Glycated Hemoglobin (HbA1c)

The term HbA1c refers to glycated hemoglobin. It develops when hemoglobin, a protein within red blood cells that carries oxygen throughout your body, joins with glucose in the blood, becoming ‘glycated’. Analysis of glycated hemoglobin (HbA1c) in blood provides evidence about an individual’s average blood glucose levels during the previous two to three months, which is the predicted half-life of red blood cells (RBCs). The HbA1c is now recommended as a standard for testing and monitoring diabetes.

Historically, HbA1c was first isolated by Huisman in 1958 and characterized by Bookchin and Gallop in 1968, as a glycoprotein. The elevated levels of HbA1c in diabetic patients were reported by Rahbar in 1969. Bunn et al. identified the pathway leading to the formation of HbA1c in 1975. Using the HbA1c as a biomarker for monitoring the levels of glucose among diabetic patients was first proposed by Koenig et al. in 1976.

Proteins are frequently glycated during various enzymatic reactions when the conditions are physiologically favorable. However, in the case of hemoglobin, the glycation occurs by the nonenzymatic reaction between the glucose and the N-terminal end of the ß-chain, which forms a Schiff base. In the primary step of glycated hemoglobin formation, hemoglobin and the blood glucose interact to form aldimine in a reversible reaction. In the secondary step, which is irreversible, aldimine is gradually converted into the stable ketoamine form.

The major sites of hemoglobin glycosylation, in the order of prevalence, are ß-Val-1, ß-Lys-66, and a-Lys-61. Normal adult hemoglobin consists predominantly of HbA (a2ß2), HbA2 (a2d2), and HbF (a2?2) in the composition of 97%, 2.5%, and 0.5%, respectively. About 6% of total HbA is termed HbA1, which in turn is made up of HbA1a1, HbA1a2, HbA1b, and HbA1c fractions, defined by their electrophoretic and chromatographic properties. HbA1c is the most abundant of these fractions and in health comprises approximately 5% of the total HbA fraction.

As mentioned above, glucose in the open chain format binds to the N-terminal to form an aldimine before undergoing an Amadori rearrangement to form a more stable ketoamine. This is a nonenzymatic process that occurs continuously in vivo. The formation of the glycated hemoglobin is a normal part of the physiologic function cycle. However, as the average plasma glucose increases, so does the amount of glycated hemoglobin in the plasma. This specific characteristic of the hemoglobin biomarker is utilized for estimating the average blood glucose levels over the previous two to three months.

So, we have described the current trends in diabetes prevalence, diagnostic and prognostic potential of HbA1c, analytical aspects in HbA1c assays, and physiological changes due to hemoglobin glycation.

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