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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Glycated haemoglobin (HbA1c)

HbA1c reflects average glucose over the past 2–3 months, because glucose sticks to haemoglobin over a red cell's life.

Updated 2026-11-01

## Key facts

- It is haemoglobin A with glucose attached non-enzymatically (glycation) to the N-terminal valine of the β chain.
- It reflects mean glucose over about 2–3 months (red cell life about 120 days), weighted towards the last month.
- Guidelines (ADA, WHO) accept an HbA1c ≥ 6.5 % by a certified method (NGSP/IFCC) as a diagnostic criterion for diabetes, confirmed on another sample.
- It is reported in % (NGSP) or mmol/mol (IFCC).
- It fails when red cell lifespan is altered: haemolysis, iron deficiency anaemia, recent transfusion, pregnancy, advanced kidney disease and some haemoglobinopathies.

## In detail

- What it measures: the glycated haemoglobin fraction by liquid chromatography (HPLC), immunoassay or enzymatic methods.
- Where it comes from: glucose forms a Schiff base with haemoglobin's free amino group, which rearranges into a stable product (Amadori rearrangement); the higher and longer the glucose, the more HbA1c.
- Why it rises: sustained hyperglycaemia (diabetes, prediabetes). It is also falsely raised by iron, B12 or folate deficiency (older red cells), splenectomy and uraemia (carbamylation with some methods).
- Why it is falsely low: haemolysis, recent bleeding, transfusion, erythropoietin, pregnancy, advanced liver disease; some variants (HbS, HbC, HbE, high HbF) interfere depending on the method.
- When it is unreliable, fasting glucose, the oral glucose tolerance test, fructosamine (reflects 2–3 weeks) or continuous glucose monitoring are used.
- Advanced glycation end products formed on long-lived proteins (collagen, lens) explain part of the chronic complications of diabetes.

**Also called**
HbA1c, A1c, Glycosylated haemoglobin

## Clinical notes

- Each rise in HbA1c is linked to more retinopathy, nephropathy and neuropathy; the target is individualised by age and hypoglycaemia risk.
- In populations with frequent sickle cell trait (Afro-descendant communities of Colombia's Pacific and Caribbean coasts) the laboratory's method matters.
- An HbA1c that does not match the patient's glucose readings suggests anaemia, a haemoglobinopathy or a method error.

## Pathways

- [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/)

## Related conditions

- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)

## Related

- [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Hemoglobin variants (HbA, HbA2, HbF, HbS, HbC)](https://durgical.com/en/biochemistry/topics/hemoglobin-variants/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why does HbA1c reflect about 3 months of glucose?

Because glucose binds haemoglobin throughout the red cell's life, which lasts about 120 days.

## What kind of reaction forms HbA1c?

Non-enzymatic glycation: glucose binds the N-terminal amino group of the β chain and rearranges into a stable product (Amadori).

## Why can HbA1c be falsely low after haemolysis?

The young red cells that replace the destroyed ones have been exposed to glucose only briefly.

## Which test is used when HbA1c is unreliable?

Fasting glucose, the oral glucose tolerance test, fructosamine or continuous glucose monitoring.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Hemoglobin A1C (HbA1c) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/hemoglobin-a1c-hba1c-test/) (MedlinePlus, US National Library of Medicine, 2026); [Blood Glucose Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/blood-glucose-test/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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