Stability of mean assay values of magnesium and iron in lyophilized quality control serum: a study based on data from the quality assurance service (QAS) of the College of American Pathologists

Abstract

Data from magnesium and iron analyses of pools of lyophilized quality control serum were used to evaluate stability of analyte mean values in the pre-reconstitution period. Information was received from laboratories in Regional Quality Control Programs between 1977 and 1981, using CAP Quality Assurance Service data processing. For magnesium, 28 of 41 (68%) pool-method combinations were stable, 11 (27%) showed decreases, and 2 (5%) showed increases. Decreases averaged 0.008 mg/dL/month (all methods). A pronounced effect of method (automated methylthymol blue) and year correlated with decreases in measurable magnesium. For iron, 45 of 56 (80%) pool-method combinations were stable, 8 (15%) showed decreases, and 3 (5%) showed increases. Decreases averaged 0.349 g/dL (all methods). In most cases, changes in measured concentrations are attributed to methodologic factors, rather than intrinsic changes in analyte concentrations.