A reference system for ionized calcium

Abstract

Reference systems in clinical chemistry, whether loosely structured or highly organized like the National Reference System for Clinical Chemistry (NRSCC) in the USA are built upon an assemblage of interrelated materials, methods, and agreements. For example, the NRSCC Council has accepted the following items for the measurement of total calcium in human serum: 1) the Reporting Unit (the SI non-coherent molar concentration unit-mmol/1), 2) a Certified Reference Material (NBS/SRM 915 CaCO₃),3) a Definitive Method (IDMS) and 4) a Reference Method (FAAS). Recently, the IDMS measured calcium value has become available on a freeze-dried human serum (NBS/SRM 909) and allows the direct accessment of the accuracy of routine methods, instrument systems, calibrators and control materials.

Utilizing the NRSCC reference method and materials for total calcium measurements and the Radiometer System (ICA1) for ionized Ca²⁺ measurements, we have begun to ask the question, “What are the essential items in a reference system for ionized calcium?” As expected, our initial explorations reveal more problems than answers, thus our very limited and unsophisticated initial data will be presented primarily as a means to ensure discussion.

Despite, the electrochemical complexity of the electrochemical interactions, and the technologic differences from one measuring system to another, it is my belief that a reference system capable of widespread acceptance for ionized Ca²⁺ must be introduced to ensure the long-term integrity and interlaboratory compatibility of this vital physiological measurement.