Glycated Hemoglobin

Glycated hemoglobin or glycosylated hemoglobin ( HbA1c ) is a blood test that accurately measures average blood glucose for the past three months. In diabetic patients, glucose is slowly attached to hemoglobin molecules by covalent bonds making Hb A1c fraction. This fraction can be measured by chromatography. HbA1c is expressed as a percentage of the normal hemoglobin. HbA1c can be used as a diagnostic test for diabetes mellitus and as an assessment test for glycemic control in people with diabetes. The test gives a three months average. The average lifespan of a red blood cell is four months. Individual red blood cells have varying lifespans, so an average of three months is taken. The normal range is 4-7 % depending on the technique of measurement.

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For people who have diabetes, the test is advised to see how well diabetes has been controlled over the last few months. People with diabetes are advised to have this test every 3 to 6 months, or more frequently if it is not under control. A higher value of Hb A1c indicates poorer control of blood glucose levels.

HbA1c and blood glucose levels

Normal, Prediabetic, and Diabetic range

HbA1c (DCCT Unit) Diabetic Category
Below 5.7 %Normal
5.7- 6.4 %Prediabetes
6.5 % or higherDiabetes

Most laboratories report the HbA1c values as percentages. IFCC (International Federation of Clinical Chemistry and Laboratory Medicine) has developed a standard method by which values are reported in mmol/mol.

Conversion between DCCT (Diabetes Control and Complication Trials) and IFCC units.

DCCT Units (%) IFCC Units (mmol/mol)
420
531
642
753
864
975
1086

HbA1c criteria for Prediabetes and Diabetes:

HbA1c (IFCC units) Diabetic Criteria
39-47 mmol/molPrediabetes
Equal or greater to 48 mmol/mol Diabetes

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Certain conditions make the HbA1c test unreliable

Results can be unreliable in some circumstances, for example after blood loss or surgery, blood transfusions, anemia, pregnancy or high erythrocyte turnover, in the presence of chronic renal failure or liver disease, after administration of high-dose vitamin C or Erythropoietin treatment.

Lower than expected levels of HbA1c can be seen in people with hemoglobinopathies with abnormal hemoglobin or if the lifespan of the red cells is reduced, such as with glucose-6-phosphate dehydrogenase deficiency, sickle-cell disease, or any other condition causing premature red blood cell death. Blood donation will result in the rapid replacement of lost RBCs with newly formed red blood cells. Since these new RBCs will have only existed for a short period of time, their presence will lead HbA1c to underestimate the actual average levels.

Conversely, higher than expected levels can be seen in people with a longer red blood cell lifespan, such as with vitamin B12 or folate deficiency.

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AUGUST 19, 2021

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