Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (400 page)

   vWF Ag and activity levels run 20–30% lower in blood group O than in individuals with other blood groups.

   VWF levels fluctuate. Both the VWF and factor VIII are acute-phase reactants. Levels may increase two to five times from baseline in the third trimester of pregnancy, with strenuous exercise and severe stress. Other preanalytic variables (collection of sample, storage) must be considered. Repeated testing may be required.

Suggested Readings

Branchford BR, Paola JD. Making a diagnosis of VWD.
Hematology Am Soc Hematol Educ Program.
2012;2012:161–167.

Favaloro EJ, Mohammed S, Mcdonald J. Validation of improved performance for the automated von Willebrand factor ristocetin cofactor activity assay.
J Thromb Haemost.
2010;8:2842–2844.

Nichols WL, Hultin MB, James AH, et al. von Willebrand disease (VWD): evidence-based diagnosis and management guidelines, the National Heart, Lung, and Blood Institute (NHLBI) expert panel report (USA).
Hemophilia.
2008;14:171–232.

FACTOR XII (F XII) DEFICIENCY

   Definition

The intrinsic pathway of coagulation is initiated by F XII in a reaction involving high molecular weight kininogen and plasma kallikrein. F XII deficiency was first described preoperatively in Mr. Hageman (hence the synonym Hageman factor) who was found to have a prolonged PTT, no bleeding history, and no deficiency of any known clotting protein. He succumbed to a thrombotic event. Affected patients have no bleeding diathesis. The fact that patients with severe deficiency of F XII, prekallikrein, and low molecular weight kininogen do not bleed, even when exposed to severe trauma or surgery, suggests that these proteins play no, or a minimal, role in hemostasis, although they may have other physiologic roles.

   Laboratory Findings Test

   Patients have normal PT, but variably prolonged PTT, depending on the severity of deficiency. A definitive diagnosis (necessary to exclude other etiologies for a prolonged PTT) is established by determining the factor’s level in plasma.

   F XII is not affected by vitamin K deficiency or oral anticoagulants.

   Decreased Levels

   Congenital: Autosomal recessive inheritance: Levels of F XII are 40–60% in heterozygous state and are undetectable in homozygotes.

   Acquired

   Septic shock

   Severe liver disease

   Nephrotic syndrome