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Ristocetin Cofactor (Functional von Willebrand Factor) 

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

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.

Contributor Information and Disclosures

Vadim Kostousov, MD Research Associate, Transfusion Medicine and Coagulation, Department of Pathology and Immunology, Texas Children’s Hospital, Baylor College of Medicine

Disclosure: Nothing to disclose.

Chief Editor

Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital

Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology

Disclosure: Nothing to disclose.

  1. Klarmann D, Eggert C, Geisen C, Becker S, Seifried E, Klingebiel T. Association of ABO(H) and I blood group system development with von Willebrand factor and Factor VIII plasma levels in children and adolescents. Transfusion. 2010 Jul. 50(7):1571-80. [Medline].

  2. Appel IM, Grimminck B, Geerts J, Stigter R, Cnossen MH, Beishuizen A. Age dependency of coagulation parameters during childhood and puberty. J Thromb Haemost. 2012 Aug 21. [Medline].

  3. Favaloro EJ, Soltani S, McDonald J. Potential laboratory misdiagnosis of hemophilia and von Willebrand disorder owing to cold activation of blood samples for testing. Am J Clin Pathol. 2004 Nov. 122(5):686-92. [Medline].

  4. 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). Haemophilia. 2008 Mar. 14(2):171-232. [Medline].

  5. Federici AB, Lee CA, Berntorp EE. Lillicrap D, Montgomery RR. Von Willebrand Disease: Basic and Clinical Aspects. Wiley-Blackwell; 2011.

  6. Flood VH, Gill JC, Morateck PA, Christopherson PA, Friedman KD, Haberichter SL. Common VWF exon 28 polymorphisms in African Americans affecting the VWF activity assay by ristocetin cofactor. Blood. 2010 Jul 15. 116(2):280-6. [Medline].

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