Differences in the Calcium Concentration of Serum and Plasma Initially and after Storage

Calcium concentrations of plasma and serum in glass and plastic containers initially and after storage for up to 8 months at —20°C were compared. The mean initial calcium concentration of the plasma samples was 2.491 mmol/l and this was significantly higher than the mean serum concentration of 2.440 mmol/l. There were no significant differences in the mean calcium concentrations attributable to differences in glass or plastic as materials for storage containers for either serum or plasma. The mean calcium concentration of serum remained stable over the entire storage period, but that of the plasma samples fell significantly between one month and six months, with no further fall between six months and eight months.