HbA1c Converter

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).

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.

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).

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.

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.

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.

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.

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.

Both measure glycated haemoglobin but use different reference standards. DCCT % (also called NGSP) is the traditional percentage format used in the USA, India, and most of Asia. IFCC mmol/mol is the newer SI-unit format adopted in the UK, Europe, and Australia. The two scales are not interchangeable by simple multiplication — the conversion formula is: mmol/mol = (% − 2.15) × 10.929. Indian laboratories report HbA1c in DCCT % under ADA and RSSDI guidelines.

Per ADA 2024 and RSSDI guidelines, an HbA1c of ≥6.5% (48 mmol/mol) on two separate occasions confirms diabetes mellitus. Prediabetes is 5.7–6.4% (39–47 mmol/mol). Below 5.7% is considered normal.

RSSDI and ADA recommend an HbA1c target of <7.0% (53 mmol/mol) for most adults with type 2 diabetes. Targets should be individualised: <6.5% for younger patients with short disease duration, no hypoglycaemia risk, and no cardiovascular disease; <8.0% for elderly patients, those with significant comorbidities, hypoglycaemia unawareness, or limited life expectancy.

HbA1c is unreliable in haemolytic anaemia, sickle cell disease (HbS), HbC disease, malaria, and recent blood transfusion (falsely low). It is falsely high in iron deficiency anaemia, vitamin B12 deficiency, and splenectomy. In CKD, both effects occur depending on severity. In these situations, fructosamine (reflects 2–3 week average) or glycated albumin is preferred. During pregnancy, use fasting and post-prandial glucose or CGMS instead.