To cite this article: Chen D, Tange JI, Meyers BJ, Pruthi RK, Nichols WL, Heit JA. Validation of an automated latex particle-enhanced immunoturbidimetric von Willebrand factor activity assay.

Objective-Left atrial blood stasis is associated with increased risk for left atrial appendage thrombus (LAAT) and stroke in atrial fibrillation (AF). Von Willebrand factor (VWF) is associated with thromboembolism in AF. VWF thrombogenic activity is proportional to multimer size, which is regulated by VWF-cleaving protease (ADAMTS13). Methods and Results-To assess the association between left atrial blood stasis and VWF-ADAMTS13 system, plasma VWF antigen (VWF:Ag), VWF activity (VWF:Act), and ADAMTS13 activity were measured in 414 consecutive patients with nonvalvular AF (age 63Ϯ13 years; 25% women) and in 100 patients (age 64Ϯ14 years; 39% women) with normal sinus rhythm. Key Words: stroke Ⅲ thrombosis Ⅲ von Willebrand factor Ⅲ atrial fibrillation C ardioembolic stroke in the setting of atrial fibrillation (AF) begins with the left atrial appendage thrombus (LAAT) formation. 1-3 The risk for LAAT development and stroke increases with left atrium mechanical function deterioration that is reflected by reduction in left atrial appendage emptying velocity (LAAEV), development of spontaneous echocardiographic contrast (SEC), and progression of left atrial distension. 4 -7 However, the underlying mechanism behind the relationship between AF, atrial stasis, and thrombotic propensity is complex and poorly understood.Von Willebrand factor (VWF) a large plasma glycoprotein that mediates platelet adhesion and aggregation, 8 has been associated with stroke risk in AF. 9 -13 Prior studies comparing VWF levels between AF patients and subjects in normal sinus rhythm (NSR) were limited by small sample size. 14 -16 Assessment of archived plasma samples from Stroke Prevention in Atrial Fibrillation III participants lacked a comparator. 9 -11 Moreover, a prior association between elevated VWF and stroke became nonsignificant after adjustment for other clinical predictors. 11 The association between VWF and AF therefore remains largely unexplored.The thrombogenic potential of VWF is directly proportional to VWF activity (VWF:Act) determined by both plasma concentration and multimer size, 17 which is regulated by VWF-specific protease ADAMTS13. 18 Two patients may have similar plasma VWF content, yet thrombotic propensity will vary considerably depending on the proportion of highmolecular-weight multimer content. This nuance would be missed if mere antigen content was assessed.Atrial distension is associated with overexpression of VWF multimers, 19 and blood stasis has been shown to downregulate ADAMTS13 activity. 20 Previous studies, however, have not systematically accounted for degrees of atrial stasis or measured complete variables within the VWF system. We hypothesized that atrial distension and stasis in AF are associated with the higher VWF and increased proportion of large multimers that lead to the development of LAAT. To address this hypothesis, transesophageal echocardiography (TEE) measures of stasis and LAAT and measures of VWF antigen (VWF:Ag), VWF:Act, and ADAMTS13 activity were compared in consecutive patients wi...

Warfarin dosing relies on accurate measurements of international normalized ratio (INR), which is calculated from the prothrombin time (PT), International Sensitivity Index international sensitivity index (ISI) of the thromboplastin, and the geometric mean of normal PT (MNPT). However, ISI assignments of certain reagent/instrument combinations are frequently unavailable, especially when the reagent and instrument are not from the same manufacturer. The effort to be in compliance with widely endorsed Clinical and Laboratory Standards Institute (CLSI) guidelines by locally verifying or assigning an ISI to an unsupported reagent/instrument combination is further hindered by the lack of US Food and Drug Administration (FDA)-approved certified plasmas designated for a particular reagent/instrument combination. The objectives of the study include development of a process to verify/assign ISI and MNPT of a single thromboplastin reagent from one manufacturer across multiple instruments including several from another manufacturer and across several campuses of a single institution, the Mayo Clinic. In this study, RecombiPlasTin 2G (R2G), was evaluated on the ACL TOP 700 (IL), STA-R Evolution, STA Compact, and STA Satellite. Random normal donor samples (n = 25) were used to verify/assign MNPT. A subset of the normal donors (n = 8) and 13 warfarin pools (INR range: 1.3-3.9), created from stable warfarin patient plasma, were used for ISI verification/assignment. The manufacturer's assigned ISI was first verified on the ACL TOP 700 (reference method), then assigned on three unsupported instruments using orthogonal regression analysis. The MNPT and manufacturer assigned ISI (11.0, 0.95) were verified on the ACL TOP 700 and subsequently assigned on the STA-R Evolution (11.6, 1.04); STA Compact (11.5, 1.02); and STA Satellite (10.9, 0.99). Linear correlations of the INR results from all the four instruments demonstrated an r2 > 0.99. This process provides a reproducible approach to assigning ISIs on unsupported reagent/instrument combinations. Our data also confirm that ISIs of the same PT reagent differ significantly on different instruments, thus confirming the requirement for evaluations and validation of ISIs for different reagent/instrument combinations.

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