HbA1c (Hemoglobin A1c): Normal Range, High & Low Causes
What HbA1c measures
HbA1c shows the percentage of your hemoglobin that has sugar attached to it. Because red blood cells live about three months, the result reflects your average blood sugar over that period rather than a single moment.
HbA1c reference ranges
| Category | Conventional units | SI units |
|---|---|---|
| Normal | Below 5.7% | Below 39 mmol/mol |
| Prediabetes | 5.7% – 6.4% | 39 – 47 mmol/mol |
| Diabetes range | 6.5% or higher | 48 mmol/mol or higher |
For adults already diagnosed with diabetes, clinicians usually set an individual target, often around 7%, based on age, other conditions and risk of low blood sugar.
Need to convert units? Use the HbA1c to average glucose calculator.
Common causes of a high result
- Diabetes or prediabetes
- Iron-deficiency anemia, which can push the value up
- Recent heavy sugar intake over weeks (not a single meal)
- Kidney failure in some people
Common causes of a low result
- Frequent low blood sugar episodes in people treated for diabetes
- Recent blood loss or a blood transfusion
- Hemolytic anemia or other conditions that shorten red-cell life
- Some hemoglobin variants, which can make the test unreliable
How to prepare for the test
No fasting is needed. You can take the test at any time of day.
How often to retest
When diabetes is being managed, HbA1c is commonly checked every 3 to 6 months. Your clinician decides the schedule.
Track HbA1c over time
Upload your reports and Labgist plots every HbA1c result on one timeline, converts units and shows what changed.
Get the appFrequently asked questions
What is a normal HbA1c level?
Below 5.7% (39 mmol/mol) is considered normal. 5.7%–6.4% is the prediabetes range and 6.5% or higher on repeat testing falls in the diabetes range.
How is HbA1c different from a blood glucose test?
A glucose test shows your blood sugar at the moment of the draw. HbA1c reflects your average over roughly three months, so one unusual meal does not change it much.
Can HbA1c be inaccurate?
Yes. Anemia, recent blood loss or transfusion, kidney disease and some inherited hemoglobin variants can make HbA1c read too high or too low. Your clinician may then use other tests.
Related tests
Sources
This page is general education and was last updated 2026-10-30. It is not a substitute for medical advice. See our editorial approach.