HAEMOSTASIS vWF:RCo FVIII vWF:FVIIIB vWFpp
Signal peptide D1
vWF:Ag
Propreptide (vWFpp)
Exons 2 3–10 11–17 FVIII Heparin binding sites 18–20 20–28 Gplb Heparin 28 Collagen binding site
Collagen binding sites 28
vWF multimers Typical aspects of the von Willebrand factor multimers in different vWD types.
of vWD, and new kits and techniques (eg HemosIL, Innovance) have tried to improve the tests. The ristocetin cofactor test uses platelets
with a natural GpIb. In the HemosIL test, the platelets are replaced by particles covered with a recombinant GpIb fragment. This test also depends on the use of ristocetin, as it is needed to promote binding to platelets and to make the A1 loop. The Innovance test also uses recombinant GpIb, but with two gain-of- function mutations. In this test, the promotion of binding is not on the vWF side, by making the loop, but it is on the GpIb side with a more avid GpIb for the vWF. This test is ristocetin- independent and is thus more physiological, although normally the body has no GpIb with a gain-of-function mutation either. These new tests are undergoing trials to
compare them with the ristocetin cofactor test to see if they can provide additional information and have a lower CV. This has been recognised by the International Society on Thrombosis and Haemostasis (ISTH) with
Propeptide vWD1
vWD type
1– 3–
2A– 2B– 2M–
2N– Frequency of different von Willebrand disease types across the von Willebrand factor. 408 AUGUST 2016 THE BIOMEDICAL SCIENTIST
C8 –1
TIL 1
D1 assembly
E 1
vWD2
C8 –2
TIL 2
D2 assembly
E 2
‘Diagnosis is made based on the patient’s personal bleeding history, which can be measured through a standardised bleeding score’
the introduction of specific nomenclature for the two tests: GpIbR for the HemosIL test using recombinant fragments, and GpIbM for the Innovance mutation test. While antigen detection is a mainstay of
diagnosis and classification, the ratio between antigen and ristocetin cofactor is also important. This looks at the ratio between activity and the antigen itself and the criterion used in the past has been 0.7. This was also used in a European type 1 study, but it subsequently suggested in the analysis that it would be better to use 0.6
Mature vWF
Furin Factor VIII vWD3
TIL *
D*
E *
C8 –3
D3 assembly SS
TIL 3
E 3
GpIb
Collagen A1
ADAMTS13 A2
Collagen A3
because it is more predictive of abnormal structures and mutations. A Canadian von Willebrand type 1 study used a cut-off of 0.6 and now the Americans have even come up with the idea of moving to 0.5. As both tests have variations and problems, it is possible that individual laboratories may have to set their own criterion. At low levels it is impossible to use the
ratio because fault margins will be large. The ratio is crucial in distinguishing between type 1 and type 2, but again results depend on the laboratory tests and their variation. They are not reliable under 15 IU/dL and can lead to misdiagnosis. The collagen binding test is another
functional assay introduced to replace the ristocetin cofactor test because it offers less inter-assay and inter-laboratory variation. It measures the effect of the high molecular weight multimers and produces little variation, but it is not a replacement for the tests mentioned above as studies have shown that variation in the ristocetin cofactor
RGD D4N vWD4
C8 –4
D4 assembly
TIL 4
C1 C2 C3 C4 C5 C6 CK S S 28–32 33–34 35–39 GplIb/IIIa 40–42 42–44 45–48 52 2N D2 D’ D3 2A 2N
2B 2M
2A
ADAMTS13 cleavage site Tyr1505-Met1506
vWF:CB A1 A2 A3 D4
B1 C1 C2 CK 2A
B2 B3 vWF genetics 2M
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