Practical hemostatic tests
The following are simple, in-house tests requiring no specialized equipment. They are quick, inexpensive, practical tests
that allow recognition and characterization of hemostatic defects. These tests are often referred to as "cage-side", in that
they provide results almost immediately.
A quick, reasonably accurate estimation of platelet numbers can be made from a stained blood smear is much quicker than an
actual platelet count. After routine preparation and staining, the blood smear is scanned to ensure even platelet distribution
and that there is no evidence of platelet clumping. The average number of platelets in approximately 5-10 oil immersion fields
is counted to estimate platelet numbers. The count is ranked as very low, low, normal, or high. One platelet per oil-immersion
field represents approximately 20,000 platelets. Approximately 8-12 platelets per oil-immersion field are considered normal.
While platelet estimation helps determine the presence of thrombocytopenia in an emergency situation, a true platelet count
is necessary to classify the severity of depletion. Ongoing platelet quantitation is helpful in monitoring the course of a
disease or the patient's response to certain therapies.
Buccal mucosal bleeding time
Bleeding time is the time it takes for bleeding to stop after severing a vessel. The bleeding time test most often used in
veterinary medicine today is the buccal mucosal bleeding time (BMBT).
The BMBT assesses platelet and vascular contribution to hemostasis, thereby evaluating primary hemostasis. A disposable template
with two spring-loaded blades is used to produce standardized incisions in the buccal mucosal surface of the upper lip. The
blades create 5mm long X 1 mm deep incisions. The duration of bleeding from these incisions is monitored.
Buccal mucosal bleeding time
- bleeding time device
- gauze strip
- filter paper or gauze sponges
- timing device
1. Place animal in lateral recumbency.
2. Expose mucosal surface of upper lip. Position a gauze strip around the maxilla to fold up the upper lip. Tie the strip
gently, just tight enough to partially block venous return.
3. The incision site should be void of surface vessels and slightly inclined so that shed blood from the incision can flow
freely toward the mouth. Place bleeding time device flush against mucosal surface, applying as little pressure as possible,
and press tab to release scalpels.
4. Let stab incisions bleed freely, undisturbed, and time until bleeding stops. Excessive blood should be blotted as often
as necessary so as not to have blood flow into patient's mouth. Place either filter paper or gauze sponge approximately 3-4
mm below the incision, taking care not to disturb the incision site and any clot that may be forming.
5. The end point is recorded when the edge of the filter paper/sponge does not soak up free-flowing blood. The bleeding time
is the mean bleeding time for the two incisions. Normal bleeding time is less than 4 minutes.
The BMBT is a screening test. As with any screening test, it is not 100% sensitive and, therefore, not all primary hemostatic
defects will be discovered. This test also will not differentiate between vascular defects or platelet function defects. The
BMBT is prolonged in cases of thrombocytopenia/pathia, von Willebrand's disease, uremia, and aspirin therapy. Obviously, BMBT
should not be performed on any patient that is known to be thrombocytopenic. Although it does have limitations, there are
several advantages to this test. Commercial bleeding time devices are readily available. The templates are standardized and
therefore, results are reproducible. It is a simple and quick test to perform, and the results are almost immediately available.
Patients seem to tolerate the procedure well, eliminating need for chemical restraint. The incisions produced are well above
the concentrated pain fibers in the lip. Sometimes the animal will reflex upon hearing the noise the scalpels make when released
from the device, but the procedure itself is not painful.
The CBT is another bleeding time test. The CBT is useful for evaluating overall hemostasis. It is sensitive to defects in
vascular integrity, platelet function, and coagulation.
Activated clotting time
The activated clotting time (ACT) is a simple, inexpensive screening test for severe abnormalities in the intrinsic and common
pathways of the clotting cascade. It evaluates the same pathways as aPTT. Some argue that ACT is less sensitive at detecting
factor deficiencies than aPTT in that factors must be decreased to less than 5% of normal in order to prolong ACT, whereas
the aPTT will be prolonged with factor deficiency less than 30% normal.
Activated clotting time test
- vacutainer sleeve
- vacutainer single collection needle
- ACT tube containing diatomaceous earth
- 370 C electric heat block (can substitute hot water bath or hold in hand)
1. Warm ACT tube in heat block to 370 C for approximately 3 minutes.
2. Perform clean venipuncture on an unthrombosed vessel. Discard the first few drops of blood to eliminate tissue thromboplastin,
the tissue factor responsible for activation of the extrinsic pathway.
3. Puncture the ACT tube with the distal needle and collect approximately 2 milliliters of blood. Begin timing as soon as
blood enters the tube.
4. After collection, invert the tube several times to mix with diatomaceous earth and place in heating block.
5. After thirty seconds from start of timing, gently tilt the tube and examine for clot formation. Return tube to heat block
and repeat procedure every ten seconds.
6. The ACT time is the time from collection of blood in the tube to initial clot formation. In the dog, the normal is 60-110
seconds. In the cat, the normal is 50-75 seconds.
Prolongation of ACT occurs with severe factor deficiency in the intrinsic and/or common clotting pathway (e.g. Hemophilia),
in the presence of inhibitors (e.g. heparin, warfarin), or in cases of severe thrombocytopenia due to the lack of platelet
phospholipid (mild prolongation of 10-20 seconds). The ACT is inexpensive, easily learned, quick to perform, reproducible,
and provides immediate results. It is a very useful measurement of coagulation in emergency situations. When compared with
the aPTT, the role that technical and laboratory error can have on the test results must be taken into consideration. This
is not to suggest that one should rely solely on the ACT. In most situations, the ACT should be followed up with an aPTT.
References available upon request