Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (407 page)

Circulating anticoagulants are antibodies that inhibit the function of specific coagulation factors, most commonly factors VIII or IX. The anticoagulants may be acquired following multiple transfusions in hemophiliacs (alloantibodies), or spontaneous (autoantibodies), most commonly against factor VIII. Lupus anticoagulant is not an anticoagulant, but prothrombotic.

   Who Should Be Suspected?

A circulating anticoagulant is suspected under two conditions:

   A patient with hemophilia A or B or, rarely, another congenital clotting factor deficiency, who has had multiple transfusions and whose bleeding does not stop on infusion of the missing factor.

   A middle-aged person, especially if suffering from lymphoma, or a postpartum patient who develops unprovoked hemorrhages: autoantibodies to factor VIII in most cases.

   Laboratory Findings and Interpretation

   In a patient with hemophilia, determinations of the missing factor show no elevations following infusions.

   In a patient with no previous bleeding history, the finding of a prolonged PTT should raise the suspicion of an acquired circulating anticoagulant. If incubation at 37°C of half normal plasma mixed with half the patient’s plasma for 1–2 hours does not correct the prolonged PTT, the presence of a circulating anticoagulant is proven.

   Specific titration of the inhibitor’s potency is performed either for factor VIII or IX inhibitors and the results reported in Bethesda Inhibitor units.

   
THROMBOTIC DISORDERS

THROMBOPHILIA

   Definition

Thrombophilia may be defined as a hereditary or acquired tendency to develop thrombotic episodes due to an abnormality in the coagulation system (i.e., a hypercoagulable state). The thrombosis may have a predilection for arteries or veins. Fifty percent of thrombotic events in patients with inherited thrombophilia occur in the setting of an acquired risk factor.

   Who Should Be Suspected?

  1.  Suspect inherited thrombophilia

A.   Venous thrombophilia

   A family history of VTE and VTE at a young age (<45 years)

   Recurrent VTE