HbA1c
Long-term blood sugar average over 2–3 months
Also known as
Definition
HbA1c (glycated hemoglobin) reflects the average blood glucose level over the preceding two to three months, as glucose binds reversibly to hemoglobin in red blood cells. It is the central diagnostic and monitoring marker for type 2 diabetes mellitus and prediabetes, and is also used in biohacking to track insulin sensitivity and the impact of dietary interventions.
| Parameter | Value |
|---|---|
| Unit | % (mmol/mol) |
| Reference Range | < 5.7% (normal) | 5.7–6.4% (prediabetes) | ≥ 6.5% (diabetes, ADA criteria) |
Agree on your personal target range with your doctor.
HbA1c is reported in % (NGSP/DCCT standard) in the US. Internationally also in mmol/mol (IFCC). Conversion: IFCC (mmol/mol) = (NGSP % − 2.15) × 10.929. Example: 5.7% ≈ 39 mmol/mol.
↓ What a low value means
A very low HbA1c can indicate a tendency toward hypoglycemia, hemolytic anemia (shorter red cell lifespan falsely lowers HbA1c), or a very low-carbohydrate diet. Your doctor interprets the result.
↑ What a high value means
HbA1c 5.7–6.4% = prediabetes (elevated risk of type 2 diabetes and cardiovascular disease). HbA1c ≥ 6.5% = diabetes mellitus (ADA diagnostic criterion; according to the ADA, to be confirmed by a repeat measurement). Elevated values correlate with micro- and macroangiopathy, neuropathy, retinopathy.
✓ What can influence this value
The HbA1c value reflects the average blood glucose of the past weeks and is influenced by diet (in particular rapidly absorbed carbohydrates), physical activity, body weight, sleep duration, and insulin resistance. A changed lifespan of red blood cells, for example with iron deficiency, anemia, blood loss, or hemoglobin variants, can shift the value independently of blood glucose. The assessment is made by the treating doctor.
When to test
No fasting required. The value is measured in diabetes screening, especially with risk factors such as overweight, family history, or hypertension, and to monitor diabetes and prediabetes. How often it is measured is decided by your doctor. Iron-deficiency anemia and hemolytic conditions can distort the value.
Frequently asked questions
Can HbA1c be falsely low or falsely high? +
Yes. Falsely low: hemolytic anemia, blood loss (shorter red cell lifespan). Falsely high: iron-deficiency anemia (longer red cell lifespan), kidney insufficiency. When HbA1c and fasting glucose do not match, CGM or fructosamine can provide additional information.
How is a low-carbohydrate diet related to HbA1c? +
A low-carbohydrate diet can significantly lower HbA1c in prediabetes and type 2 diabetes. With insulin or sulfonylureas, such a diet can lead to low blood sugar; in that case medical supervision is indicated.
What is the advantage of HbA1c over fasting glucose? +
HbA1c integrates blood glucose over 2–3 months and is not distorted by day-to-day fluctuations. Fasting glucose only shows the momentary value. HbA1c is less affected by the stress of blood draw and does not require fasting.
HbA1c
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.