HbA1c and estimated average Glucose

What is HbA1c?

Hemoglobin is the oxygen-carrying pigment that gives blood its red color and also the predominant protein in red blood cells. About 90% of hemoglobin is hemoglobin A. (The "A" stands for adult type) Hemoglobin A1c (HbA1c) is a minor component of hemoglobin A to which glucose is bound. HbA1c also is referred to as glycosylated or glucosylated hemoglobin.

In the normal 120-day life span of the red blood cell, glucose molecules join hemoglobin, forming glycated hemoglobin. Higher the amount of glucose in blood, more glucose gets attached to Hemoglobin there by increasing the percent of HbA1c.

Once a hemoglobin molecule is glycated, it remains that way. A buildup of glycated hemoglobin within the red cell reflects the average level of glucose to which the cell has been exposed during its life cycle. Measuring glycated hemoglobin assesses the effectiveness of therapy by monitoring long-term serum glucose regulation. The HbA1c level is proportional to average blood glucose concentration over the previous four weeks to three months.[ HbA1c is most dependent on the blood glucose values of last 4 weeks.So it is not a very true average of 3 months]





How is HbA1c measured?

A chemical (electrical) charge is present on the molecule of HbA1c, and the amount of the charge differs from the charges on the other components of hemoglobin. The molecule of HbA1c also differs in size from the other components. HbA1c may be separated by charge and size from the other hemoglobin A components in blood by a procedure called high pressure (or performance) liquid chromatography (HPLC). HPLC separates mixtures (e.g., blood) into its various components by adding the mixtures to special liquids and passing them under pressure through columns filled with a material that separates the mixture into its different component molecules. Because HbA1c is not affected by short-term fluctuations in blood glucose concentrations, for example, due to meals, blood can be drawn for HbA1c testing without regard to when food was eaten.

Why measure HbA1c?

HbA1c levels depend on the blood glucose concentration. That is, the higher the glucose concentration in blood, the higher the level of HbA1c. Levels of HbA1c are not influenced by daily fluctuations in the blood glucose concentration but reflect the average glucose levels over the prior six to twelve weeks. Therefore, HbA1c is a useful indicator of how well the blood glucose level has been controlled in the recent past and may be used to monitor the effects of diet, exercise, and drug therapy on blood glucose in diabetic patients.

In healthy, non-diabetic patients the HbA1c level is less than 6% of total hemoglobin. It has been demonstrated that the complications of diabetes can be delayed or prevented if the HbA1c level can be kept close to 7%.

What are the limitations to measuring HbA1c?

Since HbA1c is not influenced by daily fluctuations in blood glucose concentration, it cannot be used to monitor day-to-day blood glucose concentrations and to adjust insulin doses nor can it detect the day-to-day presence or absence of hyperglycemia or hypoglycemia.

HbA1c may be increased falsely in certain medical conditions. These conditions include uremia (kidney failure), chronic excessive alcohol intake, and hypertriglyceridemia. Medical conditions that may falsely decrease HbA1c include acute or chronic blood loss, sickle cell disease or thalassemia. Diabetes during pregnancy, commonly referred to as gestational diabetes, may falsely increase or decrease HbA1c.


HbA1c is a difficult test to perform and lab errors are common especially if done after transportation of the sample in an improper way or if done at a less experienced centre.

What is eAG [estimated average glucose]

It is the HbA1C derived estimate of average glucose using a special formula.Now eAG is considerd more patient friendly than HbA1C though it is derived from measurement of A1C itself
The relationship between A1C and eAG is described by the formula 28.7 X A1C – 46.7 = eAG