Diabetes Mellitus (DM) is a metabolic disorder characterized by chronic hyperglycemia (increased blood glucose levels). The long–term DM may lead to the development of retinopathy, nephropathy and increased risk of cardiac, peripheral arterial and cerebrovascular diseases. World Health Organisation's 2018 report says diabetes was the seventh leading cause of death in 2016.

Before 1968, diagnosis and monitoring of diabetes have always been glucose-centric, based on blood glucose values.  in 1968, Rahbar discovered a subtype of hemoglobin (coined as HbA1c) and found that it was elevated in people with diabetes. However, a lack of standardized procedures discouraged the utility of this test. 

What is HbA1c and how is it formed?

HbA1c, also termed as Glycated hemoglobin/Glycohemoglobin, is formed under physiological conditions by the Nonenzymatic glycation of specific sites of Hemoglobin.

Blood contains millions of Red Blood cells (RBC). A single RBC contains approximately 270 million molecules of the hemoglobin protein. Hemoglobin is a tetramer having four units of Globin, two alpha units (α) and two Non-Alpha (β, γ or δ) units. In adults, hemoglobin is a mixture of three forms: HbA0 (α2 ß2) 97%,  HbA2(α2δ2) 1-3%   and Hb F (α2γ2) 0.2-0.8%.

The RBC in the blood are continuously exposed to glucose in the blood, which can easily pass through the membrane. The glucose molecules are attached to hemoglobin molecules (Glycation) at a specific site, N-terminal Valine of the beta chain leading to the formation of Glycated Hemoglobin. 

As the formation of HbA1c is concentration dependent,  higher blood Glucose levels lead to raised HbA1c formation. This is the reason why HbA1c is treated as a marker of Glycemic (blood glucose) status of an individual over a period of the last 90-120 days, which is the average life span of RBC.       

Extensive studies by DCCT(Diabetes Control and Complications Trial) / UKPDS (United Kingdom Prospective Diabetes Study) trials showed a correlation between A1c  and Microvascular complications in chronic Diabetic Patients. 

Though in 1997 expert Committee recommended against using A1C values for diagnosis in part because of the lack of assay standardization, in 2009 ADA recommended that HbA1c can be adopted as one of the diagnostic criteria for diabetes.

Over a period of decades incessant efforts of international committees led to the standardization of HbA1c methods and establishment of reference values. 

What is the reference Range?

 The reference range for HbA1c is 4 - 6.5 %. Value >6.5% is the criteria for the diagnosis of diabetes. ADA suggested that people with HbA1c values between 5.7-6.4% be categorized as a high-risk group for diabetes and called it as  "Prediabetes" stage.

 Measurement of HbA1c

The A1C test should be performed using a method that is certified by NGSP  and DCCT reference assays.  At present, the following methods are in use:

1.HPLC (High-Performance Liquid Chromatography)  which is considered as a  Gold standard.

2.Immuno-turbimetric method 

3.Affinity chromatography                                                                                        

4.Electrophoretic methods

5.Methods based on chemical reactions-Enzymatic

Care should be taken while interpreting the results as many factors affect the results. Acute blood loss, blood transfusion, severe anemia, abnormal hemoglobins and the conditions where RBC survival is shortened or lengthened are some of the factors which interfere with HbA1c measurement and/or interpretation. 

  Advantages of HbA1c test over Blood Glucose Test

• It is a better index of overall glycemic exposure for over 90-120 days and risk for long-term complications. 

• Biologic variabilities like age and gender have less impact on the result. 

• The patient need not fast before the test. It is relatively unaffected by acute (in conditions like stress or illness related) perturbations in glucose levels.

However, HbA1c test has its own limitations. Besides being expensive, it cannot be an emergency room test to titrate insulin or OHA dosage. It cannot register hypoglycemia as it expresses the "average" Glucose.

Mean Plasma Glucose can be calculated from HbA1c % as follows 

Mean Plasma Glucose  = (33.3 x  HbA1C%) - 86

ADA  Recommendations

Perform the A1C test at least two times a year in patients who are meeting treatment goals (and who have stable glycemic control) and quarterly in patients whose therapy has changed or who are not  meeting glycemic goals.