Prothrombin Time (PT) Mixing Study
PT Mixing Study
Distinguish between circulating inhibitor and coagulation factor deficiency when PT is prolonged
A mixing study is used to study the cause of a prolonged screening test. This study can determine if the cause is a deficiency of one or more factors or an inhibitor.6 In a mixing study, platelet-free, normal plasma that is replete with all coagulation factors (near 100% activity for each) is mixed with the patient sample. For example, in a 1:1 mix, one part patient sample is mixed with one part normal plasma and the mixture is tested. In this case, the lowest possible concentration for any individual factor in the mixture would be approximately 50% (in the case of a patient with a factor concentration of zero) and the normal pool has an activity of 100%. In the absence of inhibitors, this mixture will produce a normal clotting time. When addition of normal plasma fails to correct the clotting time into the normal range, the cause of the abnormal test is likely an inhibitor. The performance and interpretation of mixing studies is described in more detail in the online Coagulation Appendices: Abnormal Screening Results and Lupus Anticoagulants.
Blue-top (sodium citrate) tube
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.
Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, “butterfly”), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube.Transfer the plasma into atranspak frozen purple tube with screw cap.Freeze immediately and maintain frozen until tested.
Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability
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