HbA1c is the fraction of hemoglobin that glucose has "stuck" to. The higher your sugar was over the preceding months, the more such molecules there are.
The main advantage: HbA1c shows the average glucose level over 2–3 months — roughly how long a red blood cell lives. It cannot be "prepared for" with a single day of fasting, and it does not depend on whether you ate in the morning.
Reference ranges
Diagnostic thresholds (WHO and ADA criteria):
| Normal | below 5.7% |
| Prediabetes | 5.7 – 6.4% |
| Diabetes | 6.5% and above |
| Target for most patients with diabetes | below 7.0% |
What an elevated level means
Elevated HbA1c means the average sugar has stayed high for months. The 5.7–6.4% range is prediabetes: a reversible state, and it is at this stage that a change of lifestyle gives the greatest effect.
From 6.5% it is the diagnostic threshold for diabetes (confirmed on repeat). The higher HbA1c is and the longer it stays elevated, the greater the risk of damage to vessels, kidneys, retina and nerves.
HbA1c can be falsely high with iron deficiency and B12 deficiency — red cells live longer and have time to glycate more.
What a low level means
A low HbA1c in a person without diabetes usually means good glucose control and needs no action.
But it can also be falsely low — in situations where red cells live shorter than usual: hemolytic anemia, recent blood loss or transfusion, pregnancy, severe liver disease. In those cases HbA1c underestimates the real sugar, and one should rely on glucose.
How the test is done
Venous blood. Fasting is not required — it can be taken at any time of day, regardless of food. This is the main practical difference from fasting glucose.
The result reflects the previous 2–3 months, so there is no point re-testing more often than once every 3 months. For diabetes control it is usually tested 2–4 times a year.
What affects the value
- Average glucose level over the last 2–3 months — the main factor.
- Iron and B12 deficiency — falsely raise HbA1c.
- Anemia, blood loss, transfusion — falsely lower it.
- Chronic kidney disease — distorts the result in both directions.
- Pregnancy — lowers it; other tests are used for diagnosis.
- Hemoglobinopathies — some hemoglobin variants make the test unreliable.
Calculate using this marker
Calculators where Glycated hemoglobin is used directly:
Frequently asked questions
Why is HbA1c better than fasting glucose?
It shows the average sugar over 2–3 months, not a single instant. Glucose is easily distorted by stress, lack of sleep or breaking the fast; HbA1c is independent of that and needs no fasting. But it is best to look at both together.
Is HbA1c of 5.9% already diabetes?
No, that is prediabetes (5.7–6.4%). It is not yet a disease but a warning — and the good news is that at this stage changes in diet, activity and weight can bring the value back to normal.
Can HbA1c mislead?
Yes, and this is important to know. With iron deficiency it is falsely high, and with anemia, recent blood loss or transfusion it is falsely low. If you have these conditions, rely on glucose and discuss interpretation with your doctor.