Ionized Calcium or Corrected Total Calcium?

Abstract

To the Editor: We would like to respond to the Letter of Larsson and Magnusson.1 In the Statement summarizing the deliberations of the Panel that was convened after the National Institutes of Health Workshop on the Management of Asymptomatic Primary Hyperparathyroidism,2 we recommended corrected total serum calcium over ionized serum calcium concentration as the preferred measurement technique. Larsson and Magnusson argue that the ionized serum calcium, the physiologically activity moiety of the circulating calcium concentration, should be preferred over the total calcium measurement. In their letter, they correctly point out variability in the algorithms by which the total calcium value is corrected for the prevailing albumin concentration. They also believe that ionized calcium measurement technology is readily available in hospitals these days, and finally, that the total serum calcium concentration is not without its own standardization problems. We certainly agree with their latter point, and in the Summary Statement, make a plea for greater uniformity and consistency with regard to the normal range of the total serum calcium concentration. We also acknowledge differences in the algorithms by which the total serum calcium is corrected for the prevailing albumin concentration. However, the Panel felt that these points do not argue yet for a change from the corrected total calcium concentration to the ionized calcium concentration because the ionized calcium test still requires a procedural and technical rigor that is greater than that required for the measurement of the total serum calcium concentration. Some centers have successfully instituted such measures, and can therefore, rely on the ionized calcium, but for most hospitals and reference laboratories in the United States, this is not yet the case. We are mindful that certain countries such as Sweden, have overcome the limitations of the ionized calcium to a greater extent than has the United States. In those countries, certainly one would opt for the ionized calcium concentration. However, the Panel addressed itself to issues in the United States, being careful to point out that our recommendations may not be applicable to other countries. In the United States, all things considered at this time, the Panel feels that the corrected total serum calcium concentration is the preferred measurement over the ionized calcium concentration.