A number of studies, including the Diabetes Control and Complications Trial (DCCT), have shown that good glycemic control, as assessed by GHb measurements, can reduce the chronic complications of diabetes. The National Glycohemoglobin Standardization Program (NGSP) was established to insure that GHb measurements by different methods were comparable and could be related to the candidate reference method used in the DCCT. The measurement of HbA1c in patients with Hb variants is one area not directly addressed by the NGSP. Therefore, we assessed the comparability of two DCCT-traceable methods in samples with Hb variants.


Samples containing HbAA, HbAC, and HbAS were collected from diabetic and nondiabetic patients. HbA1c concentrations were measured by a high-performance liquid chromatography method (Bio-Rad Diamat) and an immunoassay that is suitable for use in a physician's office (Bayer DCA 2000).


The two methods compared well for samples with HbAA and HbAS. However, for samples containing HbAC the immunoassay method showed relative positive biases of 8.4 and 10.4% at HbA1c levels of 7 and 9%, respectively, such that the two methods would not be judged comparable according to NGSP guidelines.


The DCA 2000 HbA1c immunoassay method showed significant positive bias in patients with HbC trait. One possible clinical implication of this overestimation is overly rigorous glycemic control with a concomitant increase in hypoglycemia. This may be especially important in certain ethnic populations, such as African-Americans, who have a relatively high prevalence of HbC trait.

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