The concentration of calcium in mammalian blood is approximately 10 mg/dL, with some variation due to species (e.g., up to 13 mg/dL in normal horses and rabbits), age, dietary intake, and analytical method. Calcium in the blood is composed of protein-bound and ionized fractions. Precise control of calcium ion levels in extracellular fluids is vital for health, as low or high levels may lead to significant diseases such as hypothyroidism, puerperal tetany, hypoparathyroidism, hypoproteinemia, renal disease, and pancreatitis.

Reliable laboratory diagnosis depends upon accurate measurement of calcium levels. Antech's Research Dept. has studied the variation of serum and plasma calcium levels when different collection tubes are used or the sample volume is too small and is diluted to obtain a result. Serum and plasma calcium levels also were measured undiluted and diluted 1:3 with water or saline. The calcium level did not vary significantly when samples were collected either in red top (RTT), serum separator (SST), sodium heparin or lithium heparin tubes. Although there was no difference between using water or saline as diluent, calcium levels of the diluted samples were almost 3 mg/dL lower than undiluted specimens.

These findings confirm that when serum or plasma samples are diluted, falsely lowered calcium values are obtained. This poses a problem for small samples, such as those submitted from avian or exotic species, that may have to be diluted prior to analysis. Antech reports include a comment whenever samples are diluted indicating that interpretation should take the lowering effect of dilution into account.