PT and TT

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  1. Most important screening tests of the primary hemostatic system are bleeding time (sensitive test of platelet function) and platelet count
    1. platelet count:
      1. counts of 50-100,000 increase bleeding time slightly
      2. counts <50,000 have easy bruising
      3. counts <20,000 have an appreciable incidence of spontaneous bleeding
    2. bleeding time – a 1mm deep and 9mm long incision with a blood pressure cuff inflated on the same arm, bleeding time is the amount of time for the cut to clot
      1. >10 minutes has an increased risk of bleeding but risk does not become substantial until >15-20 minutes
  2. PTT
    1. screens for factors XII, high molecular weight kininogen (HMWK), prekallikrein (PK), XI, IX, and VIII
    2. diseases causing prolongation: no clinical bleeding – XII , HMWK, PK; mild or rare bleeding – factor XI; severe bleeding – factors VIII and IX
  3. PT
    1. screens for factor VII and all factors after X (as does PTT)
    2. prolongation of PT and PTT after infusion of plasma suggests a coagulation inhibitor making the need for a specific test for the conversion of fibrinogen to fibrin necessary – either a thrombin time (TT) or clottable fibrinogen level
    3. diseases causing prolongation of PT: VII, vitamin K, coumadin ingestion, malnutrition
    4. disease causing prolongation of PT and PTT: factor II, V or X, vitamin K deficiency, coumadin
  4. TT – measures conversion of fibrinogen to fibrin
    1. diseases causing prolongation: mild bleeding – afibrinogenemia; severe bleeding – dysfibrinogenemia, heparin administration
      1. dysfibrinogenemia will have abnormal PT, PTT, and bleeding time
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