Magnesium ions were detected in sodium citrate solutions in several lots of evacuated
blood collection tubes. The mean concentrations ranged between 1.3 and 1.6 mmol/L.
Magnesium was also present in the rubber stoppers of the blood collection tubes and
could be leached into the citrate solution. It was shown that magnesium added to citrated
plasma shortened the prothrombin time of both coumarin and normal plasma. The effect
of magnesium was relatively greater on coumarin than on normal plasma resulting in
reduced prothrombin time ratio. Shortening of the prothrombin time was also observed
when magnesium chloride was added to dialysed plasma, i.e., in the absence of citrate.
These results indicate that magnesium contamination can interfere with accurate INR
determination in the control of oral anticoagulant therapy.
Keywords
Prothrombin time - INR - oral anticoagulant - magnesium - blood collection
