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Ristocetin: False Positives?

From Laurel Veek at Mayo Labs:

We have noticed lately when performing platelet aggregations that more often than not we have a curve that is slow to develop but between 2-3 minutes it aggregates or has an abnormal slope if it aggregates before 3 minutes. Ideally I would like the curve to be relatively flat at the lower concentrations of ristocetin.
I have spoken to the company that we get our ristocetin from. They said that some lots of ristocetin material will cause nonspecific aggregation. They said however that ristocetin should react immediately (within 30 seconds), and that reactions observed after the first three minutes are not related to the VW protein. I was wondering your thoughts on this? Have others noted this?

Hello, Laurel, and thank you for your question. I checked with the special coagulation staff at our University of Alabama at Birmingham lab, who report no problems with their current lot. I’ve also forwarded your message to a colleague who is familiar with the current lots of ristocetin available, hoping she may shed some light on this issue.

My own thoughts are that I would not use this lot of ristocetin, it seems like you run the risk of generating some false positive results for VWD. We are likely to see a few comments from others, especially if anyone else has had the same problem.

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