Biological variation of serum and urinary magnesium in apparently healthy males

The biological variations of serum (S-) magnesium and urinary (U-) magnesium concentrations and excretions have been investigated. Serum samples, 24-h and fasting urinary samples were collected from each of 60 supposedly healthy male volunteers. In addition, 12 volunteers collected additional samples 7 and 112 days after the initial sample. The reference interval for S-magnesium was 0.765-0.997 mmol l-1. The biological variation for S-magnesium was 3.2% within subjects and 7.4% between subjects. This indicated an index of indivduality of 0.5, which means that significant changes in S-magnesium can occur within the limits of the reference interval, and that serial determinations of S-magnesium might prove useful as an indicator of changes in whole body magnesium status. It is, on the other hand, unlikely that a single determination of S-magnesium can be used in assessing whole-body magnesium status in the individual. The reference interval for the 24-h U-magnesium excretion, corrected for surface area, was 1.306-4.762 mumol min-1 1.73 m-2. The 24-h U-magnesium excretion exhibited a biological within-subject variation of 36% and a between-subject variation of 26%. The 24-h U-magnesium excretion did not correlate with S-magnesium, and only slightly (r = 0.58) with the fasting U-magnesium/creatinine concentration. This, and the very large coefficients of variation, make it unlikely that the renal magnesium excretion can be used as a measure of whole body magnesium status, or that changes in the renal magnesium excretion can be used as a measure of changes in whole body magnesium status.