Evaluation of an automated platelet-based assay of ristocetin cofactor activity

von Willebrand's disease (VWD) is regarded as the most common congenital bleeding disorder, and although not available in all laboratories von Willebrand factor (VWF) activity is most frequently assessed as ristocetin cofactor (VWF:RCo). This test can be technically challenging, is subject to poor sensitivity (∼20 IU dL(-1) VWF:RCo) and has a high degree of inter- and intra-assay imprecision [coefficient of variation (cv) > 25%]. We studied an automated assay using a combined fixed platelet/ristocetin reagent (BC von Willebrand reagent, Siemens Healthcare Diagnostics) on the CS-2000i analyser (Sysmex UK Ltd). Initially inter- and intra-assay imprecision was assessed. The automated method showed good day-to-day reproducibility and linearity of standard curves. This technique, also gave low intra- and inter-assay imprecision using commercial normal (cv < 4.5%) and pathological (cv < 8.1%) control plasmas. We then compared automated technique results from 30 healthy normal subjects and 39 VWD patients to those obtained using standard aggregometry (Bio/Data, PAP4) with lyophilised fixed platelets (Helena BioSciences) and ristocetin (American Biochemical and Pharmaceutical Ltd). The automated method had a sensitivity limit of approximately 10 IU dL(-1) vs. 20 IU dL(-1) for aggregometry. Samples giving results within the aggregometry measurable range (n = 50) exhibited good correlation with the automated technique (median 70 IU dL(-1), range 7-184 IU dL(-1); and 64 IU dL(-1), 6-138 IU dL(-1) respectively; R(2) = 0.85). We subsequently compared 3 different batches of BC von Willebrand reagent, using a second group of normal subjects and VWD patients (n = 35, 55-139 IU dL(-1) and n = 30