Studies on ionized calcium in serum and plasma from normal cows. Its relation to total serum calcium and the effects of sample storing.

Ionized calcium has been determined with a new improved instrument on serum samples from 111 Swedish red-and-white cows. Simultaneous sampling of plasma and serum was performed in 32 cows for comparison of the ionized calcium level. Multiple sampling of plasma and serum from seven cows was performed to evaluate the effect of storage at 4°C and room temperature. The normal range for ionized calcium found in this study implies that the ionized calcium fraction comprises for 43.4 ± 3.0 % (mean ± 2 s) of the total serum calcium. Simultaneous analyses on plasma and serum revealed that the plasma level of ionized calcium was generally 0.ι05 mιmol/1 lower than the serum value. pH changes in stored blood samples have a direct effect on the ionized calcium levels and is therefore to be avoided. Storing samples in vacutainers for five days at 4°C or for two days at room temperature was accompanied by only small decreases of serum or plasma ionized calcium. The new instrument used in this study enables rapid analyses, and most of earlier drawbacks with calcium-ion-selective analyzers have been eliminated.