Glycosylated Hemoglobin Hemoglobin A1c

The hemoglobin A1c (HbA1c) fraction measures nonenzy-matic glycosylation of hemoglobin, which is related to level of glucose concentration over the life span of the erythrocyte. The HbA1c fraction can be used to estimate glucose control in the previous 3 months. In persons with normal erythro-cyte survival, the glucose levels in the last 30 days contribute to 50% of the HbA1c, whereas the glucose levels in the preceding 90 to 120 days contribute only 10% to the HbA1c measurement. Although previously laboratories varied considerably in the reporting of HbA1c results, virtually all U.S. laboratories now have adopted the National Glycohemo-globin Standardization Program (NGSP) methods. Recent standards propose reporting HbA1c in the familiar percentage and in international units (mmol/mol), combined with the HbA1c-derived average glucose (ADAG) (Saudek et al., 2008). Using NHANES III data that the population average for HbA1c was 5.17 with standard deviation (SD) of 0.45, the International Expert Committee (2009) selected an HbA1c of 6.5% (~3 SD above average) as the cutoff point to diagnose diabetes mellitus, with confirmation by a fasting glucose greater than 126 mg/dL or oral GTT greater than 200 mg/ dL, or a repeat HbA1c greater than 6.5%. Goals for achieving optimal control of diabetes are controversial, but a reasonable goal in most persons is an HbA1c less than 7%. Conditions that shorten erythrocyte survival, such as hemolysis or recent bleeding, give a lower HbA1c level.

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