Heparinization of blood decreases ionized calcium concentration

To evaluate whether heparin in the plasma sample may affect the ionized calcium level, we compared the ionized calcium levels of blood samples of three different heparin concentrations in 25 normal healthy volunteers: Group A contained no heparin (control group); Group B contained 4.7 IU/ml heparin; and Group C contained 37.5 IU/ml heparin. The result showed ionized calcium to be 4.99 +/- 0.28 mg/dl (Mean +/- SEM) in group A; 4.64 +/- 0.26 mg/dl (Mean +/- SEM) in group B and 4.25 +/- 0.24 mg/dl (Mean +/- SEM) in group C. These three values were significantly different (p less than 0.001). So, heparin might artificially lower the plasma ionized calcium concentration. It is concluded that: (1) samples for accurate ionized calcium measurement should contain no heparin; and (2) in spite of the careful measurement by conventional technique, an introduction of an excessive amount of heparin into the deadspace of the syringe may significantly reduce the ionized calcium concentration.