Testing for von Willebrand Disease
Order three specific plasma tests together as the initial diagnostic panel: VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and factor VIII coagulant activity (FVIII). 1
Initial Laboratory Testing Panel
The NHLBI/NIH guidelines establish these three tests as the foundation for VWD diagnosis, with all three required to establish diagnosis and determine disease type and severity 2:
- VWF antigen (VWF:Ag) - Measures the quantity of von Willebrand factor protein present in plasma 1
- VWF ristocetin cofactor activity (VWF:RCo) - Assesses the functional ability of VWF to bind platelets, which is critical for platelet adhesion 1, 3
- Factor VIII coagulant activity (FVIII) - Evaluates factor VIII levels, which are carried and protected by VWF 1
When to Order VWD-Specific Testing
If the patient has a strong mucocutaneous bleeding history, proceed directly to the three VWD-specific tests at the initial visit 2, 1. This bypasses preliminary screening tests and accelerates diagnosis in high-probability cases.
For patients with less clear presentations, consider ordering basic hemostasis tests first (CBC, PT, PTT) to rule out thrombocytopenia or other coagulation factor deficiencies, though these tests do not evaluate for VWD 2.
Interpreting Results
Abnormal results requiring further evaluation include 2:
- Low VWF:Ag, VWF:RCo, or FVIII levels - Suggests quantitative VWF deficiency
- VWF:RCo to VWF:Ag ratio below 0.5-0.7 - Indicates qualitative VWF defect (Type 2 VWD) 2, 4
The ratio cutoff used affects sensitivity and specificity for Type 2 diagnosis, with 0.7 commonly recommended by guidelines, though cutoffs range from 0.5 to 0.7 4.
Specialized Testing (Not for Initial Screening)
VWF multimer analysis should NOT be ordered as part of initial screening 2, 1. This technically complex test is reserved for reflex testing only when initial results show abnormalities or when clinical suspicion remains high despite normal initial testing 2, 1.
Additional specialized tests to consider after abnormal initial results include collagen binding assay (VWF:CB) and ristocetin-induced platelet aggregation (RIPA) 1.
Critical Pre-Test Considerations
Several factors can falsely elevate or lower VWF levels, affecting test accuracy 1:
- Stress, exercise, and inflammation - Falsely elevate VWF levels 1
- Blood type O - Associated with 25% lower baseline VWF levels 1
- Pregnancy and estrogen therapy - Increase VWF levels 1
- Timing of menstrual cycle - Can affect results in women 1
Ideally, test when these confounding factors are absent or minimal 1.
Sample Handling Requirements
Proper specimen handling is essential to avoid false results 1:
- Transport samples at room temperature 1
- Separate plasma promptly 1
- Freeze at or below -40°C if not tested within 2 hours 1
Common Pitfalls to Avoid
- Never rely on a single test - No single laboratory test can screen for all forms of VWD 1
- Do not order VWF multimer analysis initially - This is a reflex test only 1
- Avoid bleeding time and PFA-100 as screening tests - These have conflicting data on sensitivity and specificity and are not recommended for routine screening 2, 1
- Do not ignore physiological variables - Test timing matters significantly 1
Next Steps After Testing
If initial test results are abnormal, consultation with a hemostasis specialist is recommended for further evaluation, subtyping, and management planning 1.