What are the methods used to determine hemoglobin A1c?

Updated: Jul 25, 2019
  • Author: Gary L Horowitz, MD; Chief Editor: Thomas M Wheeler, MD  more...
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With the relatively recent introduction of a reference method for the measurement of hemoglobin A1c, [11] it became apparent that the methods used historically, though very highly correlated to the true hemoglobin A1c concentrations, included chemical species other than A1c; in fact, the values were roughly 2 units higher than the true value (ie, a value of 7% was closer to 5% in conventional units). [12] Different countries have approached this problem in different ways. Many countries have adopted totally new units (mmol/mol), but the United States has decided to continue to report conventional units (%). [12] The relationship between the 2 sets of units is as follows:

IFCC-A1c (mmol/mol) = [NGSP-A1c (%) – 2.15] x 10.929

However, in an effort to smooth the transition to the new units, as well as to help both physicians and patients appreciate the clinical context of various hemoglobin A1c levels, many laboratories are now reporting eAG (estimated average glucose) along with every hemoglobin A1c level. [13] Although the correlation is far from perfect and the use of this parameter has been somewhat controversial, [14] it has received support from many major organizations, including the American Diabetes Association, the American Association for Clinical Chemistry, and the College of American Pathologists. The equation to calculate eAG (in mg/dL) from hemoglobin A1c (in %) is as follows:

eAG (mg/dL) = 28.7 x NGSP-A1c (%) – 46.7

On balance, clearly, reporting an eAG of 226 mg/dL in addition to an A1c of 9.5% can enhance patient care.

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