External Peer Review Quality Assurance Testing in von Willebrand Disease: The Recent Experience of the United States College of American Pathologists Proficiency Testing Program

Abstract

The U.S. College of American Pathologists (CAP) has conducted a focused study of the proficiency testing for von Willebrand disease (vWD) analysis from 2003 to 2005. This report summarizes the findings regarding the accuracy and precision of the various assays at different analyte levels, as well as the influence of the reference material used to construct the assay standard curve. The results show that testing of von Willebrand factor (vWF):antigen (vWF:Ag) and ristocetin cofactor activity (vWF:RCo) is reasonably accurate, with all-method mean values falling within 3.2 and 5.6%, respectively, of the International Society on Thrombosis and Haemostasis Secondary Coagulation Standard (lot 2) assigned values. vWF:Ag measurements are reasonably precise (all-method coefficients of variation [CVs] = 10.7 to 15.1%), even at lower levels of vWF. The highest precision was observed for immunoturbidometric assays (CVs, 6.3 to 9.7%). vWF:RCo measurements are less precise (all-method CVs, 23.3 to 30.9%). The reference materials used in the standard curves for immunoturbidometric vWF:Ag assays appear to have accurately assigned vWF values for the majority of commercial suppliers.

Diagnostic approach to von Willebrand disease

“…57,58 The VWF:RCo, used primarily to determine abnormalities in function of VWF, has a high coefficient of variation between laboratories, and this variation increases significantly when the level of VWF:RCo is below 15 IU/dL. 59,60 Novel automated systems that test VWF activity can minimize operator imprecision [61][62][63] and provide timely results useful in therapeutic trials 64 but may still show relatively wide coefficients of variation. 65 Standardization of all assays for VWF is an ongoing effort 66 led by the ISTH.…”

Gain-of-function GPIb ELISA assay for VWF activity in the Zimmerman Program for the Molecular and Clinical Biology of VWD

“…Whereas the VWF:Ag has excellent reproducibility, with a low CV, the VWF:RCo does not demonstrate the same interlaboratory reliability. 2,3,41 Type 2A and type 2B VWD classically present with a decrease in highmolecular-weight multimers, in contrast to the relatively normal multimer distribution seen in type 2M VWD. Analysis of multimer distribution, however, is restricted to certain laboratories, and the sensitivity dependent on laboratory skill.…”

Diagnosis and classification of von Willebrand disease

“…LIA currently comprises the most popular methodology for VWF:Ag, given the ability to automate assays on most existing haemostasis analysers. All current VWF:Ag assays, be they ELISA or LIA, have similar and moderate assay imprecision, generally around 5-15% [21][22][23][26][27][28]37,38]. LOD issues depend somewhat on methodology, with LIA-based technology tending to have slightly worse LOD (5-10 U/dl) than ELISA (0-5 U/dl) ( Table 2) [24].…”

“…The main limitations for VWF:RCo (particularly manual agglutination procedures) include assay time and complexity, imprecision (often around 20-40%, and hence currently the worse performing VWF assay of its class) [21][22][23][26][27][28]37,38] and its poor LOD performance (sometimes being as high as 20 U/dl, and hence again the worse performing VWF assay of its class) [24]. This is a serious limitation for an assay that is used to help characterize functional VWF discordance, given that the vast majority of severe VWD subtypes have levels of VWF below 20 U/dl.…”

“…Third, levels of VWF vary between individuals according to the ABO blood group [19,20], and this similarly challenges the diagnosis of some patients with VWD, particularly those with VWF levels around the normal reference range cut-off values. Fourth, many of the assays used to identify VWD, particularly VWF:RCo, show a high degree of assay variability, meaning different test results on different occasions in the same or different laboratories [21][22][23]. Fifth, lower limit of detection (LOD) issues are evident for many VWF assays, again particularly for VWF:RCo or when VWF is measured by latex immunoassay (LIA) technology [24].…”