Overview
Glycosylated haemoglobin (HbA1c) is formed when glucose molecules in the blood bind non-enzymatically to haemoglobin. The extent of this binding remains directly proportional to the blood glucose concentration.
Since the average lifespan of red blood cells is about three months, HbA1c reflects the average blood glucose levels over the previous 2–3 months. It is widely used to monitor long-term glycaemic control in people with diabetes. HbA1c (Glycosylated Haemoglobin)
Biochemical Basis
Glycosylated haemoglobin is formed through irreversible non-enzymatic glycation of the beta chain of haemoglobin A. This process continues throughout the lifespan of red blood cells. Because the degree of glycation depends on circulating glucose levels, HbA1c serves as a reliable biochemical marker for long-term blood sugar control.
Pathophysiology
Glycosylated haemoglobin increases the production of reactive free radicals within blood cells. This alters cell membrane properties, leading to increased blood viscosity and impaired blood flow.
Once formed, HbA1c remains in circulation for the entire lifespan of red blood cells. Poorly controlled diabetes results in elevated HbA1c levels, which are associated with an increased risk of complications such as neuropathy, retinopathy, cardiovascular disease, and nephropathy.
Advantages
Glycosylated haemoglobin testing does not require fasting and can be performed at any time of the day. It provides a long-term assessment of blood glucose levels over 2–3 months and is standardized, reproducible, and more reflective of overall glycaemic control than single blood glucose measurements.
Indications
Glycosylated haemoglobin estimation is used to diagnose diabetes and to assess overall diabetic control. It helps manage known diabetics, evaluate suspected diabetes, monitor patients every three months, and assess the risk of diabetes-related complications.
Sample Collection
Fasting is not required for HbA1c testing. A 2.0 ml blood sample is collected in an EDTA (lavender-capped) tube at any time. The sample can be transported at ambient temperature and refrigerated for up to seven days if required.
Methods of Estimation
Several methods are available for HbA1c estimation, including high-performance liquid chromatography (HPLC), immunoassays, capillary electrophoresis, and boronate affinity chromatography. Each method has its own advantages and limitations, and selection depends on laboratory facilities and clinical needs.
Reference Range and Interpretation
Glycosylated haemoglobin values below 5.7% are considered normal. Values between 5.7% and 6.4% indicate prediabetes, while values above 6.5% are diagnostic of diabetes. Higher HbA1c levels reflect poorer glycaemic control and increased risk of complications.
Clinical Significance
HbA1c is a key marker for long-term glycaemic monitoring and is used to guide treatment decisions. It helps predict the risk of diabetic complications and allows assessment of treatment efficacy. However, conditions affecting red blood cell turnover or haemoglobin structure can influence test accuracy.
HbA1c and Fructosamine
HbA1c reflects long-term glucose control, whereas fructosamine reflects short-term glycaemic status over 2–3 weeks. Fructosamine testing is useful when HbA1c results are unreliable, such as in conditions affecting red blood cells.
Limitations
HbA1c does not reflect short-term fluctuations in blood glucose. Variations due to age, race, and certain medical conditions may affect accuracy. Conditions such as haemoglobinopathies, haemolytic anaemia, chronic renal failure, and pregnancy can interfere with HbA1c interpretation.
