ZINC HEPARIN INTRODUCES A PREANALYTICAL ERROR IN THE MEASUREMENT OF IONIZED CALCIUM-CONCENTRATION

In this study we determined how zinc heparin affected ionized calcium concentration determinations. Zinc heparin produced a positive bias (0 .020-0.067 mmol l(-1)) in ionized calcium measurement in serum/whole b lood and a negative bias (-0.035 to -0.110 mmol l(-1)) with aqueous io nized calcium standards. To test if the positive bias was due to zinc ions displacing calcium from its protein-calcium complexes, we reprodu ced the effect by adding ZnCl2 to pooled serum. Changes in [ZnCl2] (0. 1-0.5 mmol l(-1)) caused a dose-dependent increase, a constant bias ([ ZnCl2], 0.5-1.25 mmol l(-1)) or a dose-dependent decrease ([ZnCl2] > 1 .25 mmol l(-1)) in ionized calcium concentration, independent of pH. S imilar results were obtained when zinc concentration of the specimen w as changed by varying the volume of blood collected into syringes cont aining zinc heparin. We conclude that zinc heparin produces a positive bias in ionized calcium measurement in protein-based matrices.