Utility of Various Von Willebrant Factor Laboratory Tests in Assessment of Acquired Von Willebrant Syndrome in Patients with Aortic Stenosis

Background: Aortic stenosis-associated acquired von Willebrand syndrome (AS-AVWS) is caused by an accelerated degradation of the highest molecular weight von Willebrand factor (VWF) multimers (HMWM). We recently reported that the severity of the HMWM loss correlates with the severity of AS and patients’ bleeding tendency (Am J Cardiol, 2013;111:374-81). As for the missing HMWM, it is still uncertain if they are cleared from circulation or redistributed with medium-low MWMs, which can be assessed by VWF propeptide (Pro)/antigen (Ag) ratio. Besides the tedious VWF multimer analysis, it is uncertain if VWF activity by immunoturbidic method (Lx)/ Ag ratio or VWF collagen binding activity (CBA)/Ag ratio can be used as a screening test to predict AS-AVWS. It has been speculated in the literature that VWF:CBA is more sensitive than VWF:Lx for detecting a HMWM loss. Our goal was to exam the laboratory characteristics of various VWF tests in evaluating AS-AVWS and the potential mechanism of HMWM loss.

Method: Sixty-six patients (between years 2010-2012, 43% male) with varying degrees of AS (16 mild, 20 moderate, and 30 severe) and separate 21 patients who had aortic valve replacement were assessed with VWF:Ag, VWF:Lx, VWF:CBA, platelet function analyzer collagen plus adenosine diphosphate (PFA-CADP), VWF multimer analysis and multimer ratio by densitometry (Am J Cardiol, 2013;111:374-81) after their echocardiography. The clinical endpoints of this study are AS severity measured by mean gradient (MG) by echocardiography and clinically significant bleeding. Statistical analyses including Spearman rank correlation test, estimation of the area under the receiver operating characteristics (ROC) curve, and Wilcoxon rank sum tests were performed.

Results: VWF:Lx and VWF:CBA had strong correlation (Spearman r=0.94, P<0.001), and the correlation of VWF:CBA/Ag and VWF:Ltx/Ag was 0.49, p<0.001). As previously published, in patients with AS, MG correlated with VWF:Lx/Ag (Spearman r = -0.41, p <0.001), PFA-CADP (r = 0.49, p <0.001), and VWF multimer ratio (r = -0.76, p <0.001). MG also correlated with VWF:CBA/Ag (r= -0.38, p=0.002), but not with VWF: Pro/Ag (r=-0.20, p=0.12). Among AS and AVR patients, the area under the ROC curve for detection of the loss of HMWM was 0.88 (95% CI: 0.80-0.95) by VWF multimer ratio, 0.76 (95% CI: 0.65-0.86) by PFA-CADP, 0.70 (95% CI: 0.58-0.82) by VWF:CBA/Ag ratio, 0.67 (95% CI: 0.56-0.70) by VWF:Lx/Ag ratio, and 0.53 (95% CI: 0.41-0.65) by VWF:Pro/Ag . In addition to previously published results, clinically significant bleeding, in 14% of AS patients, was not associated with VWF:CBA/Ag (p=0.25) or VWF:Pro/Ag ratio (p=0.83). VWF: CBA/Ag ratio was significantly lower in patients with severe AS (MG > 40 mmHg) compared to AVR patients (median: 0.84 vs. 1.06, p<0.001), while there was no evidence of a difference in VWF:Pro/Ag (p=0.45).

Conclusion: HMWM losses are highly prevalent in patients with moderate to severe AS. Of the VWF tests, PFA-CADP and VWF multimer analysis and ratio had the highest predictive ability for AS-AVWS followed by VWF:CBA/Ag and VWF:Lx/Ag. The VWF:CBA is not overly superior than VWF:Lx for predicting a HMWM loss. The normal VWF:Pro/Ag suggests that AS-AVWS is different from other types of AVWS due to accelerated clearance of HMWM.