Routine recommended tests in acquired von Willebrand disease are vWF:Ag assay, vWF:Ag activity assay, and ristocetin cofactor
assay (vWF:RCo/Ag ratio) (normal = 1) [2, 7].
 Nonstandard abbreviations: FVIII, factor VIII; PT, prothrombin time; APTT, activated partial thromboplastin time; AT, antithrombin; VWF, von Willebrand factor; VWF:Ag, VWF antigen; VWF:CB, VWF collagen binding; VWF:RCo, VWF ristocetin cofactor
activity; FVIII:C, FVIII coagulant activity; FIX:C, FIX coagulant activity; PC-chrom, protein C chromogenic; PS-act, protein S activity; PS-total, protein S total; PS-free, protein S free; PC-clot, PC clotting; [CV.sub.A], analytical variation; [CV.sub.I], within-individual variation; CVG, between-individual variation; EFLM, European Federation for Laboratory Medicine; EQA, external quality assessment.
Differential sensitivity of von Willebrand factor (VWF) 'activity' assays to large and small VWF molecular weight forms: a cross-laboratory study comparing ristocetin cofactor
, collagen-binding and mAb-based assays.
vWF-mediated platelet aggregation was measured via the biomarker RICO (ristocetin cofactor
For patients needing vWF screening, a vWD profile was performed which included: aPTT, factor VIII quantitative functional assay, von Willebrand factor antigen (vWF:Ag) using latex immunoassay (Liatest--Diagnostica Stago-Paris/France), vWF ristocetin cofactor
(vWF: Rcof) (9,10) activity using vWFactor assay (Bio-Data-PA/USA), and ristocetin induced platelet aggregation (RIPA) using platelet aggregometry (Bio-Data-PA/USA ).
Although a history of vWD was reported only by the family of infant A (Table), testing at CDC's hemostasis laboratory revealed that both infant A and infant B had laboratory evidence of vWD, and infant D had borderline yon Willebrand factor antigen and ristocetin cofactor
results consistent with a vWD diagnosis.
For more than two decades, the ristocetin cofactor
(RCo) assay, which measures the vWF-mediated agglutination of platelets in the presence of the antibiotic ristocetin, has been the most common method for measuring the functional activity of vWF.
Patients with von Willebrand disease may have decreased VWF antigen, decreased ristocetin cofactor
activity, decreased ristocetin aggregation, decreased factor VIII levels, normal or increased APTT, and increased bleeding times with normal platelet counts.
Contract notice:provision of reagents and consumables for the determination of the activity citybrand ristocetin cofactor
dependent with provision of automaton
 Nonstandard abbreviations: VWD, von Willebrand disease; PT, prothrombin time; aPTT, activated partial thromboplastin time; VWF:Ag, von Willebrand factor antigen level; VWF:RCo, von Willebrand factor ristocetin cofactor
activity; HMW, high molecular weight; IMW, intermediate molecular weight; LMW, low molecular weight; CBC, complete blood count; PT-VWD, platelet-type VWD; LD-RIPA, low-dose ristocetin-induced platelet aggregation.
Results: Based on the von Willebrand factor antigen (vWF:Ag) and ristocetin cofactor
activity (vWF:RCo) levels and bleeding symptoms, vWD type-1 was diagnosed in 14 individuals (4 males, 10 females; prevalence: 1.04%).
Other laboratory findings which aid in diagnosis of vWD type 2B are relative decrease in plasma high molecular weight vWFmultimers (6), disproportionately low ristocetin cofactor
activity compared to vWFlevels (7) and mildly reduced or normal vWF assay.
von Willebrand factor ristocetin cofactor
(VWF:RCo) assay: implementation on an automated coagulometer (ACL).
The selected purification processes isolates the VWF/FVIII complex under highly protein-protecting conditions, resulting in a 1:1 ratio of VWF:RCo (ristocetin cofactor
) and FVIII activities that is similar to normal plasma.
ABBREVIATIONS: ELISA = Enzyme Linked Immuno-sorbent Assay; EQA = external quality assurance; LIA = Latex Immuno-Assay; LLS = Lower limit of sensitivity; QAP = Quality Assurance Program; VWD = von Willebrand disease; VWF = von Willebrand Factor; VWF:Act = von Willebrand Factor Activity' (assay); VWF:Ag = von Willebrand Factor Antigen (assay); VWF:CB = von Willebrand Factor collagen binding (assay); VWF:RCo = von Willebrand Factor Ristocetin Cofactor