Interpretation of a Von Willebrand Disease Panel

Initial Testing

• The American Society of Hematology recommends ordering the three core assays—VWF antigen, VWF ristocetin‑cofactor (or modern GPIb‑binding) activity, and factor VIII activity—simultaneously as the initial panel to avoid incomplete diagnosis. 1
• VWF antigen quantifies the amount of circulating VWF protein. 1
• VWF ristocetin‑cofactor (or VWF:GPIbR/GPIbM) measures the functional platelet‑binding activity of VWF. 1
• Factor VIII activity is frequently reduced in VWD and helps differentiate VWD from hemophilia A. 1

Diagnostic Thresholds

• A VWF level < 30 IU/dL is the primary cutoff for a definitive VWD diagnosis (American Society of Hematology). [2][3]4
• This cutoff incorporates the ~25 % lower baseline VWF seen in blood‑type O individuals. [2][3]
• VWF levels of 30–50 IU/dL may still indicate VWD when supported by a strong personal or family bleeding history; borderline results should not be dismissed without clinical correlation. 4
• The normal reference range for VWF is 50–200 IU/dL. [2][3]4

Interpretation of the VWF:RCo/VWF:Ag Ratio

• A ratio < 0.5–0.7 signifies a qualitative defect (Type 2 VWD), indicating that VWF is present but dysfunctional. (American Society of Hematology) [2][3]4
• A ratio ≥ 0.7, together with proportionally reduced VWF:Ag and VWF:RCo, denotes a quantitative defect (Type 1 VWD). (International Society on Thrombosis and Haemostasis) [5][2]3
• The 0.7 cutoff is the most widely endorsed, although some guidelines accept a broader 0.5–0.7 range. (American Society of Hematology) [2][3]4

VWD Type Classification (based on pattern of results)

Type 1 (Quantitative deficiency; ~75 % of cases)

• VWF:Ag < 30 IU/dL and VWF:RCo < 30 IU/dL (both proportionally reduced).
• Factor VIII is normal to mildly decreased.
• Ratio > 0.7 (normal).
• Multimer pattern is normal. (International Society on Thrombosis and Haemostasis) [5][2]3

Type 1C (Increased clearance subtype)

• Same laboratory pattern as Type 1, but the VWF pro‑peptide/VWF:Ag ratio > 3.2 and a rapid clearance on a DDAVP trial identify this subtype. (International Society on Thrombosis and Haemostasis) 5

Type 2A (Loss of high‑molecular‑weight multimers)

• VWF:Ag may be normal to decreased (30–200 IU/dL); VWF:RCo < 30 IU/dL.
• Ratio < 0.7 (discordant).
• Multimer analysis shows loss of high‑molecular‑weight multimers and VWF collagen‑binding is abnormal. (International Society on Thrombosis and Haemostasis) [5][2]3

Type 2B (Increased platelet affinity)

• VWF:Ag decreased (30–200 IU/dL); VWF:RCo < 30 IU/dL.
• Ratio usually < 0.7.
• Multimers show loss of high‑molecular‑weight multimers; enhanced ristocetin‑induced platelet aggregation at low concentrations and possible thrombocytopenia may be present. (International Society on Thrombosis and Haemostasis) [5][2]3

Type 2M (Platelet‑binding defect with normal multimers)

• VWF:Ag decreased (30–200 IU/dL); VWF:RCo < 30 IU/dL.
• Ratio < 0.7.
• Multimer pattern is normal, and VWF collagen‑binding is normal, distinguishing it from Type 2A. (International Society on Thrombosis and Haemostasis) [5][2]3

Type 2N (Factor VIII‑binding defect)

• VWF:Ag and VWF:RCo are in the normal‑to‑mildly decreased range (30–200 IU/dL).
• Factor VIII is markedly decreased (<10 IU/dL).
• Ratio > 0.7 (normal).
• Multimers are normal; VWF:FVIIIB binding is reduced, mimicking hemophilia A. (International Society on Thrombosis and Haemostasis) [5][2]3

Type 3 (Severe, near‑complete deficiency)

• VWF:Ag < 3 IU/dL and VWF:RCo < 3 IU/dL (virtually absent).
• Factor VIII < 10 IU/dL.
• Multimers are absent. (International Society on Thrombosis and Haemostasis) [5][2]3

When to Perform Multimer Analysis

• The American Society of Hematology advises that multimer analysis should not be used as a first‑line screening test; it is reserved for reflex testing when:

Pre‑Analytical Considerations (factors that can invalidate results)

• Avoid stress, crying, anxiety, vigorous exercise, acute illness, inflammation, pregnancy, and estrogen/oral‑contraceptive use before phlebotomy, as these elevate VWF and FVIII levels. (American Society of Hematology) [2][3]
• Document ABO blood type; type O individuals have ~25 % lower VWF levels than non‑O types. (American Society of Hematology) [2][3]
• Transport whole blood at room temperature; never refrigerate, to prevent cryoprecipitation of VWF. (American Society of Hematology) [2][3]
• Process samples promptly at room temperature, centrifuge to remove platelets, and assay within 2 hours if fresh. (American Society of Hematology) [2][3]
• For delayed testing, freeze plasma at ≤ ‑40 °C, thaw at 37 °C, and keep at room temperature < 2 hours before assay; expect a 10–20 % reduction in FVIII activity in frozen‑thawed plasma. (American Society of Hematology) [2][3]

Assay Quality and Variability

• VWF assays exhibit high coefficients of variation (10–30 %), with VWF:RCo being the least precise. (American Society of Hematology) [2][3]4
• Inter‑laboratory variability is substantial; results from different laboratories may not be directly comparable. (American Society of Hematology) [2][3]4
• WHO‑standardized calibrators should be used, and results reported in IU/dL. (American Society of Hematology) [2][3]4

Common Pitfalls to Avoid

• Ordering incomplete panels (e.g., only VWF:Ag or only FVIII) leads to missed diagnoses. (American Society of Hematology) 1
• Using multimer analysis as a first‑line test wastes resources and delays diagnosis; reserve it for reflex testing. (American Society of Hematology) 1
• Testing during acute illness, pregnancy, or while on estrogen can normalize VWF levels and mask underlying VWD. (American Society of Hematology) [2][3]
• Improper sample handling (refrigeration, delayed processing) invalidates results and necessitates repeat testing. (American Society of Hematology) [2][3]
• Failing to repeat testing in borderline cases (30–50 IU/dL) may miss fluctuating VWF levels; a single normal result does not exclude VWD in symptomatic patients. (American Society of Hematology) 4
• Ignoring blood‑type O status can lead to misclassification, as type O individuals naturally have lower VWF. (American Society of Hematology) [2][3]

Tests Not Recommended for Routine Screening

• Bleeding time and the PFA‑100 platelet function analyzer are not advised for VWD screening due to poor sensitivity, specificity, and reproducibility. (American Society of Hematology) 1

Guideline for Laboratory Evaluation of von Willebrand Disease

Initial Hemostasis Screening

• Obtain a complete blood count, prothrombin time, and activated partial thromboplastin time before VWD‑specific assays to detect thrombocytopenia/ thrombocytosis, factor deficiencies, or other bleeding disorders. 6

Core Diagnostic Panel (Grade B Recommendation)

• Order the three essential plasma tests—VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and factor VIII coagulant activity (FVIII)—simultaneously for every patient being evaluated for VWD. 6, 7
• VWF:Ag quantifies the amount of von Willebrand factor protein in plasma. 8
• VWF:RCo measures the functional activity of VWF for platelet binding. 9
• FVIII activity assesses factor VIII levels, which are frequently reduced in VWD. 8

Diagnostic Thresholds & Ratio Interpretation

• A plasma VWF level < 30 IU/dL is the primary cutoff for a definitive VWD diagnosis, accounting for the lower baseline in blood‑type O individuals and the possibility of normal‑range VWF in symptomatic patients. 10, 11
• VWF:RCo values between 30–50 IU/dL may still support a VWD diagnosis when accompanied by a strong personal or family bleeding history. 11
• Calculate the VWF:RCo / VWF:Ag ratio:
  
  • Ratio < 0.5–0.7 indicates a qualitative defect consistent with Type 2 VWD.
  • Ratio ≥ 0.5–0.7 with proportionally reduced VWF:Ag and VWF:RCo suggests a quantitative defect (Type 1 VWD). 10, 7, 9

Specialized Reflex Testing (VWF Multimer Analysis)

• Perform multimer analysis only when initial testing is abnormal, such as:
  
  • Abnormally low VWF:RCo,
  • VWF:RCo / VWF:Ag ratio < 0.5–0.7, or
  • Clinical suspicion of Type 2 VWD despite borderline initial results.
• Multimer analysis is technically complex, qualitatively interpreted, and reserved for subtype determination. 6, 7

Pre‑analytical Patient Factors

• Avoid stress, vigorous exercise, acute inflammation, pregnancy, or estrogen/oral‑contraceptive use before phlebotomy, as these conditions can falsely elevate VWF and FVIII levels. 8
• Record ABO blood type; individuals with blood‑type O have VWF concentrations ≈ 25 % lower than non‑O groups. 8

Sample Handling & Transport

• Transport whole blood at room temperature (never refrigerated) to prevent cryoprecipitation of VWF. 11, 8
• Process samples promptly at room temperature: separate plasma, centrifuge thoroughly to remove platelets, and assay within 2 hours if possible. 12
• For delayed testing or referral to reference laboratories, freeze plasma ≤ ‑40 °C, thaw at 37 °C, and keep at room temperature for < 2 hours before assay. 8
• Frozen‑thawed plasma shows a 10–20 % reduction in FVIII activity compared with fresh plasma; suboptimal handling worsens this loss. 11, 12

Assay Quality & Reporting

• Use calibrators traceable to WHO plasma standards; report results in IU/dL when WHO‑linked calibrators are employed. 11, 12
• VWF assays exhibit high coefficients of variation (10–30 %), especially VWF:RCo, and inter‑laboratory variability is substantial. 11, 12

Tests Not Recommended for Routine Screening

• Bleeding time and the platelet function analyzer (PFA‑100) are not advised as routine VWD screening tools because of inconsistent sensitivity and specificity. 6, 7

Common Pitfalls to Avoid

• Ordering only one or two of the three core tests (VWF:Ag, VWF:RCo, FVIII) leads to incomplete diagnosis and classification. 6, 7
• Using multimer analysis as an initial screen is costly, complex, and unnecessary without abnormal core test results. 6, 7
• Testing during acute illness or pregnancy can mask VWD by artificially elevating VWF levels. 8
• Improper sample handling—such as refrigeration or delayed processing—can invalidate results. 11, 8
• Failing to repeat testing in borderline cases may miss a diagnosis, as VWF levels can fluctuate. 10

Timing and Conditions for Accurate VWF Testing in Women with Menstrual Bleeding

Recommended Timing of Testing

• The 2025 VWD diagnosis guidelines advise postponing VWF testing until after active heavy menstrual bleeding and any anemia have resolved, to avoid falsely elevated VWF levels that can mask von Willebrand disease. 13
• Samples should be drawn at least several days after menstruation has completely stopped; testing during the first two days of the cycle (active bleeding phase) should be avoided. 14

Physiologic Factors That Influence VWF Levels

• VWF is an acute‑phase reactant; stress, exercise, estrogen exposure, and pregnancy can transiently raise its concentration, so testing should be performed when the patient is in a physiologically stable baseline state. 13
• Testing during pregnancy, acute illness, or while on high‑dose estrogen therapy is discouraged because these conditions can artificially increase VWF levels. 14

Need for Repeat Testing

• When initial VWF results are obtained under suboptimal conditions (e.g., during active bleeding or before anemia resolves), repeat testing is often required to achieve a definitive diagnosis. 13
• A single normal VWF result does not exclude von Willebrand disease in symptomatic patients; repeat testing is essential for confirmation. 14

Age‑Related Changes

• VWF levels increase by approximately 1–2 % per year with aging, which should be considered when interpreting results in older patients. 13