Ristocetin Cofactor (Functional von Willebrand Factor)

Reference ranges for age groups are as follows ^{[1, 2]} :

• Newborn (< 6 mo) - 50-200% (IU/dL)

• Children (1-10 y) - 40-130% (blood type O); 50-180% (non-O blood type)

• Adults - 50-150% (blood type O); 60-180% (non-O blood type)

von Willebrand factor deficiency (inherited)

See the list below:

• Type 1 - Decreases are noted in both von Willebrand factor (vWF) antigen (vWF:Ag) and vWF ristocetin cofactor (vWF:RCo) levels (vWF:RCo to vWF:Ag ratio >0.7).

• Type 2 - Decrease only noted in vWF:RCo levels. vWF:Ag levels are largely normal (vWF:RCo to vWF:Ag ratio < 0.7) in von Willebrand disease (vWD) types 2A and 2B, but vWF:RCo may be normal or decreased in types 2N and 2M.

• Type 3 - Severe deficiency or absence of both vWF:RCo and vWF:Ag (< 5%).

von Willebrand factor deficiency (acquired)

Autoimmune clearance or inhibition causes are as follows:

• Lymphoproliferative diseases (lymphoma, leukemia)

• Monoclonal gammopathies (multiple myeloma, Waldenstrom macroglobulinemia)

• Systemic lupus erythematosus and other autoimmune disorders

• Some cancers (Wilms tumor, Ewing sarcoma, carcinoma)

Causes of vWF deficiency due to increased shear-induced proteolysis are as follows:

• Small-sized (pinhole) ventricular septal defect

• Severe aortic stenosis

• Primary pulmonary hypertension

• Extracorporeal life support

Other causes include the following:

• Hypothyroidism

• Drug-induced (hydroxyethyl starch, valproic acid)

• Myeloproliferative diseases (polycythemia, thrombocythemia)

• Angiodysplasia, glycogen storage disease

Increased vWF:RCo activity and increased vWF:Ag are observed in acute phase reactions, which can be associated with the following:

• Stress, extensive exercise

• Inflammation

• Cancer

• Obesity

• Postoperative period

• Diabetes

• Atherosclerosis and atherothrombosis

• Pregnancy

Collection and panel details are as follows:

• Specimen - Citrated plasma

• Collection - Tube with sodium citrate 3.2% citrate, blue top

• Centrifugation - 2000-2500 g for 15 minutes or similar regime to produce platelet-poor plasma

• Storage - As long as 6 hours at 18-25ºC or plasma sample should be frozen; specimen stable for 1 month at -20ºC (Whole blood after collection should not be stored at refrigerator temperature [2-4ºC] due to cold-induced binding of vWF to platelets and selective loss of vWF:RCo activity in plasma. ^[3] )

Description

von Willebrand factor (vWF) is a multimeric glycoprotein (molecular weight varies from 1000-20,000 kDa) that is assembled from identical monomers in endothelial cells and megakaryocytes; it is possibly released from endothelium and platelets upon their activation. The half-life of vWF is approximately 12 hours (range, 9-15 h), and its clearance is faster in persons with blood type 0. ^{[4, 5]}

The main function of vWF is to support platelet adhesion to injured subendothelium in order to form a hemostatic plug. Also, vWF is a carrier protein for factor VIII and prevents its proteolytic degradation in plasma. vWF:RCo assay evaluates protein functional activity; platelet aggregation to vWF in the presence of ristocetin is proportional to the hemostatically active fraction of vWF. ^{[6, 3]}

In order to overcome poor sensitivity of the standard assay at lower activity of vWF:RCo (less than 20-30%), a modified protocol of vWF:RCo assay was implemented and validated. ^{[7, 8]} Also, assays of vWF activity measurement were developed that utilize microparticles covered with recombinant platelet receptors to glycoprotein Ib, which bind to vWF (vWF:GPIb) in the presence or absence of ristocetin; ^[9] these could replace the standard vWF:RCo activity assay in the future.

Indications/Applications

vWF:RCo (in conjunction with vWF:Ag and factor VIII activity) is indicated for the following:

• Diagnosis of vWD

• Differentiation of vWD subtypes

• Differentiation of vWD from hemophilia A

• Monitoring therapy of vWD

For more information, see the Medscape Drugs & Diseases article von Willebrand Disease.

Considerations

Repeated vWF:RCo activity testing is sometimes needed to identify low levels of vWF compatible with vWD. A retrospective study showed that up to 30% of pediatric patients required a second test for vWD before the diagnosis was established. ^[10]  A vWF:RCo level of less than 30% is designated as the level for a definitive diagnosis of vWD, although some patients with type 1 or type 2 vWD have vWF:RCo levels of 30-50%. ^[4] Bucciarelli et al reported that adults with initial vWF:RCo levels of 30-40% were very likely to have vWD, which was confirmed in these patients by second‐level tests. In the same study, among individuals with vWF:RCo levels of 40-60%, vWD was diagnosed in 20-30%, with female gender and non-O blood type being independent predictors of vWD diagnosis. ^[11]

vWF ristocetin binding–site polymorphism, which is more frequently detected in African Americans, can cause artificially low vWF:RCo activity and lead to a misdiagnosis of vWD type 2; ^[7] other tests (vWF collagen-binding assay, vWF:GPIb without ristocetin, vWF multimer analysis) may be useful to confirm the diagnosis in these patients.

During the third trimester of pregnancy, the vWF:RCo level can reach normal ranges in the majority of patients with vWD type 1. However, vWF:RCo activity is usually low in pregnant women with vWD type 2 or 3. ^{[12, 13]}

Evidence-based guidelines produced by a multidisciplinary panel of the American Society of Hematology (ASH), the International Society on Thrombosis and Haemostasis (ISTH), the National Hemophilia Foundation (NHF), and the World Federation of Hemophilia (WFH) include the following recommendation regarding ristocetin ^[14] :

• It is suggested that vWD be diagnosed using newer assays that measure the platelet-binding activity of vWF (such as an assay using a gain-of-function mutant of GPIb that requires no ristocetin [vWF:GPIbM] or an assay using ristocetin [vWF:GPIbR]), rather than the vWF:RCo assay