A comparison of the quantitative ristocetin von willebrand factor assay by using fresh and fixed platelets

The ristocetin induced platelet aggregation assay (VIII:RCF) using patient plasma and control platelets is a sensitive and specific test for von Willebrand's'disease. We assayed the level of VIII:RCF using fresh or formalin fixed platelets in controls and patients with von Willebrand's disease. Fresh platelets were sensitive to less than 1% VIII:RCF compared to 5% for fixed platelets. Either method gave abnormal results, but the level of VIII:RCF was significantly lower using fixed platelets than fresh platelets. Eighteen control patients were studied, and there was a close relationship between the level of VIII:RCF using both assays (r = 0.8, p < 0.01). Similarly, there was a close relationship between the level of VIII:RCF in the patients with von Willebrand's disease (r = 0.85, p < 0.01). Two normal volunteers had plasma stored for over five months and assayed 23 and 24 times. The level of VIII:RCF remained normal on all determinations. Although fresh platelets can provide a more precise quantitation of very low levels of VIII:RCF, the use of fixed platelets is useful for diagnosing von Willebrand's disease.