Measurement of von Willebrand factor (VWF) activity alone has limited diagnostic value. The diagnosis of von Willebrand disease (VWD) requires a combination of clinical and laboratory information. VWF activity assay results generally must be used together with assays of VWF antigen and factor VIII coagulant activity for optimum clinical utility and diagnostic efficiency
Results may be affected by:
Unfractionated heparin >4.0 U/mL; may cause an overestimation of the test result
Hemoglobin >70 mg/dL; may cause the result to be underestimated
Bilirubin >4.2 mg/dL; may cause the result to be underestimated
Triglycerides >1020 mg/dL; may cause the result to be underestimated
Rheumatoid factor >200 IU/mL; may cause an overestimation of the test result
Specimens from patients who have received preparation of mouse monoclonal antibody for diagnosis or therapy may contain human antimouse antibody (HAMA). The presence of HAMA may cause an overestimation of results in immunoassays that utilize mouse monoclonal antibodies. This assay contains a blocking agent against HAMA to minimize this interference.
Normal, full-term newborns may have mildly increased levels which reach adult levels by 90 days postnatal. Healthy, premature infants (30-36 weeks gestation) may have increased levels that reach adult levels by 180 days. Note: Individuals of blood group "O" may have lower plasma von Willebrand factor (VWF) activity than those of other ABO blood groups, such that apparently normal individuals of blood group "O" may have plasma VWF activity as low as 40% to 50%, whereas the lower limit of the reference range for individuals of other blood groups may be 60% to 70%.