (5.7-6.4%): Retest every 3-6 months to track progress with lifestyle changes
- Diabetes (6.5%+): Retest every 3 months to monitor treatment effectiveness
If you are actively working to reverse prediabetes, retesting at the 3-month mark is ideal because it takes about that long for lifestyle changes to fully reflect in your A1C.
Factors That Can Affect A1C Accuracy
While A1C is generally reliable, certain conditions can produce misleading results:
- Iron deficiency anemia: Can cause a falsely high A1C
- Hemoglobin variants: Conditions like sickle cell trait or thalassemia can produce inaccurate results with some testing methods
- Kidney disease: Advanced kidney disease may affect red blood cell turnover and distort A1C
- Recent blood loss or transfusions: Can lower A1C by replacing old red blood cells with new ones
- Pregnancy: A1C may not accurately reflect glucose control during pregnancy due to changes in blood volume
If you have any of these conditions, let your doctor know. They can use alternative testing methods or interpret your results with appropriate adjustments. Learn more about what constitutes a healthy result in our guide to normal A1C levels.
The Bottom Line
The A1C test is a simple, convenient blood test that reveals your average blood sugar over the past two to three months. It does not require fasting, it is not affected by a single meal, and it is one of the most reliable ways to screen for prediabetes and diabetes. If you are 35 or older — or younger with risk factors — ask your doctor for an A1C test at your next visit. Knowing your number is the first step toward taking control of your blood sugar health.
