Specific Heparin Preparations Interfere With the Simultaneous Measurement of Ionized Magnesium and Ionized Calcium

Objective: To determine whether heparin anticoagulants used for analysis of whole blood ionized calcium would influence the measurement of ionized magnesium. Methods: The effects of zinc heparin, lithium heparin, and electrolyte-balanced heparin on the simultaneous measurement of ionized magnesium and ionized calcium in serum were determined using ion selective electrodes. Results: Time-dependent biases in ionized magnesium and calcium concentrations were apparent with zinc heparin but not with lithium or electrolyte-balanced heparin. Ionized magnesium and calcium concentrations were more significantly influenced by volume-dependent changes in zinc heparin potency than with lithium or electrolyte-balanced heparin. Conclusion: Zinc heparin produces a significant positive bias in the simultaneous determination of ionized magnesium and ionized calcium concentrations