Summary
HbA1c (glycated haemoglobin) measures the percentage of haemoglobin that has glucose attached to it. Because red blood cells live for approximately 90 days, HbA1c reflects your average blood sugar over the past 2–3 months — making it far more informative than a single fasting glucose. It is used to diagnose type 2 diabetes, identify prediabetes, and monitor glycaemic control in known diabetics.
Unlike fasting glucose, HbA1c does not require fasting, is not affected by a single meal, and is not influenced by short-term changes in diet or stress. This stability makes it the preferred screening and monitoring marker for type 2 diabetes globally.
In the UK, HbA1c is reported in mmol/mol (IFCC units): a level below 42 mmol/mol is normal; 42–47 mmol/mol defines prediabetes; 48 mmol/mol and above on two separate occasions diagnoses type 2 diabetes. Earlier detection and intervention at the prediabetes stage can prevent or significantly delay progression to diabetes.
HbA1c has important limitations: conditions that shorten red blood cell lifespan (haemolytic anaemia, sickle cell disease, iron deficiency) give falsely low results; conditions that lengthen red cell lifespan can give falsely high results.
What It Is
Haemoglobin A1c is formed by the non-enzymatic glycation of the N-terminal valine residue of the haemoglobin beta-chain (Hb A1c, or HbA1c). This reaction between glucose and haemoglobin is irreversible and proceeds at a rate directly proportional to the ambient glucose concentration throughout the lifespan of the red blood cell (average 90–120 days).
The resulting HbA1c level therefore provides a weighted average of blood glucose over the preceding 2–3 months, with the most recent 30 days contributing approximately 50% of the result. Standardised to the IFCC reference method, values are reported in mmol/mol in the UK.
HbA1c is also used to calculate estimated average glucose (eAG): eAG (mmol/L) = (HbA1c × 0.0716) − 0.0946.
Functions
Long-term glucose averaging
Provides a 90-day weighted average of blood glucose — far more representative than any single glucose measurement.
Diabetes diagnosis
HbA1c ≥ 48 mmol/mol on two occasions in the absence of symptoms, or once with symptoms, diagnoses type 2 diabetes.
Prediabetes detection
HbA1c 42–47 mmol/mol identifies prediabetes — a stage where lifestyle intervention can prevent or delay diabetes.
Complication risk predictor
Every 1% (11 mmol/mol) rise in HbA1c increases the risk of retinopathy, nephropathy, neuropathy, and cardiovascular events.
Reference Ranges
HbA1c — Glycated Haemoglobin
Measured in mmol/mol| Status | Range (mmol/mol) | Range (%) | What it means |
|---|---|---|---|
| Normal | < 42 | < 6.0% | Normal blood glucose regulation — diabetes is unlikely. |
| Prediabetes | 42–47 | 6.0–6.4% | Prediabetes — significant risk of type 2 diabetes. Lifestyle intervention is effective at this stage. |
| Diabetes | ≥ 48 | ≥ 6.5% | Consistent with type 2 diabetes — confirm with a second test and seek medical review. |
A single HbA1c ≥ 48 mmol/mol in a symptomatic person diagnoses diabetes. In asymptomatic people, a second confirmatory test is required. Some conditions (haemolytic anaemia, iron deficiency, haemoglobinopathies) give unreliable HbA1c results.
Symptoms of Imbalance
Prediabetes is usually asymptomatic. Type 2 diabetes symptoms develop gradually as blood glucose climbs.
- Very low HbA1c (< 30 mmol/mol) may indicate haemolytic anaemia or over-treatment with insulin
- Hypoglycaemia symptoms: shakiness, sweating, confusion, palpitations
- Increased thirst (polydipsia)
- Frequent urination (polyuria)
- Unexplained weight loss
- Fatigue and lack of energy
- Blurred vision
- Slow-healing wounds and cuts
- Frequent infections (skin, urinary, thrush)
Causes of Imbalance
- Over-treatment with insulin or oral hypoglycaemics
- Haemolytic anaemia (shortened red cell lifespan gives falsely low result)
- Iron deficiency anaemia (falsely lowers HbA1c)
- Sickle cell disease or other haemoglobinopathies
- Type 2 diabetes (most common cause)
- Type 1 diabetes
- Impaired glucose tolerance / prediabetes
- Gestational diabetes (past or current)
- Prolonged corticosteroid use
- Pancreatic disease (pancreatitis, pancreatectomy)
- Conditions with prolonged red cell lifespan (iron deficiency if chronic)
FAQs
No — HbA1c does not require fasting. Because it reflects average glucose over 2–3 months rather than a single moment, it is not affected by what you ate that morning. This makes it far more convenient than fasting glucose as a screening test.
NICE recommends an HbA1c target of 48–53 mmol/mol (6.5–7.0%) for most people with type 2 diabetes on medication. Tighter targets (42–48 mmol/mol) may be appropriate for those managed by diet alone or at very high cardiovascular risk. Targets should be individualised — overly aggressive control increases hypoglycaemia risk, particularly in older adults.
Yes — and this is encouraging. Because HbA1c reflects a 90-day average, you can see meaningful improvement within 3 months of sustained dietary change, increased physical activity, and weight loss. A reduction in HbA1c from the prediabetes range to normal is achievable with intensive lifestyle intervention — the evidence from the Diabetes Prevention Programme is compelling.
An HbA1c of 44 mmol/mol is in the prediabetes range (42–47 mmol/mol). It means your average blood sugar has been higher than normal for the past 2–3 months. This is genuinely important — prediabetes carries a 5–10% annual risk of progression to type 2 diabetes without intervention. However, it is also highly reversible: sustained lifestyle changes (weight loss of 5–10%, increased physical activity, reduced refined carbohydrates) can normalise HbA1c in the majority of people.
HbA1c measures glucose attachment to red blood cells over their 90-day lifespan. Any condition that shortens red cell lifespan will give a falsely low result — including haemolytic anaemia, iron-deficiency anaemia (which increases red cell turnover), sickle cell trait, and thalassaemia. In these situations, fasting glucose, a 2-hour oral glucose tolerance test (OGTT), or continuous glucose monitoring provide more reliable information.
References
- WHO. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus. WHO/NMH/CHP/CPM/11.1. 2011. View source
- NICE. Type 2 diabetes in adults: management. NG28. Updated 2022. View source
- Knowler WC, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393–403. View source