Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

312
Chapter | 10 Hemostasis
disease where bleeding generally occurs during the first
year of life. The ability of vWF to mediate platelet adherence
and aggregation is a key component in primary hemostasis
(see Section II.B.3.a). In the absence, or dysfunction,
of vWF in high-flow conditions, the inability of platelets
to successfully bind collagen and form aggregates can
resemble a primary platelet disorder. As such, thrombocytopenias,
thrombocytopathias, and vWD can present in a
similar fashion; however, petechiae can be a distinguishing
feature. In contrast to platelet disorders, petechiae are not
a feature of vWD, and to the authors ’ knowledge, the reason
for this has not been explained in the literature. When
the interaction between vWF and platelets is examined, the
importance of hemorrheology quickly becomes evident.
The role of vWF is most important in platelet adherence
in high blood flow environments. As mentioned previously
(see Section II.B.3.a), the platelets are able to bind
collagen through the GP VI and GP Ia-IIa receptors independent
of vWF in areas of low blood flow. Capillaries are
sites of low blood flow, and therefore, during normal wear
and tear, platelets are able to occlude small vessel defects
without assistance from vWF. Therefore, a decreased concentration
or function of vWF would not be reflected as a
defect in this process. However, a decrease in platelet number
or function would prevent occlusion of these vessel
defects and result in petechiae.
Bleeding in vWD-positive dogs generally involves hemorrhage
from mucosal surfaces and excessive hemorrhage
after surgery or trauma ( Thomas, 1996 ). Epistaxis, hematuria,
gastrointestinal hemorrhage, prolonged estral bleeding,
and gingival bleeding at tooth eruption are some of the more
commonly reported occurrences ( Brooks, 2000 ). In some
dogs, rebleeding from incised tissues has been observed as
long as 24 h after a surgical procedure ( Brooks, 1992 ). In
general, animals that tend to be more likely to hemorrhage
have vWF concentrations below 20%; however, the concentration
of vWF:Ag does not accurately predict the risk
of hemorrhage ( Brooks, 2000 ; Thomas, 1996 ). Multiple
factors, beyond the concentration of vWF, can influence
an animal’s likelihood to bleed. The type of tissue affected
can contribute to the level of hemorrhage. For example, the
oronasal cavity and urinary tract are areas that are high in
fibrinolysins and therefore tend to produce a more severe
bleed ( Brooks, 2000 ). Concurrent platelet dysfunction can
also increase the likelihood that a vWD patient will experience
hemorrhage. Therefore, in conditions that can influence
platelet function (e.g., uremia, hypoproteinemia, anemia,
and liver disease), an increased possibility of hemorrhage
should be considered ( Brooks, 2000 ).
b. Diagnosis
Determination of both vWF concentration and function
is important in the diagnosis of vWD, as no single
test is comprehensive enough to detect all of the variants
( Favaloro et al ., 1999 ; Paczuski, 2002 ). The current gold
standard is the vWF antigen concentration (vWF:Ag)
test. Reported ranges include normal ( 70%), abnormal
( 50%), and indeterminate values that comprise the difference
( Thomas, 1996 ). This test can identify some of the
dogs with types I and III vWD, but it can leave some diagnoses
indeterminate. In addition, this test is not effective
for identifying type II disease as this is a qualitative rather
than a quantitative deficiency. Type I disease can also be
problematic to diagnose because of extragenic influences
that can temporarily increase vWF into the normal range,
as discussed later in the chapter ( Ewenstein, 1997 ).
In 1986 the use of a collagen binding assay (CBA) as an
alternate to ristocetin-induced platelet aggregation assays
(vWF:RCof) was proposed ( Paczuski, 2002 ). This assay
measures the quantity of vWF bound to immobilized collagen
in a procedure similar to that of an ELISA ( Paczuski,
2002 ). The coefficient of variation for this test has been
reported as low as 4.4% ( Callan et al ., 2005 ). When compared
to the vWF:RCof, the CBA was superior in its detection
of type II vWD and had decreased assay variability
(interassay and interlaboratory) ( Favaloro, 2000 ). Collagen
has been shown to bind vWF with a preference for the
high-molecular-weight forms, and therefore the CBA can
be used to assess the relative proportion of large vWF multimers
( Brown and Bosak, 1986 ; Duggan et al ., 1987 ). In
vWD patients the collagen binding activity is significantly
decreased compared to control patients ( Paczuski, 2002 ). In
types I and III vWD, the decrease in function of vWF parallels
the level of vWF:Ag present; in type II vWD, there is
a disproportionate decrease in activity ( Denis and Wagner,
1999 ). Collagen binding assays, in conjunction with the
antigen assay, can therefore be used to assess the quantity
of vWF as well as its function ( Paczuski, 2002 ). This makes
these assays valuable not only as screening tests for vWD
but also to distinguish between types I and II ( Paczuski,
2002 ). The ratio of vWF:Ag to vWF:CBA in normal patients
and type I vWD is generally less than or equal to 1 ( Favaloro
et al ., 1991 ). In type II vWD, this ratio has been reported to
range from 2 to 8 ( Brown and Bosak, 1986 ; Favaloro et al .,
1991 ). Although the CBA is currently utilized extensively in
human diagnostics, it is only recently becoming available for
the diagnosis of canine vWD.
c. Extragenic Influences on vWF Concentration
The value of a single sample in an animal with an indeterminate
vWF:Ag ELISA result is limited by daily and
weekly variation in vWF:Ag concentration ( Kramer et al .,
2004 ). In humans there is question as to whether other
traits may influence the phenotypic expression in type I
disease. For instance, individuals with blood type O have
levels of vWF 20% to 30% lower than individuals with
other blood types ( Ewenstein, 1997 ). Several other factors
have also been found to increase the plasma concentration
of vWF, including azotemia, liver disease, strenuous
exercise, endotoxemia, parturition, and increased plasma
vasopressin ( Thomas, 1996 ). In dogs, significant increases
in the vWF concentration occur during the last one-third