Changes in plasma ionized calcium and magnesium in blood donors after donation of 450 mL blood. Effects of hemodilution and Donnan equilibrium.

The plasma concentration of ionized calcium and ionized magnesium in 26 blood donors decreased 0.01 mmol/L during blood donation. The changes could be explained by admixture of interstitial fluid. About 162 mL or 36% of the donated blood was replaced by interstitial fluid during blood donation. From the changes in concentration and hematocrit we could estimate the composition of the added fluid. The concentration of protein was much lower than in plasma. The concentration of protein-bound and free cations was also lower, in accord with the Donnan theory. We conclude that blood donors immediately after blood donation are unsuited as a reference population for proteins and ions.