Von Willebrand's Antigen to Activity Ratio in Normal Clotting
In a person with normal clotting, the von Willebrand's antigen to activity ratio (VWF:RCo/VWF:Ag or VWF:Act/VWF:Ag) should be between 0.7-1.2. 1, 2
Normal Ratio Values
The normal ratio of von Willebrand factor activity to antigen has been established through multiple studies:
- In healthy subjects, the reference range for VWF:Act/VWF:Ag is 0.8-1.16 2
- In normal patients (with normal multimers), the reference range for VWF:Act/VWF:Ag is 0.73-1.14 2
- For VWF:RCo/VWF:Ag, the reference ranges are:
- 0.74-1.23 in healthy individuals
- 0.62-1.25 in normal patients 2
Clinical Significance of the Ratio
The VWF activity to antigen ratio is a critical parameter in:
Differentiating VWD types:
Detecting acquired von Willebrand syndrome (AVWS):
Diagnostic Cutoffs
The recommended cutoff ratio for distinguishing normal from abnormal VWF function varies slightly by guideline:
- Most commonly, a ratio <0.7 is considered abnormal 1, 3
- Some guidelines use a more stringent cutoff of <0.5-0.6 3
- For Type 2 VWD subtypes (2A, 2B, 2M), the ratio is typically <0.5-0.7 1
Technical Considerations
When interpreting VWF ratios, consider:
- Test variability can affect the ratio calculation
- VWF:Act assays may have better precision than VWF:RCo assays 2
- Multiple tests may be needed for accurate diagnosis, especially in borderline cases
- Patient factors like stress, exercise, pregnancy, and inflammation can affect VWF levels 1
Clinical Application
A normal ratio (≥0.7) in a person with good clotting indicates:
- Normal VWF function
- Normal multimer structure
- Absence of qualitative VWF defects
If the ratio is abnormal (<0.7) despite normal clotting history, further evaluation may be warranted, including:
- VWF multimer analysis
- Specialized platelet function studies
- Assessment of other coagulation factors 1
Remember that a single normal test does not definitively rule out all bleeding disorders, and clinical correlation is essential when interpreting laboratory results.