THE PT (PROTHROMBIN TIME) AND PTT (PARTIAL THROMBOPLASTIN TIME) ASSAYS
A patient’s platelet-poor plasma (it is specifically plasma and not serum, as the process of clotting plasma to create serum consumes the coagulation factors) is placed in a testing chamber. For the PT assay, the specimen is mixed with thromboplastin and calcium. For the PTT assay, partial thromboplastin, an activator to initiate the clotting cascade, and calcium are added. The end point of the reaction is clot formation which may be detected optically, because the sample becomes more turbid with clotting, or mechanically, because clot formation reduces the ability of a magnet or other movable object added to move freely.
PT AND PTT MIXING STUDIES
To determine if an elevated value for the PT or the PTT is the result of an inhibitor or one or more factor deficiencies, a sample of the patient plasma is mixed in equal parts with a sample of normal plasma. The mixed plasma is then placed at 37 °C immediately after mixing, and then again at 30, 60, and, if desired, 120 minutes of incubation, a sample is removed from the mixture, and a PT or PTT (whichever is elevated above the reference range) is performed.
COAGULATION FACTOR ASSAYS
To measure the amount of an individual coagulation factor, the test is a modified PT or PTT. The patient’s plasma is mixed with plasma deficient in the single factor being assayed. In this way, the patient’s plasma is the only source of the coagulation factor which is present in the sample. Coagulation factor assays are based upon the PT (adding thromboplastin and calcium) for factors II, V, VII, and X; or the PTT (adding partial thromboplastin, an activator of the coagulation cascade, and calcium) for factors VIII, IX, XI, and XII.
VON WILLEBRAND FACTOR ASSAYS
There are now several different assays to assess the function of von Willebrand factor. The ristocetin cofactor assay has been the primary method to measure von Willebrand factor activity for decades. It is now being replaced with a variety of assays using different methodologies (not shown here), and the need for a replacement is driven by the high complexity and imprecision of the ristocetin cofactor test. There is no dominant assay in use currently. Ristocetin cofactor is the name given to the test because ristocetin is the principal reagent in this assay. Using formalin-fixed platelets, the rate of platelet aggregation upon addition of ristocetin to the sample is measured to provide a quantitative value for von Willebrand factor function. It is important to note that it is not the ...