Ionized calcium Total calcium consists of free or ionized calcium (50%), calcium bound to protein (40-45%), principally albumin and calcium complexed to anions (5-10%), e.g. citrate, lactate, bicarbonate. Total calcium level does not give an indication of what is available at the cellular level. Ionized calcium (iCa) is the form of calcium that is readily available to cells, and measurement of iCa is a more accurate reflection of the physiological calcium state. Measurement of ionized calcium requires instrumentation with ion-specific electrodes. Unlike total calcium, ionized calcium is unaffected by albumin concentration, but it is affected by acid-base balance. Acidosis (decreased pH) increases iCa. Alkalosis (increased pH) decreases iCa. The pH of blood, serum or plasma changes rapidly with storage (usually decreases with refrigeration and increases with freezing), therefore iCa is unstable in stored blood samples. The storage stability of iCa differs depending on the species and the type of sample (whole heparinized blood, serum or plasma). Because of this pH-dependence, sample handling for iCa measurement is critical and requires anaerobic collection and storage. In general, iCa is a test that may not be readily available to the veterinarian in private practice, because of the problems with stability.

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