A1C vs HbA1c: What is the Difference?

Executive Summary

There is no clinical difference between A1C and HbA1c. HbA1c is the scientific and laboratory name for glycated hemoglobin, while A1C is the common shorthand used by patients and doctors in the United States. They both measure the same 3-month average blood sugar percentage.

The Origin of the Names

The reason two different terms exist is purely linguistic.

HbA1c: This stands for Hemoblobin A1c. The "Hb" is the chemical symbol for hemoglobin, and "A1c" refers to the specific sub-fraction of hemoglobin that glucose attaches to. This term is favored in scientific papers, lab reports, and in many countries outside the US (like the UK and Canada).
A1C: This is the simplified "patient-facing" term adopted by the American Diabetes Association (ADA) to make the test easier to talk about. It drops the "Hb" for simplicity.

Why "A1c" and not "A1a" or "A1b"?

Hemoglobin A is the main type of hemoglobin in adults. When scientists used chromatography to study it, they found several small components. They labeled them alphabetically: A1a, A1b, and A1c.

They discovered that A1c was the one that reacted most strongly with blood sugar, making it the perfect marker for monitoring diabetes.

Term	Usage Context	Scientific Meaning
HbA1c	Medical Journals / International	Hemoglobin Alpha-1-c
A1C	General Practice / USA	Common Shorthand for HbA1c
Glycated Hemoglobin	Biochemistry	The process of sugar-protein bonding
GHb	Old Medical Texts	Glycosylated Hemoglobin (Older term)

Are the Results the Same?

Yes. If you take an "A1C test" in New York and an "HbA1C test" in London, you are getting the exact same laboratory procedure. Both tests are standardized by the NGSP (National Glycohemoglobin Standardization Program) to ensure that a 7.0% in one lab is the same as a 7.0% in another.

Key Facts About A1C / HbA1c

Timeframe: Both represent the last 8–12 weeks of glucose history.
Unit: Both are reported as a percentage in the US (e.g., 6.5%).
International Units: In some regions, HbA1c is reported in mmol/mol instead of percentages. This is called the IFCC standard.

Convert A1C % to International mmol/mol →

Frequently Asked Questions

Is one more accurate than the other?

No. They are identical. The name used on your lab report (A1C or HbA1c) depends entirely on your hospital's software or your country's naming conventions.

What does the "c" stand for in HbA1c?

The "c" doesn't stand for a word like "concentration" or "calculated." It was simply the third fraction (a, b, c) found during the laboratory separation of hemoglobin molecules.

Does A1C mean "Average 100"?

No. A1C is a molecular designation, not a mathematical average of 100. However, an A1C of 5.0% does happen to correlate with an average blood sugar of roughly 97 mg/dL (close to 100).

The Biochemistry of Glycated Hemoglobin

To understand A1C, we must look at the Amadori rearrangement. This is a series of chemical reactions where glucose bonds to the N-terminal valine of the hemoglobin beta chain. Because this bond is nearly irreversible, the percentage of glycated hemoglobin remains stable throughout the 120-day lifespan of the erythrocyte. This makes A1C a superior metric for long-term glycemic control compared to transient plasma glucose tests.

References

World Health Organization (WHO) - Use of HbA1c in Diabetes Diagnosis
[NGSP - Harmonizing Hemoglobin A1C testing](http://www.ngsp.org)
Diabetes Care - International Expert Committee Report on A1C

Learn more in our comprehensive What is A1C? complete guide.