# Trends in Quantification of HbA1c Using Electrochemical and Point-of-Care Analyzers
## 1 Introduction  
Diabetes is a metabolic disorder manifested through elevated blood glucose levels and is commonly associated with complications such as microvascular and cardiovascular disorders, vision impairment (retinopathy), kidney dysfunction (nephropathy), peripheral nerve disorders (neuropathy), etc. [1; 2; 3; 4; 5]. The global prevalence of diabetes nearly quadrupled from 1980 to 2014, and the current estimates by the International Diabetes Federation suggest over 537 million diabetic individuals worldwide [6]. These numbers are expected to further increase to 643 million by the year 2030 [6; 7]. Unhealthy lifestyle and diet, as well as genetic causes, have been mainly implicated in the onset of diabetes. Late diagnosis of diabetes can be life-threatening. Early detection of diabetes can lead to lesser organ damage by introducing non-pharmacological interventions in the form of lifestyle modifications.  
Random blood glucose is a direct measure of glucose levels in the blood and has been traditionally used to obtain information on the glycemic status of an individual. However, glucose levels are prone to fluctuations owing to numerous factors, such as diet and physical and emotional stress [8]. Hence, there is a need for a more stable and reliable marker for diabetes. Glycated hemoglobin (HbA1c), a stable compound belonging to the class of advanced glycation end products (AGE), is formed as a result of the non-enzymatic addition of various sugars to hemoglobin (Hb). It comprises about 5% of the total Hb in human blood. HbA1c is further classified based on the sugar adducts attached to Hb. The HbA1a1 form has fructose-1,6-diphosphate as the substituent, HbA1a2 has glucose-6-phosphate, HbA1b has pyruvate, while HbA1c has glucose as the substituent. Amongthese, HbA1c is the most abundant form, formed by a non-enzymatic reaction, and reflects the glycemic condition of the individual over 2 to 3 months [9; 10]. Hence, it serves as a stable biomarker for diabetes in blood, unlike glucose [9; 11].  
HbA1c is formed in a two-step non-enzymatic reaction, where glucose adds to the valine residue in the \(\beta\)-chain of Hb, forming a Schiff's base, which is then rearranged yielding HbA1c, an Amadori product [10; 12] (Figure 1). The extent of glycation may vary, with the number of glycated sites up to 15 being reported [13]. Clinically, HbA1c levels are usually expressed as a percentage of total Hb to account for the variations in the Hb levels among individuals. To facilitate user-friendly interaction, a linear relationship between HbA1c and equivalent average glucose (AG) levels was proposed by the A1c-Derived Average Glucose (ADAG) Study Group using the following relation [14; 15]:  
\[\text{eAG}\left(\frac{\text{mg}}{\text{dL}}\right)=(28.7*\text{HbA1c\%})-46.7\]  
The normal range of HbA1c in healthy individuals lies between 4 and 6.0%, while in severe hypergly conditions, HbA1c values can go up to 20%. In general, individuals with HbA1c values in the range of 5.7-6.4% are classified in the prediabetic stage, while the individuals having HbA1c > 6.5% are diabetic [16; 17]. During the recent pandemic, it was shown that diabetic individuals with HbA1c levels > 8.6% had a higher risk of life-threatening complications when infected with the SARS-CoV2 coronavirus, thereby necessitating the need for a robust and rapid detection strategy for HbA1c to screen the large numbers of infected individuals [18; 19; 20; 21; 22].