Standardization of two immunological HbA1c routine assays according to the new IFCC reference method

Abstract

The measurement of HbA1c is meanwhile well established as the most important parameter in clinical chemistry for monitoring the long term metabolic control of diabetic patients. However, the comparability of HbA1c values obtained by different methods in different laboratories and different countries is limited since there was no internationally agreed reference method available. Additionally the % HbA1c values, based on the most common DCCT values are too high due to well known non specificity of the DCCT standardization protocol. Recently a new reference method for the determination of HbA1c was developed and has been approved by the IFCC as the future basis for the worldwide standardization of HbA1c routine assays. The two routine methods from Roche Diagnostics (both based on the immunoturbidimetric determination of the stable glucose adduct to the N-terminal group of the hemoglobin beta chain, Roche-Hitachi/Tina-quant [a] and COBAS INTEGRA) are directly standardized against this new reference method. Both routine methods are based on a two step approach: in a first step the total Hb is quantified by a colorimetric method and in a second step the stable glucose adduct to the N-terminal group of the hemoglobin beta chain is quantified by immunoturbidity (HbA1c mass). Consequently, first the standardization of the total Hb assay was reconfirmed by method comparisons against the cyanomethemoglobin reference method. Second, the standardization of the HbA1c mass was done by running method comparisons against the new IFCC reference method. The resulting new calibrator values for HbA1c [mass] and total Hb [mass] allow a direct estimation of % HbA1c according to IFCC just by calculating the HbA1c [mass]/total Hb [mass] ratio. Good linear correlations were obtained when comparing the routine methods against the new IFCC reference, indicating the high specificity of these immunological approaches. The following correlations are obtained: Hitachi / Tina-quant [a] (Y) versus IFCC reference method (X): Y = -0.031 + 1.009 x X; r = 0.995. COBAS INTEGRA (Y) versus IFCC reference method (X): Y = -0.156 + 1.006 x X; r = 0.997. Hitachi / Tina-quant [a] (Y) versus Integra (X); (both standardized according to IFCC): Y = 0.057 + 1.003 x X; r = 0.997. The HbA1c values obtained with this new IFCC standardization are significantly lower than the well known DCCT values. Due to the specificity of the immunological approach which is very close to the analyte as defined by the IFCC reference method, no further corrections of the obtained % HbA1c values are necessary. A slope/intercept correction formula is derived which allows the transformation of IFCC values into the DCCT numbers if requested.