A1C (Hemoglobin A1C / HbA1c)
A blood test that measures average blood sugar levels over the past 2–3 months, used to screen for and monitor diabetes.
Last updated: April 10, 2026
Over 37 million Americans have diabetes, and roughly 96 million adults have prediabetes — many without knowing it. The A1C test is one of the primary tools used to screen for both conditions and track blood sugar control over time.
Why Is A1C Tested?
The A1C test measures the percentage of hemoglobin (a protein in red blood cells that carries oxygen) with glucose attached to it. Because red blood cells typically live for about 2–3 months, the A1C provides a picture of average blood sugar levels over that time window — not just at the moment of the blood draw.
This makes A1C particularly useful because it is not affected by what was eaten that day, unlike a fasting glucose test. A healthcare provider may order an A1C test to:
- Screen for type 2 diabetes or prediabetes
- Monitor blood sugar management in people already diagnosed with diabetes
- Evaluate whether a diabetes treatment plan is working effectively
- Assess risk for diabetes-related complications (nerve damage, kidney disease, eye disease)
- Track progress after lifestyle changes or medication adjustments
The American Diabetes Association recommends A1C screening starting at age 35 for all adults, or earlier for those with risk factors such as obesity, family history of diabetes, or a history of gestational diabetes.
Normal Reference Ranges
These thresholds are established by the American Diabetes Association (ADA). Individual targets may vary, especially for people already managing diabetes.
| Category | A1C Level | Estimated Average Glucose (eAG) |
|---|---|---|
| Normal | Below 5.7% | Below 117 mg/dL |
| Prediabetes | 5.7% – 6.4% | 117 – 137 mg/dL |
| Diabetes | 6.5% or higher | 140 mg/dL or higher |
| A1C % | Estimated Average Glucose (mg/dL) |
|---|---|
| 5% | ~97 mg/dL |
| 6% | ~126 mg/dL |
| 7% | ~154 mg/dL |
| 8% | ~183 mg/dL |
| 9% | ~212 mg/dL |
| 10% | ~240 mg/dL |
What Does a HIGH A1C Mean?
An A1C of 5.7% or higher signals that blood sugar levels have been above optimal range. The higher the A1C, the greater the exposure to elevated glucose over the past 2–3 months.
Possible causes or associations with elevated A1C include:
- Prediabetes (5.7%–6.4%) — blood sugar is higher than normal but not yet in the diabetes range
- Type 2 diabetes (6.5% or higher) — the body does not use insulin effectively or does not produce enough
- Type 1 diabetes — the immune system destroys insulin-producing cells
- Poorly managed diabetes — medication, diet, or activity level may need adjustment
- Cushing syndrome or chronic corticosteroid use — cortisol raises blood sugar
Persistently elevated A1C levels increase the risk of serious complications including heart disease, stroke, kidney damage, nerve damage (neuropathy), and vision loss (retinopathy). The ADA generally recommends a target A1C below 7% for most adults with diabetes.
What Does a LOW A1C Mean?
An A1C below the normal range (roughly under 4%) is uncommon but can occur. In most cases, a low A1C simply reflects well-controlled blood sugar. However, in certain situations it may warrant further investigation.
Possible reasons for an unusually low A1C include:
- Conditions that shorten red blood cell lifespan — hemolytic anemia, sickle cell disease, and significant blood loss reduce the time glucose has to attach to hemoglobin
- Recent blood transfusion — donor red blood cells dilute the glycated hemoglobin reading
- Chronic kidney disease (advanced) — can alter hemoglobin turnover
- Hypoglycemia — frequent low blood sugar episodes (especially in people taking insulin or certain medications)
- Pregnancy — increased red blood cell production can lower A1C readings
A falsely low A1C could mask actual blood sugar problems. If A1C results seem inconsistent with daily blood sugar readings, a healthcare provider may use alternative tests such as fructosamine or continuous glucose monitoring (CGM).
Factors That Can Affect Your Results
A1C is generally reliable, but several factors can make the result higher or lower than expected:
- Hemoglobin variants — sickle cell trait, thalassemia, and other hemoglobinopathies can interfere with certain testing methods
- Iron deficiency anemia — may falsely raise A1C because red blood cells live longer when iron is low
- Recent blood loss or transfusion — can falsely lower the result
- Pregnancy — hemodilution and increased red blood cell turnover can affect accuracy
- Chronic kidney disease — altered hemoglobin metabolism can skew results in either direction
- Certain medications — high-dose vitamin C, vitamin E, and dapsone can affect some assay types
- Age and ethnicity — A1C levels may differ slightly across ethnic groups even at the same average glucose level
Should I Be Concerned?
A single elevated A1C does not automatically mean diabetes. The ADA recommends confirming a diabetes diagnosis with a repeat A1C test or a different test (fasting glucose or oral glucose tolerance test) unless symptoms are clearly present.
Results that may warrant closer attention:
- A1C of 5.7%–6.4% — prediabetes range; lifestyle changes may prevent or delay progression to diabetes
- A1C of 6.5% or higher — consistent with diabetes; typically confirmed with a second test
- A1C above 9% — indicates poorly controlled blood sugar with increased complication risk
- Significant increase from a previous result — even within the normal range, a rising trend may signal early insulin resistance
Prediabetes is often reversible with lifestyle changes. Large studies such as the Diabetes Prevention Program have shown that modest weight loss and regular physical activity can meaningfully reduce the risk of progressing to type 2 diabetes. A healthcare provider can help determine what goals are appropriate for each individual.
Common Next Steps
Depending on the result, a healthcare provider may recommend:
- Repeat testing — to confirm an elevated result, especially for initial diagnosis
- Fasting glucose or oral glucose tolerance test (OGTT) — for confirmation or if A1C accuracy is questionable
- Lifestyle modifications — dietary changes, increased physical activity, and weight management for prediabetes
- Medication adjustment — metformin or other glucose-lowering drugs if lifestyle changes are not enough
- Regular monitoring — A1C every 3–6 months for people with diabetes; annual screening for those at risk
- Referral to a diabetes educator or endocrinologist — for comprehensive management support
- Additional tests — fasting insulin, lipid panel, kidney function (creatinine/eGFR), and urine albumin to assess overall metabolic health
Frequently Asked Questions
What is a normal A1C level?
A normal A1C is below 5.7%. An A1C of 5.7%–6.4% falls in the prediabetes range, and 6.5% or higher indicates diabetes, according to the American Diabetes Association.
Does fasting affect A1C results?
No. Unlike fasting glucose, the A1C test does not require fasting. It can be drawn at any time of day regardless of when the last meal was consumed, making it a convenient screening and monitoring tool.
How often is A1C tested?
For people without diabetes, the ADA recommends screening every 3 years starting at age 35, or sooner with risk factors. For those managing diabetes, A1C is typically checked every 3–6 months depending on treatment stability and glycemic control.
What is the difference between A1C and fasting glucose?
Fasting glucose measures blood sugar at a single point in time after an overnight fast. A1C measures the average blood sugar over the past 2–3 months. Both are valid for diagnosing diabetes, but they capture different information. A1C is less affected by day-to-day variability.
Can A1C be inaccurate?
Yes. Conditions that alter red blood cell lifespan can make A1C unreliable. These include sickle cell disease, thalassemia, iron deficiency anemia, recent blood loss or transfusion, and advanced kidney disease. In these situations, fructosamine or continuous glucose monitoring may be used instead.
Can prediabetes be reversed?
In many cases, yes. Research such as the Diabetes Prevention Program has shown that moderate lifestyle changes — including weight loss and regular physical activity — can substantially reduce the risk of progressing to type 2 diabetes. Dietary improvements and stress management may also help. A healthcare provider can help tailor an approach to each individual.
What A1C level is considered dangerous?
An A1C above 9% is generally considered poorly controlled and is associated with significantly higher risk of diabetes complications. An A1C above 12% indicates severely uncontrolled blood sugar and typically requires prompt medical attention and treatment adjustment.
Can stress raise A1C?
Chronic stress can raise blood sugar over time through cortisol and other stress hormones, which may contribute to a higher A1C. However, short-term stress on the day of the test does not directly affect the result because A1C reflects a 2–3 month average.
How does A1C convert to average blood sugar?
The formula is: estimated average glucose (eAG) = (28.7 x A1C) – 46.7 mg/dL. For example, an A1C of 7% equals approximately 154 mg/dL. Most lab reports now include the eAG alongside the A1C percentage for easier interpretation.
Is A1C used for type 1 and type 2 diabetes?
Yes. A1C is used to monitor blood sugar control in both type 1 and type 2 diabetes. It is also approved for diagnosing type 2 diabetes and prediabetes. For type 1 diabetes, A1C is more commonly used for ongoing monitoring, as diagnosis typically involves autoantibody testing and other criteria.
Sources
- American Diabetes Association. Standards of Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1).
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The A1C Test & Diabetes.
- Diabetes Prevention Program Research Group. Reduction in the Incidence of Type 2 Diabetes with Lifestyle Intervention or Metformin. N Engl J Med. 2002;346(6):393-403.
- MedlinePlus. A1C Test. U.S. National Library of Medicine.
- CDC. National Diabetes Statistics Report, 2022.
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Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges vary by laboratory and individual factors. Always discuss results with a qualified healthcare provider.