Q: What is the HbA1c cut-off to diagnose diabetes?

According to WHO and ADA criteria, diabetes is diagnosed when HbA1c ≥6.5% (≥48 mmol/mol) on two separate occasions, or once with confirmed symptoms or other diagnostic criteria. Prediabetes is defined as HbA1c 5.7–6.4% (39–47 mmol/mol) by ADA, or 6.0–6.4% (42–47 mmol/mol) by WHO.

Q: What conditions cause falsely high or low HbA1c?

Falsely HIGH HbA1c: iron deficiency anaemia, vitamin B12 deficiency, splenectomy, alcoholism, chronic kidney disease (uraemia). Falsely LOW HbA1c: haemolytic anaemia, haemoglobin variants (HbS, HbC, HbE — common in India and Africa), pregnancy (increased red cell turnover), recent blood transfusion, erythropoietin therapy. In these conditions, fructosamine or continuous glucose monitoring may be more reliable.

Q: What is the difference between DCCT and IFCC HbA1c?

DCCT (%) is the older unit used primarily in the USA and many Asian countries including India. IFCC (mmol/mol) is the newer internationally standardised unit adopted by Europe, Australia, and WHO. The two scales are related but not linearly identical. Most modern analysers report both. The conversion formula is: IFCC (mmol/mol) = (DCCT% − 2.15) × 10.929.

Q: How often should HbA1c be tested?

In patients with well-controlled diabetes meeting targets, HbA1c should be tested every 6 months. In patients with poorly controlled diabetes or those whose treatment has been recently changed, every 3 months. For diagnosis and high-risk prediabetes screening, annually. Note that HbA1c reflects average glucose over 2–3 months, so testing more frequently than every 3 months is rarely useful.

Question 1

What is HbA1c and why is it measured?

Accepted Answer

HbA1c (glycated haemoglobin or glycosylated haemoglobin) measures the percentage of haemoglobin that has glucose attached to it. Because red blood cells live approximately 90–120 days, HbA1c reflects average blood glucose over the past 2–3 months. It is the gold standard for monitoring long-term glycaemic control in diabetes and is also used to diagnose diabetes (≥6.5% / ≥48 mmol/mol) and prediabetes (5.7–6.4% / 39–47 mmol/mol).

Question 2

How do I convert HbA1c % to mmol/mol?

Accepted Answer

To convert HbA1c from DCCT % to IFCC mmol/mol, use the formula: mmol/mol = (HbA1c% − 2.15) × 10.929. For example, HbA1c 7% = (7 − 2.15) × 10.929 = 53 mmol/mol. To convert back: HbA1c% = (mmol/mol ÷ 10.929) + 2.15.

Question 3

What is eAG (estimated average glucose)?

Accepted Answer

eAG is the estimated average blood glucose level corresponding to an HbA1c value, expressed in mg/dL or mmol/L. Formula: eAG (mg/dL) = (28.7 × HbA1c%) − 46.7. Convert to mmol/L by dividing by 18.016. For example, HbA1c 7% corresponds to eAG ~154 mg/dL (~8.6 mmol/L).

Question 4

What is the target HbA1c for type 2 diabetes?

Accepted Answer

For most adults with type 2 diabetes, the ADA and IDF recommend an HbA1c target of <7% (<53 mmol/mol). A less stringent target of <8% (<64 mmol/mol) is acceptable for elderly patients, those with limited life expectancy, severe hypoglycaemia risk, or multiple comorbidities. More stringent targets (<6.5%) may be appropriate for newly diagnosed patients without cardiovascular disease.

Question 5

What is the HbA1c cut-off to diagnose diabetes?

Accepted Answer

According to WHO and ADA criteria, diabetes is diagnosed when HbA1c ≥6.5% (≥48 mmol/mol) on two separate occasions, or once with confirmed symptoms or other diagnostic criteria. Prediabetes is defined as HbA1c 5.7–6.4% (39–47 mmol/mol) by ADA, or 6.0–6.4% (42–47 mmol/mol) by WHO.

Question 6

What conditions cause falsely high or low HbA1c?

Accepted Answer

Falsely HIGH HbA1c: iron deficiency anaemia, vitamin B12 deficiency, splenectomy, alcoholism, chronic kidney disease (uraemia). Falsely LOW HbA1c: haemolytic anaemia, haemoglobin variants (HbS, HbC, HbE — common in India and Africa), pregnancy (increased red cell turnover), recent blood transfusion, erythropoietin therapy. In these conditions, fructosamine or continuous glucose monitoring may be more reliable.

Question 7

What is the difference between DCCT and IFCC HbA1c?

Accepted Answer

DCCT (%) is the older unit used primarily in the USA and many Asian countries including India. IFCC (mmol/mol) is the newer internationally standardised unit adopted by Europe, Australia, and WHO. The two scales are related but not linearly identical. Most modern analysers report both. The conversion formula is: IFCC (mmol/mol) = (DCCT% − 2.15) × 10.929.

Question 8

How often should HbA1c be tested?

Accepted Answer

In patients with well-controlled diabetes meeting targets, HbA1c should be tested every 6 months. In patients with poorly controlled diabetes or those whose treatment has been recently changed, every 3 months. For diagnosis and high-risk prediabetes screening, annually. Note that HbA1c reflects average glucose over 2–3 months, so testing more frequently than every 3 months is rarely useful.

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About HbA1c — Interpretation & Diabetes Targets