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What is HbA1c?

Posted on December 20, 2018 | No Comments on What is HbA1c?

You may have heard your doctor ask for or order HbA1c in your blood tests. And when you asked him or her what it is, you got the answer “It’s your blood sugar average for 3 months.” And that was it. You may still have a few questions, so let us try to get into the nitty-gritty of HbA1c.

What exactly is the HbA1c, and how did it come to be a measurement of average blood sugar for 3 months?
The HbA1c test (hemoglobin A1c test, glycosylated hemoglobin A1c test, glycohemoglobin A1c test, or A1c test) is a laboratory test which reveals average blood glucose over a period of two to three months. It measures the number of glucose molecules attached to hemoglobin, a substance in red blood cells.

The test takes advantage of the lifecycle of red blood cells, the duration of which is approximately 120 days. So by measuring attached glucose in the red blood cell, the average blood sugar level over the previous three months can be determined. The level of HbA1c at any point in time is contributed to by all circulating erythrocytes, from the oldest (120 days old) to the youngest. However, HbA1c is a “weighted” average of blood glucose levels during the preceding 120 days, with the youngest erythrocytes contributing to a greater extent than the older ones. Approximately 50% of the glycated hemoglobin level is determined by plasma glucose levels over the previous month, and 75% during the previous 2 months.

How is HbA1c measured?
Laboratories use High Performance Liquid Chromatography or Immunoassay. In the United States, the National Glycohemoglobin Standardization Program (NGSP) began in 1996, with the goal of standardizing glycated hemoglobin tests to the high-performance liquid chromatography (HPLC) method reported as HbA1c as used in the 1993 Diabetes Control and Complications Trial (DCCT). There are several types of certified methods available for measuring hemoglobin A1c: immunoassay, ion-exchange HPLC, electrophoresis, boronate affinity HPLC, and enzyme methods. Reliable bench-top point of care analyzers, such as the one which uses a cassette-based immunoassay method, are also now available for clinic use.

Results can be unreliable in many circumstances, such as after blood loss, blood transfusions, anemia, or high erythrocyte turnover; in the presence of chronic renal or liver disease; after administration of high dose vitamin C; or erythropoeitin treatment.

Lower-than-expected levels of HbA1c can be seen in people with shortened red blood cell lifespan, such as with glucose-6-phosephate dehydrogenase deficiency, sickle-cell disease or any other condition causing premature red blood cell death. On the converse, higher¬than-expected levels can be seen in people with a longer red blood cell lifespan, such as with Vitamin B12 or Folate deficiency.

HbA1c is increased in patients with renal failure, when measured by ion-exchange chromatography. Interference by carbamylated hemoglobin can occur with uremia, hypertriglyceridemia and hyperbilirubinemia, and salicylates can cause interference by acetylated species.

In summary, HbA1c is a very useful tool in the diagnosis and monitoring of prediabetes and diabetes. Its use should be combined with data from other parameters such as a glucose tolerance test or home blood sugar monitoring, in order to guide management more precisely. It has intricate standardization measures, and is influenced by disorders of red blood cell lifespan and other biochemical interferences. Interpretation of HbA1c considers all these factors in every unique situation, and is best guided by your doctor.

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