A comparison of reference method values for calcium, lithium and magnesium with method-dependent assigned values.
Commercially available control sera were analysed by the reference methods for calcium, lithium and magnesium. Calcium: The mean deviation of the target values for routine flame atomic absorption spectrometry from the reference method values was -1.2%, in the case of flame atomic emission spectrometry -0.1%. The assigned values of determinations by methylthymol blue and cresolphthalein showed a positive bias: +2.3 and +2.2%. Lithium: Target values for routine flame atomic absorption spectrometry and flame atomic emission spectrometry were in generally good agreement with the reference method values (deviation +1.7% and +0.7%), although the differences were dependent on concentration. Magnesium: The assigned values for routine flame atomic absorption spectrometry yielded a mean bias of -0.4% from the reference method value. The target values for absorption spectrometry after reaction with Magon and Calmagite, however, differed by +8.2% and +9.3% on average. These methods seem to be less suitable for the determination of magnesium in serum than FAAS and would have failed several times to meet the requirements of quality assessment.