Evaluation of serum ionic calcium measurement in a general hospital population

Direct measurement of ionic calcium was evaluated in light of conflicting reports concerning the correlation of ionic calcium with total calcium and ionic calcium as predicted from total calcium and serum protein. Ionic calcium in the sera of normo- hypo-, and hypercalcemic adult hospitalized patients was measured anaerobically using an Orion model 98-20 calcium flow-thorugh electrode. Good correlation (r = .77) was found with the calculated ionic calcium value derived from total protein and total calcium, using the McLean-Hastings nomogram. Good correlation was also present between the measured ionic and total calcium (r = .80). In view of these correlations, the introduction of serum ionic calcium measurements into the general hospital laboratory repertoire is of questionable value.