Pathophysiology of HbA1c
Proteins are frequently glycated during various enzymatic reactions when
the conditions are physiologically favourable. However, in the case of
hemoglobin A, the glycation occurs by the nonenzymatic reaction between
the condensation of glucose and the N-terminal end of the β-chain of
hemoglobin A, commonly lysine, forming a Schiff base aldimine7. The Schiff base may then undergo a rearrangement,
converting into a stable Amadori product, otherwise known as HbA1c8. The formation of glycated hemoglobin is a routine
part of the physiologic function cycle. It was found that as the average
plasma glucose increases, so does the rate of this glycation reaction
and the total quantity of HbA1c produced in the
plasma9. The longer hyperglycaemia occurs in blood,
the more glucose binds to hemoglobin in the red blood cells. This
results in increased rates of production of glycated hemoglobin. Once
a hemoglobin molecule is glycated, it remains that way and its Amadori
arrangement is viewed as “nearly irreversible” according to a study
from Higgins and Bunn 10. As a result, this specific
characteristic of the hemoglobin is utilized as a biomarker, estimating
a humans average blood glucose levels over the previous 2 to 3
months9.
The glycation of hemoglobin results in the formation of advanced
glycation end products (AGEs). Alongside their formation is the
accompanied release of free radicals and oxidants as side products from
the Amadori rearrangement. These result in oxidative damage to cells and
the extracellular matrix of the body tissue 11. It is
found that additionally, the accumulation of these free radicals alters
erythrocyte membrane properties leading to erythrocyte aggregation,
increased blood viscosity and impaired blood flow 12.
This can result in shear stress, due to thicker abrasive blood
consistency which affects the vascular endothelium can result in a
number of inflammatory and atherogenic events should the levels become
excessive 13. Following a consultation from the World
Health Organization (WHO), it was concluded that HbA1c can be used as a
viable diagnostic test for diabetes. This was as long as “stringent
quality assurance tests are in place” and assays are kept consistent
and standardised to their stated criteria, which are aligned to their
internationally set reference values 14. It is also
important that there are no conditions present which interfere with the
accuracy of its measurement. An HbA1c of 6.5% is recommended as the cut
point for diagnosing diabetes, according to guidelines set by the World
Health Organization 14. A value less than 6.5% does
not exclude diabetes diagnosed using glucose tests 14.
According to the American Diabetes Association (ADA) Guidelines 2020,
the value of HbA1c should be kept below 7% in all non-pregnant adult
diabetics (53 mmol/mol) 15. Values greater than 7%
indicate an increased chance of progression to diabetic complications,
especially microvascular ones. The HbA1c levels appear to be closely
related to the blood glucose levels and are resultantly affected by any
forms of glycaemic control. An initially raised HbA1c levels has been
found to progressively decrease in the weeks following the introduction
of insulin and dietary therapy; with a tendency for these values to
level out after approximately seven weeks of
therapy16.