How is hemoglobin A1c testing interpreted?

Updated: Jul 25, 2019
  • Author: Gary L Horowitz, MD; Chief Editor: Thomas M Wheeler, MD  more...
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Hemoglobin A1c (glycated hemoglobin) reflects the average blood glucose concentration over the course of the RBC lifespan, roughly 120 days in normal individuals.

It provides different, and complementary, information to a single glucose concentration. Some patients may have near normal fasting glucose values but very high postprandial levels, and others may have elevated fasting levels with only moderately elevated postprandial levels. Hemoglobin A1c provides information comparable to what might be provided by having frequent glucose values throughout the day over the course of 3 months. [3, 4, 5]

Thus, elevated values give a sense of the degree of overall glucose control in patients with diabetes mellitus. Intensive glucose control in diabetic patients, reflected in lower hemoglobin A1c values, has been shown to "delay the onset and slow the progression of diabetic retinopathy, nephropathy, and neuropathy." [6] The goal of therapy is to attain a value of less than 7.0% (while minimizing hypoglycemic episodes).

Hemoglobin A1c should be monitored at least twice per year in stable patients with diabetes who are meeting glycemic goals and four times per year in patients who are not meeting glycemic goals or in whom the diabetes treatment regimen has recently been changed. [2]

As of January, 2010, the American Diabetes Association began promoting the use of hemoglobin A1c as the preferred diagnostic test for diabetes mellitus. Among its advantages over fasting glucose values (or 2-hour glucose values during an oral glucose tolerance test) is that samples can be drawn at any time and need no special handling (whereas ongoing glycolysis can falsely lower glucose values). [7, 8]

Most laboratories report a calculated eAG (estimated average glucose) along with every measured hemoglobin A1c, which is designed to facilitate communication with patients, as well as to help clinicians appreciate the degree of hyperglycemia the A1c represents.

Note that falsely low values can occur in patients whose RBC lifespan is shorter than normal (eg, hemolytic anemia, some hemoglobinopathies) as well as in patients who have received transfusions from nondiabetic patients.

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