ABSTRACT
For more than two decades, the ristocetin cofactor (RCo) assay, which measures the
von Willebrand factor (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. There is, however, general agreement among clinical analysts that
this method has major practical disadvantages in performance and reproducibility.
Today, collagen-binding assays (CBA) based on the enzyme-linked immunosorbent assay
(ELISA) technique that measure the interaction of vWF and collagen are an alternative
analytic procedure based on a more physiological function than that of the RCo procedure.
We used both assay systems in a comparative study to assess the functional activity
of vWF in plasma as well as in therapeutic preparations. We measured RCo activities
of plasma from healthy donors and patients with different types of von Willebrand
disease (vWD) and of vWF as a drug substance in factor (F) VIII/vWF concentrates using
both the aggregometric and the macroscopic methods. In addition, we measured collagen-binding
activity (vWF:CB) using a recently developed commercially available CBA system. To
investigate the relation between the structure and the functional activity of vWF,
we isolated vWF species with different numbers of multimers from FVIII/vWF concentrates
by affinity chromatography on immobilized heparin. The vWF:RCo and vWF:CB of the different
fractions were measured, and the multimeric structure of vWF was analyzed by sodium
dodecyl sulfate (SDS) agarose gel electrophoresis. (vWF:CB and vWF:RCo are part of
the nomenclature proposed by the International Society on Thrombosis and Hemostasis
Scientific and Standardization Committee [ISTH SSC] subcommittee on von Willebrand
factor, in Maastricht, Germany, June 16, 2000.) Measurement of functional vWF activity
by CBA can be carried out with substantially higher interassay reproducibility than
can measurement of RCo. Both assay systems can be used for diagnosis and subtyping
of vWD, but CBA is more sensitive than either of the two RCo methods. The analysis
of vWF multimers in the different fractions obtained by affinity chromatography on
heparin Sepharose showed that the activity measured both with RCo assay and CBA correlated
with the degree of multimerization. Our results suggest that measurement of the functional
activity of vWF by the RCo procedure can be replaced by the more reliable CBA, reflecting
the physiological hemostatic activity of vWF. The CBA method appears not only to be
more sensitive and easier to carry out than the RCo method is but also to have a higher
reproducibility and allow better standardization.
KEYWORDS
von Willebrand factor - vWF - ristocetin cofactor activity - collagen-binding activity
- FVIII/vWF concentrates - von Willebrand disease - heparin affinity chromatography