Evaluation of intrinsic and routine quality of serum total magnesium measurement

We investigated the intrinsic (as delivered by the manufacturer) and routine quality of four systems for measurement of serum total magnesium (t-Mg(2+)) by method comparison with an ion chromatography reference method. The results of the study were interpreted on the basis of analytical quality specifications derived from the biological variation of t-Mg(2+), expanded by the analytical uncertainty of the reference measurements. This resulted in limits for systematic error of 2.1% and for total error of 4.3%. The study demonstrated that those limits were challenging for all routine systems. Most of them met the total error criterium just borderline and one showed a considerable systematic error (-5.2%). Concerning the measurement quality in the routine laboratories, the study showed that many were unable to preserve the intrinsic quality of the respective manufacturer. Consequently, loss of system performance in the routine laboratory mostly led to violation of the analytical specifications. Most strikingly, the study revealed enormous quality differences between routine laboratories. This indicates that, still, many routine laboratories do not make adequate use of currently available internal and external quality control tools. Moreover, some laboratories considerably expanded the high end of the reference interval, thereby reducing the diagnostic potential of t-Mg(2+).