HAEMOSTASIS
test and in collagen binding is the same. A longstanding test has been to look at
the multimers of vWD. As mentioned previously, vWF is produced by the endothelial cells, and comprises large multimeric structures that, as a type of controlling mechanism, undergo proteolysis by ADAMTS13 to prevent the development of ultralarge multimers. This leads to a very typical pattern in the patient, with bands representing the various fragments. However, interpretation of the banding pattern is time-consuming, requires expert knowledge, and not everyone can look at them and make the right interpretation. Three studies have looked at the reliability
of multimers based on laboratories that used them to distinguish between type 1 and type 2, and a large number of cases were wrongly assigned; 20% of type 1 cases were later rediagnosed as type 2. In some laboratories there was a 15% error rate in type 1 samples.
A typical banding pattern in a vWD patient, with individual bands representing the various multimer fragments.
Mutation database A von Willebrand mutation database is held for the ISTH by the University of Sheffield and it is now also included in the European Association for Haemophilia and Allied Disorders (EAHAD) database. When a mutation is found it can be searched on these databases for further relevant information. However, there are a number of examples of inconsistencies reported. For example, a mutation in exon 26 (W1144G) was reported by workers in Wisconsin in 2006 as a type 1, supported by expression studies, and also reported by Sheffield as a type 2A. It was later reclassified as a type 2A due to the presence of multimer abnormalities. Similarly, Y1146C has also been recorded as a type 1 in Canada, but reclassified in Sheffield as type 2A. So, even when a mutation is found, typing remains unclear because the ISTH database is not certain of the type. In relation to the two examples above, workers in Antwerp believe they both represent type 2A/IIE, but this type is not mentioned in the register because the ISTH database only includes types 2A, 2M, 2N and 2B, and does not permit further subdivision to IIE, IID or IIC.
Propeptide Of special interest is von Willebrand propeptide, which is released together with vWF before cleaving, and has a constant half-life. The vWF has a variable half-life and therefore the propeptide provides a measure of the synthesis and secretion of vWF, and the ratio between the two gives an idea of the elimination of vWF. It can be used to check the antigen measurement and elimination. It is an enzyme-linked immunosorbent
‘While antigen detection is a mainstay of diagnosis and classification, the ratio between antigen and ristocetin cofactor is also important’
assay (ELISA) technique, is very stable and has important implications for the treatment of vWD.
Treatment Despite the changes in classification, testing and diagnosis of vWD, there remain three possibilities in relation to treatment. The plasma concentration of the patient’s own vWF can be increased by using DDAVP (desmopressin) or we can replace the patient’s vWF using vWF concentrates. The concentrates currently in use are still human plasma-derived but trials are in progress with recombinant versions. In addition, improving haemostasis in other ways is also a possibility, mostly using antifibrinolytics (eg tranexamic acid).
In conclusion In vWD, classification is based on laboratory tests that have large, in some cases enormous, CVs. There is also the important influence of stress, of exercise, so the patient can present with varying values, meaning that repeated testing is needed. The author has seen patients referred for further analysis from other hospitals with vWF values of
‘A von Willebrand mutation database is held for the ISTH by the University of Sheffield and it is now also included in the European Association for Haemophilia and Allied Disorders (EAHAD) database’
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20 IU/dL; they came by bus and by train, and when tested they showed vWF levels of 70 IU/dL. So, without knowing the results from the other hospitals, a diagnosis of vWD would not have been possible, and bleeding symptoms were very slight in the patients referred. The ristocetin cofactor to antigen ratio is
very important but it is influenced by test CVs and which tests are used. Currently, a ratio of 0.6 is used, (historically, 0.7) and the Americans even suggest 0.5. Importantly, it should be remembered that the tests performed are not done under physiological flow conditions. So, are the results really representative of what is happening in the body? In addition, it is possible that the introduction of new tests under stress conditions and shear stress conditions will result in the appearance of other phenotypes. Furthermore, the current classification does not correlate adequate with clinical bleeding. Clearly, laboratory testing in vWD is vital
but it is necessary to review the results in context with certain clinical aspects. Awareness of assay limitations, their availability and their detection limits is also required. In the case of the new tests, if they prove to be better then they should be used. Currently, the basic testing panel comprises factor VIII testing (antigen assay), two functional assays, normally one for platelet binding, the other for collagen binding, and also access to additional tests in difficult cases where there is a problem in arriving at a diagnosis. In addition, vWFpropeptide should be included in the list because not only is it stable, but it also has important consequence for treatment with desmopressin.
Professor Alain Gadissuer is a consultant haematologist at Antwerp University Hospital in Belgium. This article is based on a presentation given at last year’s IBMS Biomedical Science Congress.
AUGUST 2016 THE BIOMEDICAL SCIENTIST 2A/IID 2A/IIC 2A/IIE NP 2A/IIA 2B
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