Page 1031 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1006 CHAPTER 9

VetBooks.ir lected very carefully to prevent any platelet activa- scan at low magnification is first performed to evalu-
Samples for haemostasis evaluation must be col-
ate the overall distribution of cells on the smear
tion or tissue thromboplastin-induced activation of
the clotting mechanism. The initial 1–2 ml of blood and to examine for the presence of platelet clumps
or unusually large cells. On completion of a low-
collected should be discarded to avoid this problem. magnification scan, higher magnification should be
Unless the sample can reach the laboratory within used to further evaluate individual cell lines. A sys-
1 hour and be tested within 4 hours, plasma should tematic approach should be adopted that is used
be harvested, shipped on ice and frozen at –20° C each time a blood smear is evaluated (Fig. 9.2). It is
until analysis can be performed. important to make sure that all three cell lines are
represented, as an absence of platelets could easily be
Blood cell enumeration missed if not specifically evaluated.
Manual enumeration of platelets, RBCs and leuco- Equine platelets are round to oval structures and
cytes in the horse has largely been replaced with are the smallest blood cells. They are anucleate and
automated analysis. Both small in-practice and larger stain very palely with commonly used Romanowsky-
high-throughput instruments are available for quan- type stains (Wright’s, Diff-Quik). Occasional very
tifying equine blood cells. Technology utilised to fine stippling of red- to purple-coloured granules
enumerate cells may be based on impedance (Coulter may be observed. Sometimes, elongated forms can
counters), optical or laser cytometry (CELL-DYN, be found. Platelets may aggregate together and be
Advia, ProCyte, LaserCyte) or on centrifugation found along the feathered edge or in the body of the
(QBC Vetautoread). Other measured values include smear itself.
haemoglobin concentration, mean corpuscular Equine RBCs are round, stain orange to red with
volume (MCV), and mean platelet volume (MPV). Romanowsky-type stains and exhibit minimal cen-
From these measured parameters, haematocrit, tral pallor. These cells are quite uniform in size in
mean corpuscular haemoglobin (MCH), mean cor- health. Polychromatophilic RBCs or reticulocytes
puscular haemoglobin concentration (MCHC) and are not readily observed in the horse in health or
red cell distribution width (RDW) are calculated. during a response to anaemia. As mentioned above,
equine RBCs frequently exhibit rouleaux forma-
Blood smear evaluation tion, which is observed microscopically as rows or
Microscopic evaluation of a well-made blood smear ‘stacks of coins’ of RBCs (Fig. 9.3). This arrange-
is a necessary part of a complete haematological ment of cells must be distinguished from pathologi-
analysis, even with the advent of automated differen- cal agglutination.
tials. Blood smears should be prepared so that there
is a feathered edge and monolayer, which is an ideal
area to evaluate blood cell morphology. A complete
9.3

9.2

Fig. 9.2 Wright’s-stained blood smear. The arrows Fig. 9.3 Normal equine blood smear. The
indicate a systematic approach for performing a arrow indicates red blood cell rouleaux formation
leucocyte differential in the monolayer of the slide. (Wright’s stain).