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

III. Laboratory Assessment of Hemostasis
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and depending on the specific activators used. Pooled
plasma from a similar species is required as a control to
avoid interpretive errors. There can be significant variations
in the assay method and reagents used, so it is crucial that
each laboratory define its own species-specific reference
interval ( Evans and Flynn, 1992 ; Johnstone, 1984 ). Similar
to the OSPT, the test is relatively insensitive, requiring loss
of activity of 70% of at least one factor before prolongation
of the clotting time. Common interferences (lipid, hemoglobin,
bilirubin) may interfere with an optical method and
provide an erroneous result. Hemoconcentration was demonstrated
to prolong aPTT in one study, emphasizing the
importance of the blood-anticoagulant ratio for accurate testing
( O’Brien et al ., 1995 ).
The aPTT can additionally be used to monitor cats and
dogs receiving heparin therapy ( Greene and Meriwether,
1982 ; Mischke, 2003 ). The assay can be modified to detect
specific factor deficiencies ( Deniz et al ., 1995 ; Mischke,
2000 ).
c. ACT
The activated clotting time (ACT) is similar to the aPTT in
that it provides information about the same portions of the
clotting mechanism (intrinsic or common pathways), but it
is a less sensitive test. It is a point-of-care test performed
on whole blood, where a compound in the tube, typically
siliceous earth, activates the contact (intrinsic) pathway
( Stockham and Scott, 2002 ). It depends on endogenous
platelets in the sample as the phospholipid source for clotting.
Because of this, although not well documented, severe
thrombocytopenia may result in a prolonged ACT. It is
used primarily as a screening test for coagulopathies, but
it requires loss of greater than 95% loss of activity of one
or more factors before it is prolonged, so disease is often
advanced by the time the ACT is abnormal. The ACT is the
time in seconds to first visible clot formation while gently
inverting the tube after initial incubation for 60sec at 37 ° C
either in a heating block or in a human axilla. The ACT is
used in humans and sometimes in animal patients to monitor
therapeutic heparinization.
Reference values have been published for cats ( Bay et al .,
2000 ), but each facility should follow a standard protocol and
ideally define its own reference interval. Automated ACT
methods are available on some point-of-care instruments
that offer OSPT and aPTT, but they are not exactly the same
as the tube ACT (phospholipid is added) and provide no
advantage over an automated aPTT ( Tseng et al ., 2001 ).
d. TCT
The thrombin clotting time (or simply thrombin time)
assesses the ability of thrombin to convert fibrinogen
to fibrin, so it is abnormal when there are quantitative or
qualitative abnormalities in fibrinogen. Variations of the
test are available, so a reference interval specific for the
laboratory and species should be developed ( Mischke and
Jacobs, 2001 ). The Clauss method is specifically used to
derive a concentration of fibrinogen in a plasma sample
(see Section III.B.5.b) .
4. Assessment of Fibrinolysis
a. FDPs
Fibrin-fibrinogen degradation product (FDP) assays measure
the breakdown split products of either fibrinogen or
fibrin, and increased values indicate enhanced fibrinogenolysis
or fibrinolysis. Elevated FDPs are used to help confirm
the presence of DIC (see Section IV.B.4). Several human
assays are available, including both serum and plasma-based
assays, but the plasma latex agglutination test appears more
sensitive for detecting DIC in dogs ( Boisvert et al ., 2001 ;
Stokol et al ., 1999 ). FDP assays have also been used in cats
and horses.
b. D-Dimer
D-dimers and x-oligomers are small fragments created
when fibrinolysis occurs. They are products of fibrin degradation,
not fibrinogen degradation, so are more specific
for enhanced fibrinolysis. There are currently no available
animal-specific assays, but there appears to be sufficient
cross-reactivity between human and canine proteins as some
human-based semiquantitative immunological latex agglutination
methods have been used in canine patients ( Griffin et al .,
2003 ; Nelson and Andreasen, 2003 ; Stokol, 2003 ; Stokol
et al ., 2000b ). D-dimers have been assessed in horses, predominantly
in an attempt to document DIC related to colic
(Dallap et al ., 2003 ; Heidmann et al ., 2005 ; Monreal, 2003 ;
Sandholm et al ., 1995 ).
5. Assessment of Specifi c Hemostasis Proteins
a. Prothrombin, FV, FVII, FVIII, FIX, FX, FXI, FXII
Specific factor activity is traditionally assessed using coagulometric
methods. Depending on the factor, modifications
of the OPST (for extrinsic or common pathway factors) or
the APTT (for intrinsic factors) are used where the activity
of the factor in patient plasma is measured in a sample
mixed with plasma known to be deficient in the factor. If
the patient sample is deficient in the factor in question, the
OSPT or APTT, as appropriate, does not correct ( Stockham
and Scott, 2002 ). The activity of the factor in the patient’s
plasma is derived from a standard curve created from serial
dilutions of pooled homologous plasma from clinically
healthy animals that has been added to the factor-deficient
plasma. Factor-deficient plasmas for domestic animal species
are not generally available, so methods must be validated
using human reagents.
Chromogenic methods are now available and used quite
extensively in human medicine. However, the expense, the