Liquid Heparin Anticoagulant Produces More Negative Bias in the Determination of Ionized Magnesium than Ionized Calcium

The ionized calcium level in blood is known to be falsely decreased when self-prepared liquid heparin anticoagulant is used, due to dilution and binding effects. The effect of liquid heparin on the determination of ionized magnesium is not as well understood. We compared the effect of liquid sodium heparin on the determination of ionized calcium and magnesium in 44 clinical samples using two types of user-prepared heparin syringes which differed in the amount of residual heparin from the BD Preset reference syringe. With the type 1 syringe, the liquid heparin was expelled once or twice such that some heparin could be left in the dead space at the syringe hub, while the liquid sodium heparin was thoroughly expelled from the type 2 syringe. The ionized magnesium levels obtained with the type 1 syringe were significantly lower than the reference value (by 0.068 mmol/L) (p < 0.0001), while the value obtained with the type 2 syringe differed less from the reference, by only 0.014 mmol/L (p < 0.0001). The heparin binding effect resulted in more negative bias in ionized magnesium (- 0.026 +/- 0.032 mmol/L) than in ionized calcium (- 0.009 +/- 0.042 mmol/L, p < 0.0001). In conclusion, we recommend using lyophilized, calcium-balanced, heparinized syringes for the determination of ionized magnesium and ionized calcium due to the increased negative bias in ionized magnesium determinations. When user-prepared syringes are used, the thorough evacuation of heparin solution should be strictly prescribed.