Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (917 page)

Decreased In

   If factor VIII decreases below 40%, PTT becomes prolonged. In the presence of an inhibitor to factor VIII, PTT remains prolonged even after therapeutic infusions of factor VIII; mixing the patient’s plasma with normal plasma in a 1:1 proportion does not correct the prolonged PTT and does not increase the original low factor VIII. Specific methodology can report the titer of the inhibitor in Bethesda inhibitory units.

   
Congenital disorders

   Hemophilia A: usually severe deficiency in male carriers and usually mild decrease in female carriers of the hemophilia gene

   von Willebrand disease (see p. 454): especially if moderate to severe; more so in individuals with blood type B

   
Acquired disorders

   Acquired anti–factor VIII autoantibodies in previously unaffected individuals

   Acquired anti–factor VIII alloantibodies in hemophilia A patients treated with multiple infusions of factor VIII

   DIC and pathologic fibrinolysis

Increased In

   Acute-phase reactant (acute inflammatory conditions).

   Pregnancy and the use of oral contraceptives.

   If markedly increased, it may predispose to thromboembolism.

FACTOR XI
*

   Definition

   Factor XI is synthesized in the liver and megakaryocytes. Factor XI is activated by factor XIIa and by thrombin, the preferred activator on platelet surface. In turn, factor XI activates factors XII and IX in the intrinsic pathway. Factor XI is not affected by vitamin K antagonists.