Ionized calcium: Indications and advantages of its measurement

Plasma calcium exists in three forms: ionized, protein bound, and complexed with anions. Although all the forms are in equilibrium with each other, only the plasma ionized calcium has been reported to be physiologically active. Thus it is the preferred constituent for use in patient care. Despite this fact, the use of plasma total calcium concentration continues to be the predominant test for patient care in most hospitals mainly because of availability and ease of measurement. It has been documented that the use of total calcium is unreliable in cases where there is a change in the protein-calcium binding characteristics as in patients with hypergammaglobulinemia or a decrease/increase in pH. In an attempt to enhance the diagnostic accuracy of total calcium for these cases, several formulae have been developed for estimating "corrected" plasma total calcium. Although these formulae work reasonably well in normal healthy populations, they have been inadequate when applied for critically ill patients who are most prone to derangements in calcium metabolism. It has therefore been suggested that the measurement of the biologically active plasma ionized calcium be the routine rather than the exception. Ionized calcium however, has not gained widespread use for reasons that include the lack of familiarity by clinicians with the use of ionized calcium, problems historically associated with the measurement of ionized calcium, intricacies in the sample collection, and unavailability of ionized calcium measurement on the major clinical chemistry platforms. The advent of ionized calcium measurement as a standard feature in nearly all blood gas analyzers has enhanced its availability, especially in the critical care units. It is recommended that the use of ion-ized calcium measurement be adopted for the true reflection of calcium homeostasis in all patients.