Saturday, January 31, 2009

Glycosylated Hemoglobin (HbA1C)

Introduction

Glycosylated hemoglobin is the combination of hemoglobin and glucose. When the blood sugar level is higher than normal for long period, hemoglobin is glycosylated. Glycosylation of hemoglobin occurs as a two-step reaction, resulting in the formation of a covalent bond between the glucose molecule and the terminal valine of the β chain of the hemoglobin molecule. The rate at which this reaction occurs is related to the prevailing glucose concentration.

Glycosylated haemoglobin is expressed as a percentage of the normal haemoglobin (standardized range 4-6.5%). This test provides an index of the average blood glucose concentration over the life of the haemoglobin molecule (approximately 6 weeks). The figure will be misleading if the life-span of the red cell is reduced or if an abnormal haemoglobin or thalassaemia is present. There are considerable inter-individual variations in HbA1c levels, even in normal people. Although the glycosylated haemoglobin test provides a rapid assessment of the level of glycaemic control in a given patient, blood glucose testing is needed before the clinician can know what to do about it.


Important HbA1C values are;


HbA1c <6.5% Good glycemic control

HbA1C 6.5-8.5% Moderate glycemic control

HbA1C >8.5% Bad glycemic control




Assessment of Long-Term Glycemic Control

Measurement of glycated hemoglobin is the standard method for assessing long-term glycemic control. When plasma glucose is consistently elevated, there is an increase in nonenzymatic glycation of hemoglobin; this alteration reflects the glycemic history over the previous 2 to 3 months, since erythrocytes have an average life span of 120 days.

Glycated hemoglobin or A1C should be measured in all individuals with DM during their initial evaluation and as part of their comprehensive diabetes care. As the primary predictor of long-term complications of DM, the A1C should mirror, to a certain extent, the short term measurements.

In standardized assays, the A1C approximates the following mean plasma glucose values:

1. HbA1C of 6% is 7.5 mmol/L (135 mg/ dL),
2. HbA1C of 7% is 9.5 mmol/L (170 mg/dL),
3. HbA1C of 8% is 11.5 mmol/L (205 mg/dL),
4. A 1% rise in the A1C translates into a 2.0-mmol/L (35 mg/dL) increase in the mean glucose

ESTABLISHMENT OF A TARGET LEVEL OF GLYCEMIC CONTROL

Because the complications of DM are related to glycemic control, normoglycemia or near normoglycemia is the desired, but often elusive, goal for most patients. However, normalization of the plasma glucose for long periods of time is extremely difficult. Regardless of the level of hyperglycemia, improvement in glycemiccontrol will lower the risk of diabetes complications In general; the target A1C should be 7.0%.

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