| A full haematological examination should always contain
the following parameters: |
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Red cell indices - RBC count, PCV/HCT, Hb, MCV, MCHC |
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White blood cell count with manual differential |
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Platelet count |
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Morphology - RBC, WBC and platelets |
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For interpretative purposes cell counts reflect the kinetics
of entry and loss of cells from circulation, and cell morphology reflects
the status of individual cells which is a direct reflection of the health
of the bone marrow, the circulation and the tissues. |
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Hypochromic RBCs flatten more
than normochromic cells in a film and have a thin rim of haemoglobin around
an increased pale central concavity. They are deficient in haemoglobin and
therefore undergo an extra division during maturation becoming smaller cells
(microcytes). In iron deficiency the red cells become microcytic
before becoming hypochromic. |
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Stomatocytes and target cells
are RBCs with increased membrane in comparison to content and therefore
adopt different folded conformations, appearing mouth-like or like targets
when viewed in 2 dimensions. All reticulocytes have excessive surface membrane
and can appear like polychromatic stomatocytes or target cells and this
finding is irrelevant. When mature RBCs adopt these shapes it usually indicates
that the membrane lipids have altered in response to some underlying disease
process e.g. chronic inflammation or liver disease. |
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Reticulocytes are large, folded
polychromatic cells. These are the primary indicators of RBC regeneration
in cats and dogs. |
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Echinocytes or Burr cells are
crenated cells with uniform circumferential spiculation. They occur in renal
disease but the most common cause is RBC dehydration/EDTA artefact in an
aged blood sample. |
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Acanthocytes are shrunken RBCs with uneven, assymetrical spiculation.
This is a special form of RBC fragmentation Acanthocytes are important early
markers of vascular anomalies such as haemangiosarcoma or microvascular
angiopathies. They also accompany chronic liver disease. |
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Spherocytes are RBCs which have
had pieces of membrane removed by macrophages. The normal biconcave shape
is lost and the cells become small dark spheres and lose their central pallor.
Spherocytes are the hallmark of immune/mediated damage to RBCs. They are
a more powerful diagnostic criterion than a positive Coombs’ test.
Incomplete sphere formation occurs with partial loss of the biconcave shape.
Such cells have reduced central pallor when viewed in 2 dimensions. |
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Keratocytes are cells which develop
a membranous blister on one side which ruptures leaving claw-like spikes.
This is a form of fragmentation in response to chemical and physical injury. |
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Eccentrocytes are cells which
have undergone oxidative damage causing adhesion of opposing membranes.
This squeezes out the haemoglobin from one side of the cell. Such cells
appear clear at one end and dark at the other when viewed in 2 dimensions.
Their presence is a good marker for oxidative red cell damage in dogs. Whereas
cats tend to form Heinz bodies, dogs form eccentrocytes (although Heinz
bodies can often be seen on the surface of eccentrocytes). The most
common cause of eccentrocyte formation in dogs and cats is onion poisoning
and paracetamol toxicity respectively. |
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Heinz bodies are non-staining
(with Romanowsky stains) particles of denatured haemoglobin. They
stain basophillic with new methylene blue. They are common in cats, because
feline haemoglobin is particularly susceptible to oxidative damage. |
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Howell Jolly bodies are nuclear
remnants which result from incomplete extrusion of the nucleus from maturing
reticulocytes. They are present in normal blood films but increased numbers
tend to be seen in splenectomised patients and in any regenerative anaemia. |
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Nucleated RBCs. Normoblasts frequently
appear in regenerative anaemias. The presence of nucleated RBCs in the absence
of polychromasia is a strong indicator of bone marrow dysfunction. It may
also occur in patients with splenic diseases or following splenectomy. Normal
dogs sometimes have occasional late normoblasts in the circulation with
minimal evidence of polychromasia. The presence of megaloblastic RBCs or
red cells with other nuclear or cytoplasmic maturation abnormalities in
the peripheral circulation is a very useful indicator of bone marrow dysfunction/dysplasia.
This is of particular relevance in cats with FELV. The presence of nucleated
red cells +/­ basophillic stippling without polychromasia can be a feature
of lead poisoning. |
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Proerythroblasts and early normoblasts
can be markers of myelodysplasia or myeloproliferative disease e.g. erythremic
myelosis. |
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Basophillic stippling is quite
a common finding in regenerative anaemias in cats and ruminants and can
be interpreted as a feature of RBC regeneration in these species. |
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Red cell parasites. The most common
red cell parasite in the UK is Haemobartonella felis which is a very difficult
microscopic diagnosis. Microscopy is an insensitive way of detecting the
parasite because the organisms may detach from the RBCs in anticoagulated
blood. |
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Inclusion bodies. Distemper inclusions
can sometimes be seen in early distemper cases. |
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This is a group of morphological abnormalities which, when
present, help to discriminate between inflammatory/infectious and neoplastic
processes. In severe toxaemic states, granulopoiesis becomes suppressed
and neutrophil morphology is altered by maturation defects. These are termed
toxic changes. The different types of toxic change are as follows: |
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Doehle bodies are remnants of rough endoplasmic reticulum
resulting from defective cytoplasmic maturation. These can be seen as angular,
poorly defined basophillic bodies in the outer regions of the cytoplasm.
A small numbers of neutrophils containing Doehle bodies are normal especially
in cats. |
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Foaminess of the cytoplasm is due to the intracytoplasmic
release of lysosomal enzymes and restricted cytolysis. |
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Increased cytoplasmic basophilia due to increased cytoplasmic
ribosomal RNA. |
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Cytoplasmic vacuolation due to lysosomal cytolysis (note
that this is a common sample ageing change in normal neutrophi/s). |
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Inappropriate appearance of intracytoplasmic azurophillic
granules (‘toxic’ granules). These are primary granules
which are retained instead of being lost during maturation. Other maturation
defects may occur for example the nucleus may undergo maturation without
cell division resulting in the formation of giant bands, mature neutrophils
with bizarre twisted/ribbon-like nuclei, or neutrophils with ringed or doughnut
nuclei. |