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Glycosylated (or glycated) hemoglobin (hemoglobin A1c, Hb1c , HbA1c or HgA1c) is a form of hemoglobin used primarily to identify the plasma glucose concentration over time. Its name is sometimes abbreviated to A1C. Glycosylation of hemoglobin has been implicated in nephropathy and retinopathy in diabetes mellitus.
Underlying principle
In the normal 120-day life span of the red blood cell, glucose molecules join hemoglobin, forming glycated hemoglobin. In individuals with poorly controlled diabetes, increases in the quantities of these glycated hemoglobins are noted.
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 (some researches state that the major proportion of its value is related to a rather short term period of two to four weeks *).
Interpretation of results
The normal range (that found in healthy persons) is 4% to 5.9%.*. People with diabetes mellitus often have higher levels of HbA1c. While diabetic patient treatment goals vary, many include a target range of HbA1c values. A diabetic with good glucose control has a HbA1c level that is close to or within the reference range. The International Diabetes Federation and American College of Endocrinology recommends HbA1c values below 6.5%, while the range recommended by the American Diabetes Association extends to 7%. A very high HbA1c represents poor glucose control.
Interpreting results is somewhat difficult because (1) laboratory results can differ depending on the analytical technique and (2) biological variation between individuals can be up to 1 percentage point (i.e. two individuals with the same average blood sugar can have A1C values that differ by up to 1 percentage point).
The mapping between HbA1c and blood glucose average is shown in the table below.
| A1C (%) | Avg. Blood Sugar (mmol/L) | Avg. Blood Sugar (mg/dL) |
|---|---|---|
| 5 | 4.5 | 80 |
| 6 | 6.7 | 120 |
| 7 | 8.3 | 150 |
| 8 | 10.0 | 180 |
| 9 | 11.6 | 210 |
| 10 | 13.3 | 240 |
| 11 | 15.0 | 270 |
| 12 | 16.7 | 300 |
Contraindications to use of test
Glycated hemoglobin measurement is not appropriate where there has been a recent change in diet or treatment within 6 weeks. Likewise the test assumes a normal red blood cell aging process and mix of haemoglobin subtypes (predominantly HbA in normal adults). Hence people with recent blood loss or hemolytic anemia, or hemoglobinopathy such as sickle cell disease are not suitable for this test. The alternative fructosamine test may be used in these circumstances and it similarly reflects an average of blood glucose levels over the preceding 2 to 3 weeks.
References
See also
"Glycosylated hemoglobin"