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- Tissue Thromboplastin Inhibition
Qualitative detection of lupus anticoagulants (LA) in plasma6
Improper preparation of the platelet-poor plasma at collection reduces the sensitivity of this assay for LA.6,7 The screening assay can be prolonged in patients with congenital or acquired factor deficiency and will be elevated in individuals on warfarin therapy.7 The reagent contains agents that neutralize unfractionated heparin at plasma levels up to 1.0 unit/mL.7 Samples with levels higher than this can produce anomalous results.7 No single LA test can detect all LA-positive patients. The ISTH recommends that any sample suspected of having LA be tested using two or more LA screening tests.6
Clotting time is measured using a dilute prothrombin time (dPT) reagent consisting of a unique formulation of relipidated recombinant tissue factor and calcium.8 Samples with extended dPT are tested in a confirmatory assay including excess phospholipid. The sample is considered to be positive for lupus anticoagulant when the dPT ratio (ie, dPT:confirm) is >1.15.
Lupus anticoagulants are antibodies which inhibit one or more of the in vitro phospholipid-dependent tests of coagulation (eg, APPT, KCT, dRVVT, dPT). Recently, the SCC Subcommittee for the Standardization of Lupus Anticoagulants provided guidelines for the laboratory diagnosis of LA.6 No single screening test can detect all LA-positive patients. The ISTH recommends that any sample suspected of having LA be tested using two or more LA screening tests.6,9 The other screening tests commonly used to detect LA assess inhibitors of the intrinsic pathway (aPTT-LA) and the common pathways (dRVVT). The dPT assay screens for the phospholipid-dependent inhibitors of a different part of the coagulation cascade, the extrinsic pathway.10
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;11 however, in the dPT assay, the thromboplastin reagent is diluted, increasing assay sensitivity for phospholipid-dependent inhibitors of the extrinsic pathway.11 Several reports have suggested that a dPT assay using recombinant tissue factor and synthetic phospholipids can be sensitive for LA.11,12 The use of a higher phospholipid concentration confirmatory test allows for determination of the phospholipid dependence of the inhibitor.6
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 a LabCorp PP transpak frozen purple tube with screw cap (LabCorp N° 49482). Freeze immediately and maintain frozen until tested. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.
Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.
Causes for Rejection
|Order Code||Order Code Name||Order Loinc||Result Code||Result Code Name||UofM||Result LOINC|
|005200||Dilute Prothrombin Time||005201||Dilute Prothrombin Time(dPT)||sec||53748-0|
|005200||Dilute Prothrombin Time||005203||dPT Confirm Ratio||Ratio||57181-0|