Page 196 - Basic Monitoring in Canine and Feline Emergency Patients

For cases of suspected individual factor deficien- ‘global’ overview of hemostasis combined with
cies, such as hemophilia, the activity levels of spe- graphical outputs that allow for quick visual assess-
VetBooks.ir cific coagulation factors can also be measured. This ment of results has made this form of testing grow
in popularity in recent years. Viscoelastic testing is
is done by mixing the patient’s plasma with test
plasma that is known to be deficient in the specific
medicine and veterinary referral institutions.
clotting factor of interest. A PT or aPTT (depend- becoming more widely available in both human
ing on location of the factor of interest in the In general, when performing viscoelastic testing,
intrinsic or extrinsic cascade) is performed on the citrated whole blood is incubated in a chamber
pooled plasma sample. The degree of prolongation with calcium plus some form of activator (ellagic
of either PT or aPTT is used to extrapolate the acid, TF, etc.). As the blood clots, the chamber will
activity of the coagulation factor of interest. While be physically affected in some way depending on
these tests may not be readily available for the the methodology of the specific analyzer. For
acutely bleeding animal who is likely emergently example, in the TEG, the changes in rotational
receiving fresh frozen plasma or whole blood (see force applied to a pin within the chamber by the
Section 9.4), sample collection prior to administra- clotting blood is measured (e.g. initially the pin
tion of blood products can be analyzed at a later moves freely but its movement slows and changes
time for a definitive diagnosis. For stable patients, as the blood clots). The signal from the chamber is
screening for specific factor activities can help pre- translated into a characteristic tracing. Several vari-
surgical planning for elective procedures and ables can be measured from this tracing which rep-
inform breeding decisions. resent various aspects of clot formation speed, clot
strength, and rate of clot breakdown. While the
entire viscoelastic assay may run for 60–90 min-
Fibrin degradation products
utes, the early physical appearance of the tracing as
Measurement of elevations of components such as well as many of the variables are often apparent
FDPs may indicate pro-thrombotic and procoagu- within 10–20 minutes of initiation of the test.
lant states such as disseminated intravascular Please see Further Reading section for more infor-
coagulation. Fibrin degradation products are mation on viscoelastic testing and interpretation of
formed from the breakdown of fibrinogen, fibrin, the various parameters.
or crosslinked fibrin. Elevated d-dimers, which are
a subtype of FDPs created only by breakdown of 9.3 Indications
actual thrombi (i.e. crosslinked fibrin formed in the
amplification and propagation phases, see Fig. 9.4), In general, coagulation testing is pursued in
are more sensitive and specific for the diagnosis of response to the clinical presentation of the animal.
thrombosis. Both FDP and d-dimer assays are typ- Clinical clues can guide the clinician to a suspicion
ically performed by semi-quantitative latex agglu- of either a primary or a secondary hemostatic
tination. In each of these tests, antibodies against disorder.
the product of interest are bound to latex beads
suspended in solution. When the product is present,
the beads clump and fall out of solution, which is Primary hemostatic disorders
detected as decreasing turbidity of the sample. Primary hemostatic disorders involve the platelets.
With primary hemostatic disorders, animals will
bleed at gingival surfaces in the mouth or other
Viscoelastic testing
mucosal surfaces such as the nose. They will also
As blood clots, it goes from a liquid state to a more display petechia or ecchymosis which is essentially
solid ‘gel-like’ consistency. This physical change is pinpoint or larger areas of bleeding under the skin
the basis of viscoelastic testing methodologies such (Fig. 9.10). Other common areas to find bleeding
as thromboelastography (TEG) or rotational throm- are in the bladder leading to hematuria, within
boelastometry (ROTEM). Rather than measuring the anterior chamber of the eye causing hyphema
specific cell or protein activities as in previous tests, (Fig. 9.11), or in the gastrointestinal (GI) tract
these assays examine how the mechanical proper- causing melena.
ties of the sample change over time as a way to Primary hemostatic disorders can be divided
assess overall clot kinetics and strength. This into two large categories – thrombocytopenia or

188 E.J. Thomovsky and A.C. Brooks