Glycated Hemoglobin

AUGUST 19, 2021

Glycated hemoglobin, or glycosylated hemoglobin ( HbA1c ) is a blood test that provides an accurate measure of average blood glucose for the past three months. In diabetes slow non-enzymatic covalent attachment of glucose to hemoglobin increases the amount in HbA1c fraction. This fraction can be measured by chromatography. HbA1c expressed as 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 is limited to three-months average. The average lifespan of a red blood cell is four months. Individual red blood cells have varying lifespans, so the average three months is taken. Normal range is 4-7 % depending on the technique of measurement. This test provides an index of the average blood glucose concentration over the life of the hemoglobin molecule.


For people who have diabetes, the test is advised to see how well the 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. Higher amounts of glycated hemoglobin, indicating 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 HbA1c value as percentage. 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)

HbA1c criteria for Prediabetes and Diabetes:

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

Certain conditions make HbA1c test unreliable

Results can be unreliable in some circumstances, for example after blood loss, after 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 red cell 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 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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