Reference Values

1.12-1.32 mmol/L

• Critical Value:
  • Low: < 0.80 mmol/L
  • High: > 1.60 mmol/L

Venous stasis (prolonged tourniquet application) and forearm exercise may increase ionized calcium due to a decrease in pH caused by localized production of lactic acid. Exposing the sample to air will cause an increase in pH due to the loss of CO₂, which will decrease ionized calcium.

Heparin binds calcium. Each unit of heparin added per ml of blood will decrease ionized calcium by 0.01 mmol/L. Therefore, the correct ratio of heparin anticoagulant to blood must be achieved during sample collection. Intravenous injection of 10,000 units of heparin has been shown in adults to cause a significant decrease of ionized calcium of about 0.03 mmol/L.

Hemodilution of the plasma by more than 20% associated with priming cardiopulmonary bypass pumps, plasma volume expansion or other fluid administration therapies using certain solutions may cause clinically significant error on ionized calcium results. These errors are associated with solutions that do not match the ionic characteristics of plasma. To avoid these errors when hemodiluting by more than 20%, use physiologically balanced multi-electrolyte solutions containing low-mobility anions (e.g. Gluconate) such as Normosol-R (Abbott Laboratories), Plasma-Lyte-A (Baxter Healthcare oCrporation), and Isolyte-S (B Braun Medical) rather than solutions such as normal saline or ringer’s lactate.