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von Willebrand Factor Assays

von Willebrand Factor Assays is a Hematology laboratory test reference in the LabPedia Medical Laboratory Library. Panel measuring vWF antigen and activity (and often factor VIII) used to evaluate von Willebrand disease and acquired vWF abnormalities.

Also known asvWF:Ag; vWF activity; Ristocetin cofactor

Overview

von Willebrand factor assays quantify vWF protein (antigen) and its platelet-binding/functional activity; factor VIII activity is commonly measured in parallel.

Clinical Importance

To evaluate mucocutaneous bleeding suggestive of von Willebrand disease

To characterize acquired vWF deficiency states when suspected

Specimen Requirements

  • Specimen: platelet-poor plasma prepared from whole blood
  • Tube: light-blue top; 3.2% sodium citrate (0.109 mol/L)
  • Anticoagulant ratio: 9:1 blood-to-citrate (CLSI H21); fill to the tube fill line
  • Handling: invert gently; transport promptly; avoid traumatic draws and line heparin contamination
  • Processing: centrifuge to platelet-poor plasma per institutional protocol before testing
  • Stability: assay-specific (CLSI H21). aPTT is generally less stable than PT; do not exceed validated time/temperature limits for the ordered tests.

Analytical Method

Immunoassays measure vWF antigen; activity assays use ristocetin-triggered platelet binding or newer GPIb-binding methods. Multimer analysis is specialized follow-up.

Instruments Used

Automated analyzers

  • Sysmex CS / CN coagulation analyzers
  • Werfen / Instrumentation Laboratory ACL TOP coagulation analyzers
  • Siemens Healthineers BCS coagulation analyzers
  • Stago STA Compact / STA R Max coagulation analyzers
  • Immunoassay modules for vWF antigen/activity

Manual methods

  • Specialized electrophoresis for vWF multimers (reference laboratories)

Reference Values

Typical adult SI examples below follow widely published international hematology practice (e.g., Dacie and Lewis; Henry's). Always interpret against the method- and demographics-specific interval printed on the patient report (CLSI EP28-A3c). Neonatal, pediatric, pregnancy, and altitude intervals differ.

Intervals are method-specific; ABO blood group O individuals often have lower vWF levels; many laboratories use ABO-adjusted interpretation.

Factor VIII often parallels vWF.

Interpretation

Common Causes of Increased Results

  • Elevated vWF as acute-phase reactant; can mask mild deficiency during illness

Common Causes of Decreased Results

  • Reduced antigen and/or activity patterns guide VWD subtype workup with multimers when indicated

Clinical Significance

  • von Willebrand disease evaluation
  • Investigation of unexplained mucocutaneous bleeding with normal platelet count

Factors Affecting Results

Pre-analytical

  • Underfilled citrate tube (excess anticoagulant prolongs clotting times)
  • Overfilled tube or clot in specimen
  • Hematocrit >55% may require adjusted citrate volume (CLSI H21 guidance)
  • Delayed processing or improper storage beyond assay stability
  • Heparin contamination from line draws
  • Stress, pregnancy, and acute illness raise vWF and can normalize mild deficiency

Analytical

  • Reagent lot / instrument calibration differences
  • Lipemia, hemolysis, or icterus interfering with photo-optical endpoints

Post-analytical

  • Applying another laboratory's reference interval
  • Misreading INR therapeutic targets as untreated reference intervals

Advantages and Limitations

Advantages

  • Directly assesses the most common inherited bleeding disorder pathway

Limitations

  • Requires panel interpretation, not a single number
  • Acute-phase elevation can hide mild disease

Clinical Pearls

Never diagnose or exclude mild VWD from a single stressed outpatient draw; repeat testing in a baseline state when results and history conflict.

Related Laboratory Tests

References

Authoritative textbooks, guidelines, and reviews supporting this laboratory test reference. Verify reference intervals, critical limits, and method details against institutional protocols and current guideline editions.

Textbooks

  • McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021.
  • Rodak BF, Carr JH. Rodak's Hematology: Clinical Principles and Applications. 6th ed. Elsevier; 2020.
  • Hoffbrand AV, Moss PAH, Pettit JE. Essential Haematology. 8th ed. Wiley-Blackwell; 2019.
  • McKenzie SB, Williams JL. Clinical Laboratory Hematology. 4th ed. Pearson; 2022.
  • Bain BJ, Bates I, Laffan MA. Dacie and Lewis Practical Haematology. 12th ed. Elsevier; 2017.
  • Kaushansky K, et al. Williams Hematology. 10th ed. McGraw-Hill; 2021.
  • Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.

CLSI and Professional Guidelines

  • Clinical and Laboratory Standards Institute. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays; Approved Guideline. CLSI document H21-A5. CLSI; 2017.
  • Clinical and Laboratory Standards Institute. One-Stage Prothrombin Time (PT) Test and Partial Thromboplastin Time (PTT) Test; Approved Guideline. CLSI document H47-A2. CLSI; 2008.
  • Clinical and Laboratory Standards Institute. Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory; Approved Standard. CLSI document EP28-A3c. CLSI; 2010.
  • Clinical and Laboratory Standards Institute. Management of Critical- and Significant-Risk Results. CLSI guideline GP47-Ed1. CLSI; 2015.

Frequently Asked Questions

Common questions about using this von Willebrand Factor Assays laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.

What is the von Willebrand Factor Assays laboratory test reference used for?

This Medical Laboratory Library page summarizes the clinical importance of von Willebrand Factor Assays. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.

Where can I find von Willebrand Factor Assays specimen requirements?

See the Specimen Requirements section on this page for specimen information related to von Willebrand Factor Assays. Confirm collection details against your laboratory protocol.

Where are von Willebrand Factor Assays reference values listed?

See the Reference Values section on this Laboratory Library page. Reference ranges can vary by method and population, so verify intervals with your laboratory.

How do I interpret von Willebrand Factor Assays results?

Use the Interpretation section for laboratory interpretation framing, including increased and decreased result patterns when provided. Do not treat educational text as a patient-specific diagnosis.

What analytical method is used for von Willebrand Factor Assays?

See the Analytical Method section for principle and method notes. Method details may differ by instrument and institutional SOP.

Are related laboratory tests linked from von Willebrand Factor Assays?

Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.

How can I continue learning after reading von Willebrand Factor Assays?

Use Continue learning links on this page for related Academy lessons, medical laboratory quizzes, or laboratory case studies when available for this Hematology topic.

What are the clinical pearls for von Willebrand Factor Assays?

See the Clinical Pearls section for concise laboratory practice notes associated with von Willebrand Factor Assays. Apply pearls alongside institutional protocols.

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