Page 195 - Basic Monitoring in Canine and Feline Emergency Patients

activates the intrinsic and common pathways (see plasma is more sensitive than a visual exam, the
Fig. 9.1). Cats generally clot in < 8 minutes and aPTT can detect when factor activity drops to
VetBooks.ir dogs within 3–13 minutes, although times up to 21 <35%. The PT is similar, but since the activator
used is TF, the PT test looks for deficiencies in the
minutes have been reported in healthy dogs. As this
method is highly subjective and prone to significant
has a similar sensitivity to the aPTT, detecting
variation, additional methods to standardize sur- extrinsic and common pathway of the cascade. It
face activation have been developed. when factors in the extrinsic and common path-
ways drop below approximately 35% activity.
For both tests, the time from addition of the acti-
Activated clotting time (ACT)
vator to the first formation of fibrin within the
The ACT test uses a tube containing diatomaceous sample is the result. The formation of fibrin can be
earth (gray top tube) to more consistently activate detected by changes in optic density or electrical
coagulation than in the Lee-White method. Similar impedance of the samples. The PT and aPTT are
to glass, diatomaceous earth is negatively charged not affected by primary hemostatic disorders.
and can activate factor XII to initiate coagulation However, because the test result is reported once a
via the intrinsic pathway, but the larger surface area small amount of fibrin is created, they do not reflect
activates coagulation more effectively than glass disorders of hemostasis that may occur after this
alone. Two milliliters of whole blood is added to the point (e.g. abnormalities in the amplification or
gray top tube, mixed by inversion, and then incu- propagation of coagulation that lead to the throm-
bated at 37°C (body temperature) for 60 seconds. bin burst [see Fig. 9.4B] or alterations in fibrinolysis
After 60 seconds, the tube is checked by inversion [see Fig. 9.5]).
every 5 to 10 seconds for visual evidence of develop- The reference ranges specific to the individual
ment of a clot. The time from initial contact of blood laboratory or analyzer should be used for interpret-
with the tube until visual development of a clot is the ation of these tests as the duration to form fibrin is
ACT; in dogs, reported normal values range from specific to the strength of the activator used. The
60–110 seconds, and in cats 50–75 seconds. standardization of the PT to the strength of activa-
The ACT is not a very sensitive test and is only tor used is the basis of the ‘international normal-
abnormal in patients that have severe (<10% fac- ized ratio’ or INR. To calculate an INR, the PT
tor activity) hemostatic deficits. Also, because it value generated at the laboratory is divided by the
depends on the platelets present in the blood mean PT value in that lab’s reference range. This
sample to provide procoagulant phospholipid allows comparison of the degree of PT elevation
surfaces for amplification and propagation, it can versus normal between laboratories. While the INR
be prolonged in cases of severe (<10,000/μL) is commonly used in human medicine to monitor
thrombocytopenia. warfarin-type anticoagulant therapy, it is rarely
used in veterinary medicine.
Activated partial thromboplastin time (aPTT)
and prothrombin time (PT) Specific factor testing
More sensitive tests of secondary hemostasis While tests such as the PT and aPTT test the func-
include the aPTT and PT. For these tests, blood tionality of larger pathways of the coagulation
anticoagulated with 3.2% citrate (blue-top tube) is system, it is possible to measure amounts or activity
used. The sample in the blue-top tube is combined of certain individual factors as well. Unfortunately,
with standardized quantities of calcium, phospho- these tests are often performed at reference labora-
lipids, and an activator, and all testing is per- tories or only available at research or referral insti-
formed at 37°C. tutions. This limits their clinical utility if the
The aPTT tests the intrinsic and common path- patient’s status is time sensitive. As an example,
way (see Fig. 9.1), because a surface activator (kao- fibrinogen quantities (factor I) are most commonly
lin, ellagic acid, or silica) is used, similar to the directly measured by the Clauss assay. In this test,
diatomaceous earth of the ACT. However, because a high level of thrombin is added to diluted plasma
the various components of the aPTT (phospholip- and the change in optical density caused by the
ids, calcium, activator) are more standardized and precipitation of fibrin is compared to samples of
the optical detection method for fibrin formation in known concentration.

Coagulation 187