» VWF Inhibitor Assay

VWF Inhibitor Assay

Dear Sir,
I would like to know is there any kit for VWD inhibitor study.
with regards, Vilas Hiremath.

Hello to Vilas Hiremath, and thank you for your question. I assume you are looking for a means to detect and titer a von Willebrand factor (VWF) inhibitor in a patient with acquired von Willebrand disease (VWD), and not an assay for VWF protease (ADAMTS 13) inhibitor, which is used to support the diagnosis of acquired thrombotic thrombocytopenic purpura. The latter is provided by the reference lab at the Blood Center of Wisconsin and other coagulation specialty reference laboratories, and Mayo Medical Laboratories provides a ristocetin inhibitor assay that may meet your needs. I don’t know of a kit for determining VWF inhibitor concentration, perhaps one of our participants can help. Geo.

Bleeding Disorders | GeorgeFritsma | July 22, 2010 4:22 pm

2 Comments

By Kentos, July 23, 2010 @ 5:56 am

Hi Vilas,
It should be noted that it is often very difficult to prove via means of an inhibition assay the presence of a VWF inhibitor. A simple mixing study where VWF:Ag, VWF ristocetin cofactor (VWF:RCo) or VWF collagen binding assay (VWF:CBA) is assayed on a 1:1 mix of patients plasma and normal plasma both immediately and after an incubation at 37oC (much like a Bethesda assay for measuring factor VIII or IX inhibitors) often fails to show the presence of the inhibitor. An ELISA assay may also be employed to detect antibodies–this is also not always successful, not to mention technically quite challenging. I believe there was a method described in Blood for such an ELISA (sorry can’t remember the exact reference).
I assume you have a patient with acquired von Willebrand syndrome (aVWS). To prove there is increased clearance of VWF the best assay (in my opinion) is the VWF propeptide assay. In this assay a discordant VWF:Ag to VWF propeptide level is supportive of the diagnosis of aVWS with the relevant clinical supportive evidence (e. g., decreased survival of infused VWF concentrates in a patient with MGUS). Hope this helps.

Kent Chapman
By emmanuel, August 5, 2010 @ 12:58 am

I basically agree with Kent. You could try doing mixing studies like you do for FVIII inhibitors but using VWF assays, but since the VWF protein is so large and complex you would need to perform multiple assays - you are more likely to see a VWF inhibitor using functional assays such as VWF:RCo or VWF:CB than with VWF:Ag. You can also try doing this with a RIPA, but this is time consuming and you will need some normal blood. You might detect an inhibitor with these, but if you don’t, then this doesn’t mean that one isn’t present, since I would expect a false negative rate of at least 50%. You can also look for inhibitors directly by ELISA (ie anti-VWF assay), but there is a high risk of false positives with that assay. As Kent mentions, some inhibitors will not be evident by mixing studies, but will cause clearance of VWF which may be detected with the VWF propeptide assay. There is no kit available for identifying inhibitors, but some commercial VWF propeptide assays are available.

Some references listed below related to identification of VWF antibodies:

1. Coleman R, Favaloro EJ, Soltani S, Beng T. Acquired von Willebrand disease: potential contribution of the VWF:CB to the identification of functionally inhibiting auto-antibodies to von Willebrand factor. J Thromb Haemost 2006; 4: 2085–8.

2. Guerin V, Ryman A, Velez F. Acquired von Willebrand disease: potential contribution of the von Willebrand factor collagen-binding to the identification of functionally inhibiting auto-antibodies to von Willebrand factor: a rebuttal. J Thromb Haemost. 2008 Jun;6(6):1051-2.

3. Siaka C, Rugeri L, Caron C, Goudemand J. A new ELISA assay for diagnosis of acquired von Willebrand syndrome. Haemophilia. 2003 May;9(3):303-8.