Automated Methods Evaluated for Von Willebrand Factor Activity

The laboratory evaluation for VWD requires an assessment of plasma VWF activity, which is commonly done using an aggregometer to assess VWF ristocetin cofactor activity which is a quantitative method that uses the antibiotic ristocetin to induce plasma VWF binding to the VWF receptor on target platelets and platelet agglutination.

Scientists at the McMaster University (Hamilton, ON, Canada) evaluated 262 samples from 217 patients that included 188 samples from 153 females and 73 samples from 63 males. There were 197 samples from adults (ages 18 to 84) and 64 from children (ages 0 to 17 years), and 67 samples were from 34 patients with previously diagnosed VWD. After validating that the assay could be performed on an instrument from a different manufacturer, they compared VWF activity assay (VWF:Ac) to VWF ristocetin cofactor activity (VWF:RCo) findings, including ratios of activity/antigen.

Plasma was tested by the VWF:Ac on an STA-R Evolution (Diagnostica Stago; Parsippany, NJ, USA) and using the Innovance assay on a Sysmex CS2000i instrument (Siemens; Erlangen, Germany). VWF:RCo was performed by aggregometry on a Helena AggRAM instrument (Helena Laboratories, Beaumont, TX, USA). There was excellent correlation between VWF:Ac results run at two different sites on two different instruments. VWF:Ac had greater precision and sensitivity to low levels of VWF than the VWF:RCo method.

Although there was good correlation between VWF:Ac and VWF:RCo results among healthy controls and patient subjects, VWF:Ac results were undetectable and/or significantly lower than VWF:RCo among patients who had types 2A, 2B, or 2M VWD. Additionally, a higher proportion of patient samples were classified as showing qualitative defects using the VWF:Ac compared with VWF:RCo method.

The authors concluded that the Innovance VWF:Ac method is an acceptable, automated alternative to the VWF:RCo method for assessment of VWF binding to the platelet receptor glycoprotein GPIbα that is sensitive to both quantitative and qualitative defects of VWF. Laboratories and clinicians need to be aware that some patients with VWD have much lower levels of VWF measured by the VWF:Ac assay than by VWF:RCo. The study was published on April 18, 2014, in the International Journal of Laboratory Hematology.

Related Links:
McMaster University 
Diagnostica Stago
Siemens Healthcare