ACCURACY OF CONVERSION FORMULAS FOR ESTIMATION OF GLYCOHEMOGLOBIN

To analyse the accuracy of the conversion formulae for estimation of g lycohaemoglobin (GHb) measured by different methods, we analysed 210 s amples for HbA(1c) using HPLC. Fifty of these specimens were analysed by micro-column chromatography (MC), 43 by electrophoresis (EP), 50 by IMX system (Abbott Laboratories), 38 by Primus HPLC and 29 by Diamat HPLC. Regression analyses were performed and the equations were used t o estimate HbA(1c) values (HbA(1c) calc) for the five methods. The 95% limits of agreement between HPLC and the converted results were -1.77 to 1.71%, -1.54 to 2.54%, -0.92 to 0.88%, -0.46 to 0.56%, and -0.39 t o 0.41% for MC, EP, IMX, Primus and Diamat equations, respectively. Th e mean relative errors were 3.4 (-28.2 to 35%), 1.3 (-22.9 to 25.5%), 0.4 (-14.6 to 15.0%), 0.51 (-6.55 to 7.57%), -0.20 (-5.8 to 5.4%), for MC, EP, IMX, Primus and Diamat, respectively. These results show that conversion formulae based on methods that do not measure HbA(1c) (MC, EP and IMX) are inaccurate and can mask a clinically relevant variati on of HbA(1c). However, GHb results obtained by HPLC methods could be interchangeably converted with an absolute variation of less than 1%. Converted HbA(1c) results from non-standardized methods should be inte rpreted with caution.