Laboratory assessment of von Willebrand factor: differential influence of prolonged ambient temperature specimen storage on assay results

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Abstract
Summary. The laboratory assessment of von Willebrand factor (VWF) is typically performed at specialist laboratory sites, particularly when performed as a battery of laboratory tests in a thorough workup for the diagnosis of von Willebrand's disease (VWD). In these cases, specimens could derive from a variety of off‐site sources, including smaller laboratories, and general clinical practitioners. Because of the potential for lack of control by the specialist laboratory over the method of specimen handling and transport from these sources, and recent VWF methodological advances, we investigated the effect of prolonged ambient temperature specimen storage on laboratory assay results. Thus, specimens were collected from 10 separate individuals, and each variably processed to provide an ideal (‘control') plasma specimen, and additional specimens that were then stored at ambient temperature for up to 7 days. Specifically, specimens were stored either as whole blood, or as separated plasma, and VWF tested in isolated plasma, post 24 h, post 48 h and post 7 days storage. Three separate laboratory assays for VWF were performed; (i) a standard ‘antigen’ (antisera‐ELISA‐based) assay (VWF:Ag), (ii) a standard ristocetin‐dependent‐platelet‐agglutination procedure (VWF:RCof) and (iii) a more recently described ELISA‐based functional collagen‐VWF binding assay (VWF:CBA). Results can be summarized as follows, (i) Plasma storage: there was no (statistically significant) change in VWF:Ag or VWF:RCof assay results over the 7‐day storage period; however, there was a small but statistically significant fall (P= 0.009) in VWF:CBA assay results after 7 days storage of plasma. (ii) Whole blood storage: there was no (statistically significant) change in VWF:Ag, VWF:CBA or VWF:RCof assay results over the 7‐day storage period, although the data suggested a trend towards increasing VWF:Ag over time. As a result of the change in assayed VWF:CBA following prolonged plasma storage, a similar small but statistically significant (P= 0.009) change (increase) in the VWF:Ag to VWF:CBA ratio was observed. This ratio has previously been determined to be useful in the differential diagnosis of VWD subtypes, with high VWF:Ag to VWF:CBA ratios potentially indicative of Type 2 VWD. Fortunately, the absolute magnitude of the altered ratio following prolonged plasma storage is unlikely in practice to affect the diagnosis of VWD in most testing cases. Nevertheless, there will be occasional borderline normal cases in whom the change in VWF:CBA, or in the calculated VWF:Ag to VWF:CBA ratio, may otherwise influence a clinical diagnosis of VWD. Caution is therefore suggested in the interpretation of laboratory‐derived VWF data, particularly if the specimen is derived as an off‐site referral.