Page 496 - Problem-Based Feline Medicine

488 PART 7 SICK CAT WITH SPECIFIC SIGNS

– Bleeding into body cavities (pleural space, peri- If the blood sample is being collected by using a winged-
toneal cavity, joints). infusion set (“butterfly”), jugular catheter, or vacuum
– Scleral hemorrhage. tube draw, then the first 2–5 ml should be discarded or
● Excessive bleeding may be characterized by: used for serum chemistry determinations. This may
– Delayed bleeding and rebleeding because the ini- reduce platelet clumping and premature activation of
tial platelet plug is not stabilized by a fibrin clot. coagulation. In most cases a blood sample is obtained by
– Detection of a bruise following venepunc- venepuncture using a syringe and needle, in which case
ture is one of the most common initial signs the EDTA (platelet count) and citrate tubes (coagulation
of coagulopathy. It should not be dismissed tests) should be filled first, and the remaining blood used
as a result of venepuncture technique in a cat for serum chemistry determinations.
at risk for a bleeding disorder.
Routine tests of hemostasis include:
– Increased rate of bleeding if a larger vessel is
● Platelet count.
injured.
– Used for detecting thrombocytopenia.
Differences are not absolute between bleeding due – Platelet counts may be obtained by estimation
to platelet disorders and coagulopathy, and clinical from a blood smear, manual counting, or with
signs overlap. Bleeding patterns seen with both automated cell counters.
include cutaneous and subcutaneous bleeding at –To estimate platelet count, prepare a routinely
venepuncture sites, gingival bleeding, retinal hemor- stained blood smear. Examine the blood smear
rhages, cerebral hemorrhages, pulmonary hemor- monolayer where red cells are close together
rhages and widespread bleeding. and 50% are touching. Normally there are
about 10–30 platelets/oil immersion field
(magnification = × 1000); each platelet ≈ 20 ×
WHAT? 10 /L (20 000/μl). This requires a properly pre-
9
pared blood smear. An alternative method that
To diagnose the cause of bleeding:
avoids a blood smear uses a disposable count-
● Rule out a local lesion.
ing chamber and supravital stain (ZynoStain,
● Confirm abnormal bleeding with tests of hemostasis.
Zynocyte, MA, USA) (Tasker et al, 2001).
● Characterize abnormal bleeding as a defect in pri-
– Manual counting may be performed using a
mary and/or secondary hemostasis based on clinical
Unopette microcollection system (Becton
signs and tests of hemostasis.
Dickinson) and a hemocytometer.
● Determine if the hemostatic defect is inherited or
– Automated cell counters use impedance
acquired.
(Coulter principle), quantitative buffy coat
● Determine the mode of inheritance if inherited.
(QBC) or light scattering properties (flow
● Determine an underlying cause if defect is acquired.
cytometry).
The most common causes of bleeding are local – Platelet counting in cats has historically been
lesions – trauma, inflammation and neoplasia. problematic because cats are prone to artifac-
tual thrombocytopenia due to platelet clump-
The most common causes of impaired hemostasis
ing. Platelet clumping results from difficulties in
are liver diseases, neoplasia and infectious diseases.
obtaining blood samples and increased aggre-
Most of the alterations in hemostasis are subclinical.
gatability of platelets.
The most common causes of abnormal clinical bleed- – Citrate samples are superior to EDTA samples,
ing are hepatic lipidosis, retroviral-induced megakar- although platelet clumping may still occur.
yocytic hypoplasia and vitamin K antagonist poisoning. – If a citrated blood sample has been
obtained for coagulation testing (see
below), consider using this sample for
TESTS OF HEMOSTASIS
platelet counting.
Tests of hemostatic function are necessary to determine – A solution containing citrate, theophylline,
what stage of the hemostatic process is abnormal. adenosine and dipyradamole (CTAD) con-