Glycated (Glycosylated) Hemoglobin: HbA1c: New Directions to Diagnose Diabetes: 2nd Place Winner 2010 AMT Technical Writing Contest
Balatbat, Joseph, Journal of Continuing Education Topics & Issues
Also known as hemoglobin A1c, HbA1c, A1C or Hb1c, Glycated (Glycosylated) Hemoglobin is a form of hemoglobin used primarily to identify the average plasma glucose concentration over a prolonged period of time. Increased levels of glycated hemoglobin has been associated with cardiovascular disease, nephropathy, and retinopathy in diabetes mellitus. Monitoring the level of HbA1c in juvenile onset (type 1--autoimmune) diabetes may improve treatment. (1)
In 1958, hemoglobin A1C was first separated from other forms of hemoglobin (Huisman and Metering) using a chromatographic column. 2 Ten years later, hemoclobin A1C was characterized as a glycoprotein (non-enzymatic attachment of glucose to protein) by Bookchin and Gallop. (3) Ralibar and coworkers (4) wrote that the blood level of A1C is found to be elevated in diabetes and seven years thereafter, formally recommended its use to monitor the degree of control of glucose metabolism in diabetic patients (Cerami, Koenig, et al.). (5)
Principle and Measures
During the normal 120-day life span of the red blood cell (RBC), glucose molecules react with hemoglobin, forming glycated hemoglobin. Once a hemoglobin molecule is glycated, it remains in this form. In people without diabetes, about 4% to 6% of their hemoglobin is glycosylated. Red blood cells (RBCs) that contain the hemoglobin circulate in the bloodstream for three to four months before being broken down and replaced. During that time, the RBC can bond, irreversibly, to glucose in the bloodstream. A buildup of glycated hemoglobin within the red blood cell therefore reflects the average level of glucose to which the cell has been exposed during its life cycle. Thus, A1C readings higher than about 6% indicate higher than normal amounts of glucose roaming the blood stream in the past 120 days.
Other studies state that the major proportion of its value is related to a rather shorter period of two to four weeks. (6) Measuring glycated hemoglobin assesses the effectiveness of therapy by monitoring long-term serum glucose regulation. In individuals with poorly controlled diabetes, the quantities of this glycated hemoglobin are much higher than in healthy people.
Using the conversion table (See table 1) from the American Diabetes Association's (ADA) 2005 position statement on Standards of Medical Care in Diabetes, the 7.5% A1C reading would equate to an average blood glucose of about 168mg/dL. Bear in mind that the correlation between mean plasma glucose levels and A1C levels is an estimation only, dependent on methodology used for the calculation as well as other factors, such as the red blood cells' life span. A 1 percent change in an A1C result reflects a change of about 30mg/dL (1.67 mmol/L) in average blood glucose.
Indications and Use
Glycated hemoglobin values reflect the 2-3 month average endogenous exposure to glucose including postprandial spikes in the blood glucose level, and have low intraindividual variability, particularly in persons without diabetes. (7,8) These characteristics may contribute to the superiority of glycated hemoglobin over fasting glucose for long-term macrovascular risk stratification.
Recommendations for the diagnosis of diabetes are based on the relations of fasting glucose and glycated hemoglobin with microvascular disease, typically retinopathy. (9,10) Nonetheless, cardiovascular disease is the leading cause of illness, death, and hospitalization in persons with diabetes. (11,12) In a recent study by Selvin et al, (13) it was suggested that glycated hemoglobin values in the normal range can identify persons at increased risk for coronary heart disease, stroke, and death before the diagnosis of diabetes, indicating that glycated hemoglobin is a useful marker of cardiovascular risk and death from any cause.
There is a significant proportion of people who are unaware of their elevated HbA1C level before they have blood lab work. …