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Subject:

Re: DCCT standardization and DCCT fudge

From:

Philippe Marquis

Date:

Mon, 8 Nov 1999 18:50:13 +0100

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 ```> I have just done a small local comparison of results from 81 samples > measured using a Menarini HA-8140 and our Eurogenetics Tosoh HbA1c > analyser. Both are standardized against DCCT, according to their > manufacturers. The analyzers are also among the best performers on our > external QA schemes. > > Their results agree on average at about 7% but show increasing > discrepancy as concentrations move away from this, e.g. a Tosoh result > of 5.5% equates to a Menarini result of about 6%, whilst a Tosoh result > of 12.3% equates to a Menarini result of about 11.7%. Your results can be explained by a better specificity of Tosoh method. Let us assume that Menarini method includes in its HbA1c peak a coeluting impurity amounting to 1%. With a DCCT calibration material, if Tosoh peak is 7%, Menarini peak is 7+1=8%. This figure is reset to 7% thanks to a calibration factor of 7/8=0.87. With patient samples 1) if Tosoh peak is 5.5%, Menarini peak is 5.5+1= 6.5%, but it is reset by the calibration factor to 6.5x0.87 = 5.7%. 2) If Tosoh peak is 12.3%, Menarini peak is 12.3+1=13.3%, but it is reset by the calibration factor to 13.3x0.87=11.6% You see that opposite discrepancies appear between results higher and lower than the calibration point. > If a chromatographic method suffers from co-elution of minor fractions > with HbA1c, then calibration of these will still be a fudge, bearing in mind > that there is inter-individual variation in these fractions. > We need better methods in addition to a true HbA1c calibrant. Surely ! Even with a pure Hb A1c calibrator, the previous error should persist. Philippe Marquis Centre hospitalier regional Metz - France %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% ```

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