What Is A1C? A Complete Guide to HbA1c Testing
A1C (HbA1c) is a blood test that shows your average blood sugar over the past 2–3 months. A normal A1C is below 5.7%. Prediabetes is 5.7–6.4%. Diabetes is diagnosed at 6.5% or higher. According to the American Diabetes Association (ADA), A1C is the standard test for diagnosing and monitoring diabetes.
A1C Ranges at a Glance
| A1C % | Category | eAG (mg/dL) | eAG (mmol/L) |
|---|---|---|---|
| Below 5.7% | Normal | Below 117 | Below 6.5 |
| 5.7% – 6.4% | Prediabetes | 117 – 137 | 6.5 – 7.6 |
| 6.5% or higher | Diabetes | 140 or higher | 7.8 or higher |
| Below 7.0% | ADA target (with diabetes) | Below 154 | Below 8.6 |
Source: American Diabetes Association (ADA) 2024 Standards of Care
What Does A1C Actually Measure?
A1C measures the percentage of hemoglobin in your red blood cells that has glucose (sugar) attached to it. Hemoglobin is the protein inside red blood cells that carries oxygen throughout your body.
When blood sugar is high, more glucose attaches to hemoglobin — a process called glycation. Because red blood cells live for approximately 2–3 months before being replaced, an A1C test captures a rolling average of your blood sugar across that entire period, not just a single moment in time.
This is what makes A1C so clinically valuable: a single fasting glucose test shows where your blood sugar is right now, but A1C shows where it's been. A person could fast the night before a blood draw and appear to have normal glucose — but a high A1C would reveal that blood sugar has been running elevated for months.
Why A1C Matters for Diabetes
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), the A1C test is used for three main purposes:
- Diagnosing prediabetes and type 2 diabetes — an A1C of 6.5% or higher on two separate tests confirms a diabetes diagnosis
- Monitoring diabetes management — regular A1C tests show whether treatment plans are working over time
- Guiding treatment decisions — doctors use A1C trends to adjust medications, diet plans, and lifestyle recommendations
Chronically elevated A1C is associated with a higher risk of diabetes-related complications including nerve damage (neuropathy), kidney disease (nephropathy), eye disease (retinopathy), and cardiovascular disease. Keeping A1C within your target range is one of the most important steps in reducing these risks.
How the A1C Test Works
The A1C test is a simple blood test that doesn't require fasting. A healthcare provider draws a small blood sample — either from a vein in your arm or a fingerstick — and sends it to a laboratory. Results are typically available within a few days.
Some clinics use point-of-care A1C analyzers that can produce results in about 10 minutes during your appointment. These devices are FDA-cleared and provide results comparable to laboratory tests, though lab tests remain the gold standard for diagnosis.
What to Expect
- No fasting required — you can eat and drink normally before the test
- The test takes just a few minutes
- Results are reported as a percentage (e.g., 7.2%)
- Some labs also report eAG (estimated average glucose) alongside A1C
A1C vs. Daily Blood Sugar — What's the Difference?
Daily blood sugar readings from a glucose meter or CGM show your glucose at one specific moment. A1C gives the 3-month average. Both are important — they provide complementary information:
| Feature | A1C Test | Daily Blood Sugar |
|---|---|---|
| Time period | Past 2–3 months | Right now |
| Fasting required | No | Sometimes |
| Can be done at home | No (lab or clinic) | Yes (meter or CGM) |
| Frequency | 2–4 times per year | Multiple times daily |
| Best for | Long-term trends | Day-to-day management |
For a deeper comparison, see our guide: A1C vs Blood Sugar — What's the Difference?
What Is eAG — and How Does It Relate to A1C?
eAG (estimated Average Glucose) translates your A1C percentage into the same units you see on your glucose meter — mg/dL or mmol/L. The ADA introduced eAG in 2008 to make A1C results more intuitive for patients.
For example, an A1C of 7.0% corresponds to an eAG of approximately 154 mg/dL (8.6 mmol/L). This is the same number format you'd see on your meter after a meal.
The formula used is: eAG (mg/dL) = (28.7 × A1C%) − 46.7 — developed in the ADAG (A1C-Derived Average Glucose) study. Use our A1C Calculator to convert your A1C to eAG instantly.
Who Should Get an A1C Test?
The ADA recommends A1C testing for:
- Adults 35–70 years old who are overweight or have obesity — screening for prediabetes and type 2 diabetes
- People with risk factors at any age — including family history of diabetes, gestational diabetes, high blood pressure, or PCOS
- People already diagnosed with diabetes — tested 2–4 times per year depending on how well-controlled blood sugar is
- Pregnant women — gestational diabetes screening typically uses different tests, but A1C may be used in some cases
When A1C May Not Be Accurate
A1C is reliable for most people, but certain conditions can cause results that don't accurately reflect true average glucose. According to the NIDDK, these include:
- Hemoglobin variants — such as sickle cell trait (HbS) or HbC, which can falsely lower or raise A1C
- Iron deficiency anemia — can falsely elevate A1C
- Hemolytic anemia — destroys red blood cells faster than normal, reducing A1C
- Chronic kidney disease — can affect red blood cell lifespan and distort A1C readings
- Recent blood transfusions — introduce new red blood cells, affecting the average
If you have any of these conditions, your doctor may use alternative tests — such as fructosamine or continuous glucose monitoring — to assess blood sugar control.