RESUMEN
The annual inter- and intra-individual biological variation, including the circannual rhythmic variation, of the serum concentrations of magnesium and ionized calcium has been investigated in a group of 51 apparently healthy volunteers. Venous blood specimens were collected on intervals of once a month within a one-year period, using a standardized protocol. The inter-individual coefficients of variation were 5.12% for magnesium and 1.58% for ionized calcium. The medians of the intra-individual coefficients of variation were 1.93% for magnesium and 2.18% for ionized calcium. These data were used to determine the allowable imprecision, the allowable systematic error, the critical difference for significant change detection, and the usefulness of population reference values (index of individuality). Of the quantities studied, only the serum concentration of ionized calcium shows a significant annual rhythmic variation (amplitude 1.3%), although this result may be due to the between-run metrological variance, considering that the concentration of ionized calcium of the control material used during the study possesses a similar significant rhythmic variation.