von Willebrand Factor (vWF) Activity

CPT: 85245
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  • Ristocetin Cofactor

Test Includes

Quantitative platelet agglutination with ristocetin

Special Instructions

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.

Expected Turnaround Time

1 - 3 days

Related Documents

For more information, please view the literature below.

Procedures for Hemostasis and Thrombosis: A Clinical Test Compendium

Specimen Requirements


Plasma (platelet poor), frozen


1 mL


Blue-top (sodium citrate) tube


Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples unless the sample is collected using a winged (butterfly) collection system. With a winged blood collection set a discard tube should be drawn first to account for the dead space of the tubing and prevent under-filling of the evacuated tube.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternative anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes.

Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

Storage Instructions


Patient Preparation

Avoid warfarin (Coumadin®) therapy for two weeks and heparin therapy for two days prior to the test. Do not draw from an arm with a heparin lock or heparinized catheter.

Causes for Rejection

Gross hemolysis; clotted specimen; frozen specimen thawed in transit; improper labeling

Test Details


Screen and diagnose von Willebrand factor (vWF) deficiency6,8-10


vWF ristocetin cofactor activity may be spuriously low in individuals with certain polymorphisms in the vWF gene (eg, Asp1472His) that alter the binding of ristocetin to vWF. These polymorphisms have been reported in 17% of whites and 63% of African Americans without a history of a bleeding disorder.11 A number of transient clinical conditions can raise the vWF levels of individuals with congenital deficiency into the normal range.8 vWF is an acute phase reactant and levels can increase due to stress, inflammation, acute infection, physical exercise, and following surgery.8 Levels can also increase with estrogen administration for contraception or hormone replacement.8 vWF levels are increased two- to threefold in the second and third trimesters of pregnancy.10 Individuals with type O blood tend to have approximately 30% lower vWF levels than those with other blood types.10


In this ristocetin cofactor assay, the patient plasma is mixed with formalin-fixed platelets in the presence of ristocetin.12 The extent of platelet agglutination detected photo-optically is proportional to the vWF activity of the sample.

Reference Interval

50% to 150%. Average vWF levels tend to vary by blood type.6 One study found the mean vWF levels by blood type to be as follows:7

• Type O: 74.8%

• Type A: 105.6%

• Type B: 116.9%

• Type AB: 123.3%

Additional Information

See von Willebrand Factor (vWF) Profile [084715] for more clinical information.


1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997; 107(1):105-110. 8980376
2. Reneke J, Etzell J, Leslie S, et al. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998; 109(6):754-757. 9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997; 107(6):681-683. 9169665
5. McGlasson DL, More L, Best HA, et al. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999; 12(3):137-139. 10539100
6. Van Cott EM, Laposata M. Coagulation. In: Jacobs DS, DeMott WR, Oxley DK, eds. Laboratory Test Handbook With Key Word Index. Hudson, Ohio: Lexi-Comp; 2001:327-358.
7. Gill JC, Endres-Brooks J, Bauer PJ, et al. The effect of ABO blood group on the diagnosis of von Willebrand disease. Blood. 1987; 69(6):1691-1695. 3495304
8. Adcock DM, Bethel MA, Macy PA. Coagulation Handbook. Aurora, Colo: Esoterix−Colorado Coagulation; 2006.
9. Brandt JT. Laboratory evaluation of platelet disorders. In: McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Philadelphia, Pa: Lippincott Williams and Wilkins; 2002:1010-1032.
10. Rick ME. von Willebrand disease. In: Kitchens CS, Alving BM, Kessler CM, eds.Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002:91-102.
11. Flood VH, Gill JC, Morateck PA, et al. Common VWF exon 28 polymorphisms in African Americans affecting the VWF activity assay by ristocetin cofactor. Blood. 2010 Jul 15; 116(2):280-286. 20231421
12. Liu MC, Kessler CM. A systemic approach to the bleeding patient. In: Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002:181-196.


Nichols WL, Hultin MB, James AH, et al. von Willebrand disease (VWD): evidence-based diagnosis and management guideline. The National Heart, Lung, and Blood Institute (NHLB) Expert Panel Report (USA). Haemophilia. 2008; 14(2):171-232.18315614
Tefferi A, Nichols WL. Acquired von Willebrand disease: Concise review of occurrence, diagnosis, pathogenesis, and treatment. Am J Med. 1997; 103(6):536-540.9428838


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
164509 vWF Activity 6014-5 164509 vWF Activity % 6014-5

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