Hemoglobin A1C

What It Is

Hemoglobin is the protein in red blood cells that carries oxygen. When glucose (sugar) circulates in the blood, some of it attaches to hemoglobin. The higher the average glucose, the more hemoglobin becomes "glycated."

Red blood cells live for about three months, so the share of glycated hemoglobin gives a picture of average glucose over that period, weighted toward the most recent weeks.

A1C results can also be expressed as an estimated average glucose (eAG) in mg/dL or mmol/L. Outside the US, A1C is often reported in mmol/mol.

Common Terminology

HbA1c

Another name for hemoglobin A1C, the glycated form of hemoglobin.

Glucose

The main sugar in the blood and a key energy source for the body.

Prediabetes

Blood sugar levels higher than normal but not in the diabetes range.

eAG

Estimated average glucose, an A1C result converted to the same units as a glucose meter.

Hemoglobin variant

An inherited difference in hemoglobin, such as sickle cell trait, that can affect some A1C tests.

mmol/mol

The international unit for A1C used in the UK and many other countries.

Why the Test Is Done

A1C is a widely used blood sugar test.

• Screening adults for type 2 diabetes and prediabetes, which often have no early symptoms.
• Helping diagnose diabetes, often confirmed with a repeat A1C or another glucose test.
• Following blood sugar levels over time in people with diabetes.
• Checking how blood sugar has changed after changes in lifestyle or treatment.

What the Results Generally Describe

The ADA categories below are a general reference for non-pregnant adults. Other organizations may use different units or cutoffs, individual goals for people with diabetes vary, and a clinician interprets individual results.

• Below 5.7%: generally considered in the normal range.
• 5.7% to 6.4%: generally classified as prediabetes.
• 6.5% or higher: generally consistent with diabetes, usually confirmed by repeating the test or with a different glucose test.
• As a rough reference, an A1C of 7% corresponds to an estimated average glucose of about 154 mg/dL.
• A1C goals for people with diabetes are individualized and are not the same as diagnostic cutoffs.

How the Test Is Done

The test uses a blood sample.

• Blood is usually drawn from a vein in the arm and analyzed in a lab.
• Some clinics use a finger-stick sample with a point-of-care device that gives results during the visit.
• Labs use standardized methods so that results can be compared across labs.
• Home A1C kits exist, but results may differ from lab testing.

Preparation and What to Expect

A1C is a simple test for most people.

• No fasting or special preparation is generally needed.
• The blood draw takes a few minutes and may cause brief discomfort or a small bruise.
• Recent blood loss, a blood transfusion, pregnancy, iron deficiency, and kidney disease can affect results.
• It is helpful for the care team to know about any known anemia or hemoglobin conditions.

When Results Are Discussed With a Clinician

A1C is usually considered alongside other information.

• Results at or above the diabetes threshold are commonly confirmed with repeat testing.
• Results in the prediabetes range are often discussed in terms of follow-up testing and general prevention.
• When A1C and fingerstick or continuous glucose readings do not match, other tests may be considered.
• For people with diabetes, A1C trends over time are often reviewed during regular visits.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

• Which A1C categories and units does this lab use?
• Could any of my health conditions affect the accuracy of my A1C?
• How often is A1C generally rechecked?
• Would another glucose test add useful information?
• How does A1C relate to the readings from a glucose meter or sensor?
• How are A1C goals set for people with diabetes?

Sources

This guide was written in plain language from the following authoritative references.

1. Hemoglobin A1C (HbA1c) Test — MedlinePlus (U.S. National Library of Medicine)
2. The A1C Test & Diabetes — National Institute of Diabetes and Digestive and Kidney Diseases (NIH)
3. Diabetes — MedlinePlus (U.S. National Library of Medicine)

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