Dilute Prothrombin Time
Tissue Thromboplastin Inhibition
Qualitative detection of lupus anticoagulants (LA) in plasma
Lupus anticoagulants are antibodies which inhibit one or more of the in vitro phospholipid-dependent tests of coagulation (eg, APPT, KCT, dRVVT, dPT). No single screening test can detect all LA-positive patients. The other screening tests commonly used to detect LA assess inhibitors of the intrinsic pathway (aPTT-LA) and the common pathways (dRVVT).
The standard prothrombin time (PT) is usually normal in patients with LA because the thromboplastin reagent used in this assay typically contains a high concentration of phospholipids;however, in the dPT assay, the thromboplastin reagent is diluted, increasing assay sensitivity for phospholipid-dependent inhibitors of the extrinsic pathway. Several reports have suggested that a dPT assay using recombinant tissue factor and synthetic phospholipids can be sensitive for LA. The use of a higher phospholipid concentration confirmatory test allows for determination of the phospholipid dependence of the inhibitor.
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. Do not draw from an arm with a heparin lock or heparinized catheter.
Citrated plasma samples should be collected by double centrifugation. The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. 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 a transpak 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