Ionised calcium is sometimes indicated in the investigation of hypocalcaemia and hypercalcaemia, often before PTH measurement.
Ionised calcium is the component of total calcium which is not bound to serum proteins or chelated with anions such as citrate. Essentially it is the metabolically active component which is available to cells involved in neuromuscular transmission and most accurately reflects calcium status.
If total calcium is reduced on a profile this can reflect a true reduction in ionised calcium or a reduction in bound calcium while the metabolically active component remains unaffected. In dogs there are formulae to correct total calcium for hypoalbuminaemia and we use one of these to generate ”corrected calcium” results on reports where albumin is reduced.
However, in other species formulae are not generally available and even in dogs a direct measurement is preferable. Increased total calcium can reflect a true increase in ionised calcium, dehydration or increased chelated calcium. The latter can occur in chronic renal failure and only measurement of ionised calcium can reliably differentiate these possibilities.
Given the diagnostic significance of genuinely increased calcium concentrations with regard to the presence of hypercalcaemia of malignancy and parathyroid gland neoplasia the direct measurement of ionised calcium is a valuable additional test before undertaking extensive further investigation.