For decades, the diagnosis of diabetes has been based on glucose fasting test.  American Diabetes Association (ADA) has not previously recommended the use of A1c for diagnosing diabetes, in part due to lack of standardization of the assay. However, A1c assays are now highly standardized, and their results can be uniformly applied both temporally and across populations[1].  In this article, we will examine why Reuters reported that A1c diabetes test is a better indicator of risk[2].

What's Hemoblogin A1c?

Hemoglobin A1c reading is a measure of the average blood sugar level over the preceding three months. Sugar sticks to protein, and when it's around for a long time, it gets harder and harder to remove.  The red blood cells that circulate in the body live for a hundred days before they die, and when sugar sticks to these cells, it give doctors an idea of how much sugar has been around for the preceding three months[3].

Advantages and Disadvantages of A1c

Compared to glucose fasting test, A1c test has the following advantages:[3]

• Greater convenience

  • Fasting is not required

• Greater preanalytical stability

• Less day-to-day perturbations during periods of stress and illness

and the following disadvantages:

• Greater cost

• Limited availability of A1c testing in certain regions of the developing world

• Incomplete correlation between A1c and average glucose in certain individuals

• Can be misleading in patients with certain forms of anemia and hemoglobinopathies

  • For patients with a hemoglobinopathy but normal red cell turnover, such as sickle cell trait, an A1c assay without interference from abnormal hemoglobins should be used (an updated list of A1c assays and whether abnormal hemoglobins impact them is available here). 

  • For conditions with abnormal red cell turnover, such as pregnancy or anemias from hemolysis and iron deficiency, the diagnosis of diabetes must use glucose fasting test exclusively.

Sample Test Report

Estimated Average Glucose (eAG) is calculated using the equation[5]:

• eAG = (28.7 x HbA1c) - 46.7

  • • For example, if HbA1c is 5.6, eAG is (28.7 x 5.6) - 46.7 (or 114).

Note that the diagnostic test should be performed using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized or traceable to the Diabetes Control and Complications Trial (DCCT) reference assay.

References

1. Standards of Medical Care in Diabetes—2010

2. A1c diabetes test is a better indicator of risk (Reuters)

   1. The End of Illness by David B. Agus, MD

3. Experts recommend a more personal approach to type 2 diabetes

4. Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ, the A1c-Derived Average Glucose (ADAG) Study Group: Translating the A1C assay into estimated average glucose values. Diabetes Care 31:1473–1478, 2008

5. How Our Bodies Age and What to Expect?

• • Impaired Glucose Tolerance

6. Diabetes and Your Diet: Busting 7 Myths

7. Which drugs can increase women's risk of diabetes by 48%?