Glycosylated (or glycated) hemoglobin (hemoglobin A1c, Hb1c , HbA1c or HgA1c) is a form of hemoglobin used primarily to identify the plasma glucose concentration over prolonged periods of time. Its name is sometimes abbreviated to A1C. It is formed in a non-enzymatic pathway by hemoglobin’s normal exposure to high plasma levels of glucose. Glycation of hemoglobin has been implicated in nephropathy and retinopathy in diabetes mellitus. Monitoring the HbA1c in diabetic patients may improve treatment.<ref name=”pmid2215560″></ref>
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 researchers state that the major proportion of its value is related to a rather short term period of two to four weeks [1].)
This phenomenon was first identified in 1976<ref name=”pmid934240″></ref>.
Interpretation of results
Laboratory results may differ depending on the analytical technique, the age of the subject, and biological variation among individuals. Two individuals with the same average blood sugar can have A1C values that differ by as much as 1 percentage point).<ref name=”pmid12089188″></ref> In general, the reference range (that found in healthy persons), is about 4%–5.9%.[2].
Higher levels of HbA1c are found in people with persistently elevated blood sugar, as in diabetes mellitus. While diabetic patient treatment goals vary, many include a target range of HbA1c values. A diabetic person 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 recommend HbA1c values below 6.5%, while American Diabetes Association recommends that the HbA1c be below 7.0% for most patients. A high HbA1c represents poor glucose control. Persistent elevations in blood sugar (and therefore HbA1c) increase the risk for the long-term vascular complications of diabetes such as coronary disease, heart attack, stroke, heart failure, kidney failure, blindness, erectile dysfunction, neuropathy (loss of sensation, especially in the feet), gangrene, and gastropathy (slowed emptying of the stomach). Poor glucose control also increases the risk of short-term complications of surgery such as poor wound healing.
Lower than expected levels of HbA1c can be seen in people with shortened red blood cell life span, such as with glucose-6-phosphate dehydrogenase deficiency, sickle-cell disease, or any other condition causing premature red blood cell death. Conversely, higher than expected levels can be seen in people with a longer red blood cell life span, such as with Vitamin B12 or folate deficiency.
The approximate mapping between HbA1c values and average blood glucose measurements over the previous 4-12 weeks is shown in the table.
| HbA1c (%) |
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 |
Indications and use
Glycosolated hemoglobin is recommended for monitoring blood sugar control in patients with diabetes mellitus. The American Diabetes Association guidelines are similar to others in advising that the test be performed at least two times a year in patients who are meeting treatment goals (and who have stable glycemic control)and quarterly in patients whose therapy has changed or who are not meeting glycemic goals. [3]
Glycosylated hemoglobin measurement is not appropriate where there has been a change in diet or treatment within 6 weeks. Likewise the test assumes a normal red blood cell aging process and mix of hemoglobin 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.
Because of variation among laboratories and a lack of consensus on a diagnostic threshold, glycosolated hemoglobin is not recommended for use as a screening test to detect diabetes in patients with neither a prior diagnosis nor symptoms suggestive of the disease. For diagnosis, fasting plasma glucose or an oral glucose tolerance test is preferred.
References
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See also
- Diabetes mellitus
- Hemoglobin
External links
- National Diabetes Information Clearinghouse
- Hemoglobin A1c Test
- MDRTC Hemoglobin A1C Fact Sheet
- Simple Online A1c Calculator
- Metrika: A1C test for professionals and consumers.