Ristocetin Cofactor (Functional von Willebrand Factor) 

Updated: Jan 15, 2014
  • Author: Vadim Kostousov, MD; Chief Editor: Eric B Staros, MD  more...
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Reference Range

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 risocetin 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:

Causes of increased shear-induced proteolysis are as follows:

Other causes include the following:

  • Drug-induced (hydroxyethyl starch, valproic acid)
  • Myeloproliferative diseases ( polycythemia, thrombocythemia)
  • Angiodysplasia, glycogen storage disease

Increased vWF:RCo activity and increased vWF:Ag is observed in acute phase reactions, include the following:

  • Stress, extensive exercise
  • Inflammation
  • Cancer
  • Obesity
  • Postoperative period
  • Diabetes
  • Atherosclerosis and atherothrombosis
  • Pregnancy

Collection and Panels

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 18-25 º C or plasma sample should be frozen; specimen stable for one month at -20 º C (Whole blood after collection should not be stored in refrigerator due to cold-induced binding vWF to platelets and selective loss of vWF:Rco activity in plasma. [3] )



von Willebrand factor (vWF) is multimeric glycoprotein (molecular weight varies from 1000-20,000 kDa) that is assembled from identical monomers in endothelial cells and megakaryocytes 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]

The main function of vWF is to support platelet adhesion to injured subendothelium in order to form haemostatic plug. Also vWF is a carrier protein for factor VIII and prevents its proteolysis degradation in plasma. vWF:RCo assay evaluates protein functional activity; platelet aggregation to vWF in the presence of ristocetin is proportional to hemostatically active fraction of vWF. [5, 4]


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.


Repeated vWF:RCo activity testing is sometimes needed to identify low levels of vWF found in vWD. vWF:RCo levels of less than 30% is designated as the amount for a definitive diagnosis of vWD; some patients with type 1 or type 2 vWD may have vWF:RCo levels of 30-50%. [4]

vWF ristocetin binding site polymorphism that is more frequently detected in blacks could cause artificially low vWF:RCo activity and be misdiagnosed as vWD type 2. [6] Other tests (vWF collagen-binding assay, vWF multimer analysis) might be useful to confirm the diagnosis.