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.