Hemoglobin A1c
HbA1c· also: A1c, Glycated Hemoglobin, Glycosylated Hemoglobin
Clinical Overview
HbA1c (glycated hemoglobin) measures the percentage of hemoglobin that has glucose permanently attached to it, reflecting average blood glucose over the past 2–3 months. It is the gold standard test for long-term glycemic control in diabetes management and is used for both diagnosis and treatment monitoring.
Why This Test Matters
HbA1c predicts the risk of diabetic complications (retinopathy, nephropathy, neuropathy) better than any single glucose measurement. Each 1% reduction in HbA1c reduces the risk of microvascular complications by approximately 35–40% (UKPDS). The target HbA1c for most adults with diabetes is below 7% (53 mmol/mol), though targets are individualized based on age, comorbidities, and hypoglycemia risk.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 4 – 5.6 | % | Normal (non-diabetic) |
| 5.7 – 6.4 | % | Prediabetes range |
| 6.5 – 99 | % | Diabetes (diagnostic threshold) |
Also reported in: mmol/mol.
What Causes Abnormal Results?
High HbA1c Causes
- Poorly controlled type 2 diabetes
- Type 1 diabetes with inadequate insulin dosing
- Iron deficiency anemia (falsely raises HbA1c)
- Splenectomy (longer RBC lifespan = more glycation)
Low HbA1c Causes
- Hemolytic anemia (shorter RBC lifespan = less glycation — falsely low HbA1c)
- Sickle cell disease and thalassemia
- Recent blood transfusion (donor cells not glycated)
- Excellent diabetes management
- Hypoglycemia risk (treating diabetes too aggressively)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required — this is a key advantage of HbA1c over fasting glucose. Can be drawn at any time of day. Inform your doctor of any blood transfusions or conditions affecting red blood cell lifespan.
Factors That Can Affect Results
- Hemolytic anemia (falsely low — RBCs are destroyed before they can accumulate glycation)
- Iron deficiency anemia (falsely high — RBCs live longer, more time for glycation)
- Blood transfusion (dilutes glycated cells, falsely lowering HbA1c)
- Hemoglobin variants (HbS in sickle cell, HbC, HbE) — interfere with some HbA1c assays
- Chronic kidney disease (accumulation of carbamylated hemoglobin interferes with some assays)
- Pregnancy (falsely lowered by increased RBC turnover)
Related Topics
Frequently Asked Questions
What HbA1c level is considered good control of diabetes?
The ADA recommends an HbA1c below 7% (53 mmol/mol) for most non-pregnant adults with diabetes. However, targets are individualized: a target below 6.5% may be appropriate for newly diagnosed patients with long life expectancy; a target of 7.5–8% may be more appropriate for older adults with comorbidities, limited life expectancy, or hypoglycemia unawareness. The key is balancing glucose control against hypoglycemia risk.
Can HbA1c be used to diagnose diabetes in patients with anemia?
HbA1c is unreliable in conditions that alter red blood cell lifespan. In hemolytic anemia or sickle cell disease, RBCs are destroyed earlier, giving less time for glycation — HbA1c is falsely low. In iron deficiency anemia, RBCs live longer — HbA1c is falsely high. In these situations, fasting glucose or a 2-hour oral glucose tolerance test (OGTT) should be used for diagnosis.