Clinics and Departments

Laboratory Services

Lab Dept:


Test Name:


General Information

Lab Order Codes:



Willebrand Factor Activity; VW Factor Assay

CPT Codes:

85245 - Factor VIII vW Factor, Ristocetin cofactor

Test Includes:

Ristocetin cofactor resulted as a %.


Test Indications:

Aid with the differential diagnosis of hemophilia and type of von Willebrand disease.

Lab Testing Sections:

Coagulation (Minneapolis Campus)

Phone Numbers:

Minneapolis: 612-813-6280

St. Paul: 651-220-6550

Test Availability:

Twice Weekly (Tuesday and Friday)

Turnaround Time:

3 – 5 Days

Special Instructions:

See Patient Preparation
Deliver immediately to the laboratory at room temperature.


Specimen Type:

Whole blood


Light Blue (Buffered Na Citrate 3.2%) top tube

Draw Volume:

2.7 mL blood

Processed Volume:

0.9 mL plasma


● A clean venipuncture is essential, avoid foaming.
● Entire sample must be collected with single collection, pooling of sample is unacceptable.
● Capillary collection is unacceptable.
● Patient’s with a hematocrit level >55% must have a special tube made to adjust for the hematocrit; contact lab for a special tube.
● Mix thoroughly by gentle inversion. Deliver immediately to the laboratory at room temperature via courier or pneumatic tube.

Off campus collections:
● Must be tested within 4 hours.
● Do not refrigerate.
● If not received in our lab within 4 hours of collection, sample must be centrifuged and *platelet-poor plasma removed from cells and transferred to an aliquot tube being careful not to disturb the cell layer. Centrifuge the plasma a second time and transfer into a clean aliquot tube being careful not to include any residual platelets on the bottom of the tube. Freeze at -20C and deliver to the lab on dry ice within 2 weeks.
*Validation of your lab’s centrifuge for platelet poor plasma is required.

Special Processing:

Lab Staff: All testing will be performed in Minneapolis.

St. Paul Lab: Send whole blood specimens to Mpls for processing via courier. Must be processed within 4 hours of collection. Contact Mpls prior to sending.

Mpls Lab: Spin sample collected in blue top tube for 5 minutes on the Stat Spin centrifuge, remove plasma and transfer to a 4 mL BCS sample cup. Spin plasma again for 5 minutes in the Stat Spin Centrifuge. Transfer plasma to a new BCS sample cup for analysis leaving approximately 200 uL in the bottom of the original cup to discard. Store at frozen (-70 degrees C) temperatures if not run immediately.

Patient Preparation:

No Heparin for 2 days prior to testing.

Sample Rejection:

Improper tube; clotted specimen; underfilled tube; mislabeled or unlabeled specimens


Reference Range:


Healthy Preemie
Range (%)

Full Term Infant Range (%)

Day 1

78 – 210%

86 – 220%

Day 5

72 – 219%

83 – 197%

Day 30

66 – 216%

69 – 187%

Day 90

75 – 184%

84 – 162%

Day 180

54 – 158%

62 – 152%


Range (%)

1 – 5 years

60 – 120%

6 – 10 years

44 – 144%

11 – 16 years

46 – 153%


50 – 158%

Critical Values:





Measurement of agglutination of formalin-fixed platelets by patient’s plasma with ristocetin as compared to agglutination produced by a standard plasma used to prepare a curve.


Harmening DH (1997) Clinical Hematology and Fundamentals of Hemostasis


5/24/2010: Tubing of patient specimens is no longer prohibited.
12/14/2010: Processing information updated.
9/15/2014: Added off campus collection info.

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