von Willebrand factor (vWF) is a large multimeric glycoprotein that serves two critical hemostatic functions: mediating platelet adhesion to exposed subendothelium at sites of vascular injury, and carrying and stabilizing factor VIII in the circulation. Von Willebrand disease (VWD) is the most common inherited bleeding disorder, affecting up to 1% of the population.
von Willebrand Factor Structure and Function
vWF is synthesized in endothelial cells (Weibel-Palade bodies) and megakaryocytes (alpha granules). It is assembled into multimers ranging from 500 to 20,000 kDa, with high-molecular-weight multimers (HMWM) being the most hemostatically active. vWF functions as a molecular bridge: the A1 domain binds platelet glycoprotein Ib (GPIb), the A3 domain binds exposed collagen, and the D’/D3 domain binds factor VIII. Under high shear stress (arterial flow), vWF undergoes conformational change, exposing the GPIb binding site, enabling platelet capture from flowing blood. ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13) cleaves large vWF multimers to regulate their activity.
Laboratory Assessment of vWF
The vWF antigen (vWF:Ag) assay measures the quantitative level of vWF protein by immunoturbidimetric or ELISA methods. The normal range is approximately 50–200 IU/dL. The ristocetin cofactor activity (vWF:RCo) is the functional assay: ristocetin (an antibiotic) induces vWF binding to GPIb, agglutinating fixed platelets. The rate of agglutination is proportional to vWF activity. The ratio of vWF:RCo to vWF:Ag is < 0.6–0.7 in type 2 VWD (qualitative defect). The collagen binding assay (vWF:CB) measures vWF binding to collagen and is used for subtype classification. Factor VIII binding assay evaluates the ability of vWF to bind factor VIII, relevant for type 2N (Normandy) VWD. Multimer analysis by agarose gel electrophoresis visualizes the multimer distribution pattern, distinguishing quantitative (type 1, type 3) from qualitative (type 2) defects. Capillary gel electrophoresis (CGE) is an alternative platform that automates multimer separation with laser-induced fluorescence detection.
von Willebrand Disease Classification
Type 1 VWD (60–80% of cases) is a partial quantitative deficiency with proportional reduction of vWF antigen and activity, and a normal multimer pattern. Bleeding is usually mild to moderate. Type 2 VWD (15–30%) is a qualitative defect with four subtypes. Type 2A is caused by loss of HMWM, with decreased platelet-dependent function. Type 2B involves gain-of-function mutations causing spontaneous binding to platelets, leading to platelet clearance and thrombocytopenia. Type 2M has decreased platelet-dependent function with normal multimer distribution. Type 2N (Normandy) has decreased factor VIII binding, mimicking hemophilia A (low FVIII, prolonged aPTT). Type 3 VWD (< 5%) is a severe quantitative deficiency (undetectable vWF), causing severe bleeding similar to moderate hemophilia.
Diagnosis of VWD
Diagnosis requires a compatible bleeding history (ISTH-BAT score), low vWF levels on at least two occasions (vWF levels fluctuate with stress, inflammation, estrogen, and blood group), and exclusion of other bleeding disorders. Blood group O individuals have intrinsically lower vWF levels (25% lower than non-O), complicating diagnosis. Repeat testing (including non-stress conditions) and exclusion of acquired von Willebrand syndrome (associated with lymphoproliferative disorders, hypothyroidism, valvular heart disease, and certain drugs) are essential.
Treatment of VWD
Desmopressin (DDAVP) stimulates release of stored vWF from endothelial cells, effective in type 1 and some type 2A/2M VWD, but contraindicated in type 2B (can worsen thrombocytopenia) and ineffective in type 3. vWF-containing factor concentrates (Humate-P, Wilate) are used for major bleeding, surgery, and in patients unresponsive to DDAVP. Tranexamic acid (antifibrinolytic) is useful for mucosal bleeding. Platelet transfusions are rarely needed except in type 2B.
Platelet Function Assays
The platelet function assay (PFA-100/200) is a screening test that measures the time for platelets to occlude a membrane coated with collagen and either ADP or epinephrine, under high shear. Closure time is prolonged in VWD, platelet function defects, and thrombocytopenia. It is sensitive, but not specific — a normal PFA-100 does not exclude mild platelet disorders, and abnormalities require confirmation by light transmission aggregometry. Light transmission aggregometry (LTA) is the gold standard for platelet function testing. Platelet-rich plasma is stimulated with agonists (ADP, collagen, arachidonic acid, ristocetin, epinephrine, thrombin), and the increase in light transmission as platelets aggregate is recorded. The pattern of aggregation defects identifies the specific disorder. Whole blood aggregometry (impedance-based) is an alternative requiring less sample preparation. Flow cytometry for platelet surface glycoproteins (CD41/CD61 for GPIIb/IIIa, CD42b for GPIb) confirms specific receptor deficiencies (Glanzmann thrombasthenia, Bernard-Soulier syndrome).
Inherited Platelet Disorders
Glanzmann thrombasthenia is caused by deficiency of GPIIb/IIIa (fibrinogen receptor), with absent aggregation to all agonists except ristocetin, normal platelet count, and mucocutaneous bleeding. Bernard-Soulier syndrome is caused by deficiency of GPIb-IX-V (vWF receptor), with large platelets, thrombocytopenia, absent ristocetin agglutination but normal response to other agonists. Storage pool defects (dense granule or alpha granule deficiency) show specific aggregation patterns. Secretion defects are common and heterogeneous, often with borderline LTA results.
