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The term glycated hemoglobin refers to hemoglobin molecule that has been modified by non enzymatic addition of glucose moiety.

The term “ HbA1c “ specifically refers to hemoglobin molecule that has been modified by non enzymatic addition of a glucose moiety to the N terminal of its beta chain.

Glycated haemoglobin provides an accurate and objective measure of glycaemic control over 8-12 weeks. In, diabetes the rate of formation of HbA_1c is directly proportional to the ambient blood glucose level. A rise in 1% HbA_1c corresponds to approx 36mg/dl in blood glucose.

HbA1c reflects average plasma glucose over the previous 2–3 months in a single measure which can be performed at any time of the day, irrespective of meal intake, exercise, emotional stress just administered diabetic drugs or doesn't require any special preparation such as fasting. These properties have made it the gold standard for assessing glycaemic control in people with diabetes.

GLYCATED HAEMOGLOBIN (HbA1c) FOR THE DIAGNOSIS OF DIABETES

Latest WHO Recommendation :

HbA1c can be used as a diagnostic test for diabetes providing that stringent quality assurance tests are in place and assays are standardised to criteria aligned to the international reference values, and there are no conditions present which preclude its accurate measurement.

An HbA1c of 6.5% is recommended as the cut off point for diagnosing diabetes. A value of less than 6.5% does not exclude diabetes diagnosed using glucose tests.

Levels of HbA1c just below 6.5% may indicate the presence of intermediate hyperglycaemia. The precise lower cut-off point for this has yet to be defined, although the ADA has suggested 5.7 – 6.4% as the high risk range.

HOW OFTEN TO DO THE TEST ?

• Perform HbA1c test at least 2 times a year in patient who are meeting treatment goals.

• Perform HbA1c test quarterly in patient whose glycemic goals are not met.

DRAWBACKS

False high values are seen in iron, vitamin B12 deficiency, decreased erythropoiesis, alcoholism, chronic renal failure, Splenectomy, hyperbilirubinaemia , carbamylated haemoglobin.

False low values are seen in administration of erythropoietin, iron, vitamin B12, reticulocytosis, chronic liver disease, haemoglobinopathies, splenomegaly.

CLINICAL VALUE OF A1C

• It is used as a diagnostic tool .

• It helps to remove discrepancies between SMBG and actual degree of overall blood glucose control.

• Helps to differentiate stress induced transient glucose tolerance from true diabetes.

REFERENCE

• Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus ; Abbreviated Report of a WHO Consultation

• Davidson's Principles & Practice of Medicine 21^st edition.

• ADA Guidelines 2012.

Foot note:This has helped me to understand that HbA1c is a major tool for assessing patients long term glycaemic control but also can be now used for diagnosing diabetes.