Which factors affect the accuracy of a von Willebrand factor ristocetin cofactor (vWF:RCo) assay?

Updated: Feb 12, 2020
  • Author: Vadim Kostousov, MD; Chief Editor: Eric B Staros, MD  more...
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Repeated vWF:RCo activity testing is sometimes needed to identify low levels of vWF compatible with vWD. A retrospective study showed that up to 30% of pediatric patients required a second test for vWD before the diagnosis was established. [9]  A vWF:RCo level of less than 30% is designated as the level for a definitive diagnosis of vWD, although some patients with type 1 or type 2 vWD have vWF:RCo levels of 30-50%. [4] Bucciarelli et al reported that adults with initial vWF:RCo levels of 30-40% were very likely to have vWD, which was confirmed in these patients by second‐level tests. In the same study, among individuals with vWF:RCo levels of 40-60%, vWD was diagnosed in 20-30%, with female gender and non-O blood type being independent predictors of vWD diagnosis. [10]

vWF ristocetin binding–site polymorphism, which is more frequently detected in African Americans, can cause artificially low vWF:RCo activity and lead to a misdiagnosis of vWD type 2; [6] other tests (vWF collagen-binding assay, vWF:GPIb without ristocetin, vWF multimer analysis) may be useful to confirm the diagnosis in these patients.

During the third trimester of pregnancy, the vWF:RCo level can reach normal ranges in the majority of patients with vWD type 1. However, vWF:RCo activity is usually low in pregnant women with vWD type 2 or 3. [11, 12]

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