Other Assays: Clotting Tests

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The page below is a sample from the LabCE course Laboratory Evaluation of the Lupus Anticoagulant found in Antiphospholipid Syndrome (APS). Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Other Assays: Clotting Tests

Kaolin clotting time: This is an aPTT-based test with no added phospholipid that uses kaolin as the activator. The kaolin activates the coagulation cascade via the intrinsic pathway. Since there are no added phospholipids, the test depends on the presence of residual cell membranes and plasma lipids, making the preanalytical conditions very important.
The problem with this test is that kaolin is a particulate substance and can be problematic in coagulation analyzers using optical detection systems. Replacing the kaolin with micronized silica has been suggested to eliminate the issues with sedimentation.
Dilute prothrombin time (PT): This is performed with a recombinant PT reagent. Clotting is initiated by tissue factor via the extrinsic pathway. This activation bypasses the intrinsic pathway and any interferences of factor XII. The test uses a dilute PT reagent which should result in a prolonged clotting time compared with a higher concentration of phospholipid, which will result in a shortened clotting time. The result is calculated as a lupus ratio.
Textarin/ecarin time assay: A ratio is performed using these two tests based on the differential of these snake venoms on phospholipids to activate coagulation. Textarin is a prothrombin activator which requires FVa, phospholipids, and calcium. In the presence of APS the clotting time will be prolonged. Ecarin activates prothrombin to form meizothrombin in the absence of phospholipid. In the presence of LA, the ecarin time will be normal.