Researchers in Wisconsin recently reviewed the puzzling case of laboratory testing for von Willebrand disease (VWD). The lead investigator of the study was Veronica Flood, MD, pediatric oncologist and assistant professor, Division of Hematology/Oncology, Department of Pediatrics, Medical College of Wisconsin, Milwaukee.
VWD and its subtypes are characterized by either quantitative or qualitative defects in von Willebrand factor (VWF). Bleeding symptoms, which depending on the type, can be mild, moderate or severe. While VWD types I and III signify partial or complete lack of VWF, type II and its variation subtypes (2A, 2B, 2M, 2N) denote a dysfunction associated with VWF. Accurate diagnosis of these subtypes is harder to achieve than in hemophilia because VWF levels often fluctuate, requiring repeat lab tests before a precise diagnosis can be achieved. VWD is further complicated by its “working relationship” with another protein crucial to coagulation, factor VIII (FVIII). VWF operates as a carrier protein for FVIII and also promotes platelet binding.
A battery of tests that measure VWF quantity and function are often required to confirm a VWD diagnosis. Flood’s study focused on one specific case of type 2M VWD and the limitations in a particular lab test, the VWF ristocetin cofactor activity (VWF:RCo). VWF:RCo is a relatively simple lab test in which blood cells are separated from a patient’s plasma (liquid component of blood). Ristocetin, an antibiotic that prompts the binding of VWF and platelets, is then added to the plasma. Blood with viable VWF will clot, blood with deficient VWF will not.
While the patient featured in this study showed a significant decrease in VWF:RCo, the other assay tests of VWF function were normal. In addition, the patient exhibited no bleeding symptoms. Flood and colleagues discovered that while VWF:RCo tests registered a deficiency in VWF function, in vivo (inside the body), its actual functioning did not appear to be affected.
Investigators then questioned the initial diagnosis of type 2M VWD, likely pointing to an inaccurate diagnosis. “This disparity in low ‘functional’ VWF with absence of clinical bleeding not only affects the case reported here, but calls into question the reliance on VWF:RCo as a means of assessing VWF function,” explained Flood and fellow authors.
Flood’s research was supported, in part, by a Career Development Award from the National Hemophilia Foundation. Her work was also supported by a grant from the National Institutes of Health and a Mentored Research Award from the Hemophilia and Thrombosis Research Society.
Source: Flood V, Friedman K, Gill J, et al. Limitations of the Ristocetin Cofactor Assay in Measurement of von Willebrand Factor Function. Journal of Thrombosis and Haemostasis. 2009; 7(11): Pages 1832 – 1839.