['To determine whether heparin anticoagulants used for analysis of whole blood ionized calcium would influence the measurement of ionized magnesium.', '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.', '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.'] Do specific heparin preparations interfere with the simultaneous measurement of ionized magnesium and ionized calcium?