Overview
Blood Composition
and Examination
The examination of blood cells is known as haematology. This will
include checking the number of cells, but also other features such as
their size and shape.
These days blood samples are often examined using automated analysers,
but
Top 5 abnormalities missed when a blood film is not examined (2012)
Ryan K & Pohlman LM Clinician's Brief explains how abnormalities
may be missed if a blood film is not also examined using a microscope.
Blood consists of the following:
White Blood Cells (Leukoctyes)
These are the body's defence system. They are discussed in more detail
below.
Red Blood Cells (Erythroctyes)
These carry oxygen around the body. They are discussed in more detail on
the Anaemia page.
Platelets (Thrombocytes)
These are cells in the blood that help the blood to clot, which stops us
bleeding to death if we get cut. There is more information about
platelets
here.
Serum
You may sometimes see reference to blood serum. This is the pale, straw
coloured product that is left after blood has clotted. It may contain a
number of different compounds, including proteins such as albumin,
potassium, sodium and urea.
White Blood Cells (WBC)
or Leukocytes: Infection or
Inflammation
White blood cells, sometimes called leukocytes, are the body's defence
system. If inflammation or infection is present anywhere in the body,
or if cancer tries to strike, white blood cells will accumulate near the
source of the assault to fight the invaders, and
therefore the total number of white blood cells will increase.
As part of this process, white blood cells also remove damaged cells and
tissue.
Pet Place has some information about WBCs.
RnCeus has a good overview of WBCs.
Cornell University College of Veterinary Medicine
explains more about WBCs.
How to get the maximum information out of feline
haematology
(2011) Ishida T Presentation to the World Small Animal
Veterinary Association World Congress discusses WBCs.
There are five main types of white blood cells, divided into two groups:
Granulocytes
So called because they absorb the stain when they are viewed under a
microscope:
Agranuloctyes
So called because they do not absorb the stain when they are viewed under
a microscope:
Differential Count
There are many different types of white blood cell, so in order to work out where the problem lies, it is necessary to
examine how many of each type there are. This is called the
differential count, and it usually shows two sets of numbers:
Normally it is the absolute count that is assessed.
Cornell University College of Veterinary Medicine
explains more about the differential count.
The Merck Veterinary Manual also provides
some information.
White Blood Cells: Increased
White blood cells are often increased when fighting an infection but
elevated white blood cells do not always mean an infection is present.
Critical Care DVM
(2015) explains why
this is the case.
One common cause of elevated white blood cells is inflammation.
There can be inflammation without infection but not vice versa.
In some cases, inflammation may be visible i.e. pus, which consists
largely of white blood cells.
There are other possible causes of increased white blood cells, such as the use of
corticosteroids.
A general review of the mechanisms for steroid or
glucocorticoid induced increases in the white blood cell (WBC) count
(2015) Busti AJ (ed) Evidence Based Medicine Consult explains why
this happens.
WBCs are often
very high in cases of
acute kidney
injury (because of the toxins build up and associated inflammation).
Cats with very high WBC levels may also have increased
potassium levels.
Low White Blood Cells
Occasionally, and rather confusingly, cats with inflammation or an
infection may have a decreased number of white blood cells. This is known
as neutropaenia, and is often
the case with a viral (rather than bacterial) infection.
It may also be seen if the cat has been suffering from a chronic or severe
infection, to such an extent that the white blood cells are depleted.
Cats taking
methimazole
to control
hyperthyroidism
may have low white blood cell counts.
Neutropenia on routine CBC in cats (2024) Webb J Clinician's
Brief discusses the causes of neutropaenia in cats.
Neutrophils
Neutrophils are the most numerous white blood cell, accounting for 60-75%
of the total. They are produced in
the bone marrow.
Biochem Web has a great video of a
neutrophil hunting a bacterium.
Cornell University College of Veterinary Medicine
has an overview of neutrophils.
Segs and Bands
Mature neutrophils are called segmented cells or segs.
Immature neutrophils are called bands.
If severe inflammation or infection is present, more bands are released into the
blood than would normally be the case to help fight it, so the percentage
of bands increases compared to segs. This is sometimes called "a shift to
the left." The higher the number of bands, the more severe the
infection or inflammation.
If total neutrophil levels are high with a left shift, so overall there
are still more segs than bands, this is called a
regenerative left shift. If neutrophil levels are normal or low with a
left shift, and overall there are more bands than segs, this is called a degenerative left shift.
Top 5 leukogram patterns associated with abnormal neutrophil
concentrations (2022) Shi H & Pohlman LM Clinician's Brief
discusses various scenarios suggested by different leukogram patterns.
Degenerative left shift as a prognostic tool in cats
(2014) Burton AG, Harris LA, Owens SD & Jandrey KE Journal of
Veterinary Internal Medicine 28(3) pp912-7 found that cats
admitted to a veterinary teaching hospital over a fifteen year period who
had a degenerative left shift were 1.57 times more likely to die or be put
to sleep. Cats with leukaemia were most at risk. However, the study does
state "each situation needs to be evaluated individually, as some patients
with DLS can and will recover."
Neutrophilia: High Neutrophils
High neutrophil levels (neutrophilia) often indicate that the body is fighting a bacterial
infection.
Other possible causes include inflammation or
stress.
The use of
corticosteroids
may lead to high neutrophil levels, as may
immune mediated
haemolytic anaemia (IMHA).
Neutropaenia: Low Neutrophils
Neutrophils are
often low (neutropaenia) in cases of viral infection.
They may also
be low in cases of severe inflammation or bacterial infection where the
body has struggled to keep up with the demand for them.
CKD cats with
uraemia (which tends to apply to most CKD cats)
may have low neutrophil levels.
Cats can
occasionally have idiopathic neutropaenia, i.e. no obvious cause can be
found.
Neutropenia on routine CBC in cats (2024) Webb J Clinician's
Brief discusses the various causes of neutropaenia in cats.
A lack of
vitamin B12
has been known to cause neutropaenia in some breeds of dog.
Neutrophils: Toxic
Stress Leukogram
A stress
leukogram means that the cat has a high neutrophil count with an increase in
segs rather than bands, together with a low lymphocyte count (see
below).
The word
"stress" confuses people. Contrary to what you might expect, it does not
mean the cat is stressed by the vet visit, but refers more to chronic
physiological stress. It occurs when a cat has a chronic illness (such as
CKD), so the body is releasing more steroids.
It may also be seen when a cat is being medicated with
corticosteroids.
Cornell University College of Veterinary Medicine
explains more about a stress leukogram.
Eosinophils
Eosinophils are also produced in the bone marrow. Their primary role is to
fight allergies or parasites.
Cornell University College of Veterinary Medicine
has some information about eosinophils.
Eosinophils: High
Levels are often high because of acute or chronic inflammation, such as
that caused by:
If both eosinophils and basophils are high, this can indicate an allergic
response.
Eosinophils: Low
Cats on
steroids may have low eosinophil levels.
Rather confusingly, cats with acute or
chronic inflammation or infection may also have low eosinophil levels
(even though inflammation may also cause the opposite, high eosinophils).
Basophils
Basophils are another type of white blood cell. Their precise function is
still unknown.
Basophils:
High
If basophils are high (which is rare), you will normally also see
elevated eosinophils. Allergies and parasites such as fleas or heartworm are common
causes.
If basophils are high but eosinophils are normal, this may occasionally be
a sign of bone marrow problems, and may possibly indicate cancer.
Basophils may also be elevated in cats with liver disease.
Cornell University College of Veterinary Medicine
has some information about basophils.
Basophils:
Low
Basophils are not seen often in cats, so it is actually normal for the
value to be zero.
Lymphocytes
As the name suggests, lymphocytes are produced by the lymph glands, and
also by the
spleen. They consist of B cells and T cells:
Occasionally
you may see reference to atypical lymphocytes. This means your cat's
immune system is reacting to something. This might simply be recent
vaccinations or it might be something more serious such as cancer. Your
vet will probably ask a clinical pathologist to review your cat's results
to look for clues as to the possible cause.
Cornell University College of Veterinary Medicine
has some information about lymphocytes.
Lymphocytosis: High Lymphocytes
Lymphocytes may be increased (in terms of absolute numbers, not
percentages) in cases of chronic infection or
inflammation (such as inflammatory bowel disease), autoimmune disease or leukaemia (cancer of the blood).
Cats taking
medications for hyperthyroidism may also have elevated lymphocyte
levels.
Stopping long term
corticosteroids may lead to a temporary increase in lymphocytes.
Lymphocytes: Low
Lymphocytes are often low in cases of viral infection or when using
steroids. They may also be low in cases of chronic bacterial infections.
CKD cats with
uraemia (which
tends to apply to most CKD cats) often have low lymphocytes.
A
stress leukogram,
which occurs when a cat has a chronic illness, means the cat may
have a low lymphocyte count with a
high neutrophil count.
Lymphopenia
(2020) Allen J Clinician's Brief Aug 2020 p41 outlines
possible causes of low lymphocyte levels.
Monocytes
Monocytes can be produced in either the bone marrow or the spleen. They
are capable of leaving the blood stream to enter surrounding tissues in
order to reach hostile bacteria; when they do that, they are known as
macrophages. They also remove damaged body cells, so
are often present when there is tissue necrosis (dying tissue).
Their numbers do not usually vary much unless leukaemia (cancer of the
blood) is present.
