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Ideally, the patient should not be on anticoagulant therapy. Avoid warfarin (Coumadin) therapy for two weeks prior to the test and heparin, direct Xa, and thrombin inhibitor therapies for about three days prior to testing.
Factor XII activity is determined using an aPTT-based one-stage clotting time assay.
Direct Xa or thrombin inhibitor therapy may cause factitiously low results.
Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability