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Blood Composition

White Blood Cells (Leukocytes):

Signs of Infection or Inflammation






Red Blood Cells (Erythrocytes): Signs of Anaemia



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Home > Diagnosis > Complete Blood Count



  • A complete blood count, also known as haematology, examines the blood cells in the body.

  • There are two types of blood cell, red blood cells (RBCs) and white blood cells (WBCs).

  • White blood cells help to determine whether a cat has an infection or inflammation.

  • Red blood cells help to determine whether a cat is anaemic.

Blood Composition


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.


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.



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 are 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:



So called because they absorb the stain when they are viewed under a microscope:


So called because they do not absorb the stain when they are viewed under a microscope:

Differential Count

Because there are so many different types of white blood cell, in order to work out where the problem lies, it is necessary to differentiate between how many of each type there are. This is called the differential count, and it usually shows two sets of numbers:

  • the actual amount of each type of WBC (abbreviated as absolute or ABS);

  • the percentage of each type.

Normally the absolute count is what 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.


White Blood Cells: Decreased

Occasionally, and rather confusingly, cats with inflammation or an infection may have a decreased number of white blood cells. This 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 receiving methimazole for hyperthyroidism may have low white blood cells.




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 are 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.


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."


Neutrophils: High (Neutrophilia)

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).


Neutrophils: Low (Neutropaenia)

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.


A lack of vitamin B12 has been known to cause neutropaenia in some breeds of dog.


Neutrophils: Toxic

People can be worried when they see this expression, but don't panic, it does not mean your cat is being poisoned in some way.


Basically it means that a relatively severe inflammatory process or infection is present that is driving the bone marrow to work very hard to produce white blood cells to fight the problem. The Merck Veterinary Manual states "The term is misleading in that it implies neutrophil injury. The cells are not injured and have normal function. Toxic change is best defined as a set of morphologic changes observed on the blood smear that occur as a result of accelerated marrow production of neutrophils. The accelerated production is in response to relatively severe inflammatory states that maximally stimulate the bone marrow."


Toxic neutrophils often (but not always) are accompanied by a shift to the left (see Segs and Bands).


Cornell University College of Veterinary Medicine has some information about toxic neutrophils.


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 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:

  • allergies

  • asthma

  • parasites such as fleas

  • inflammatory bowel disease (IBD)

  • parasites such as fleas

  • occasionally lymphoma, a type of cancer

I'm a very allergic person and my eosinophils are often high.


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 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.




As the name suggests, lymphocytes are produced by the lymph glands, and also by the spleen. They consist of B cells and T cells:

  • B cells work by producing antibodies which neutralise the threat

  • T cells work with other cells to do the same thing.

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.


Lymphocytes: High

Lymphocytes may be increased 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 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.


Cornell University College of Veterinary Medicine has some information about monocytes.


Monocytes: High

Monocytes may be elevated (monocytosis) when any condition that produces inflammation is present. However, they may also be high because of stress e.g. vet visits.


Monocytes: Low

Monocytes are usually only seen in low numbers in cats, so this is nothing to be concerned about.


Red Blood Cells (Erythrocytes): Anaemia


Red blood cells carry oxygen around the body. Examining the red blood cells enables your vet to check for anaemia. S/he will be looking at:

  • packed cell volume (PCV) or haematocrit (HCT)

  • reticulocytes

  • red blood cells

  • measures of iron levels:

    • iron

    • mean cell volume (MCV)

    • ferritin

    • MCHC

    • TIBC

Since anaemia is an important topic for CKD cats, it has a dedicated page which includes an explanation of all the above.


Cornell University College of Veterinary Medicine explains more about red blood cells.






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This page last updated: 13 September 2020


Links on this page last checked: 20 June 2020








I have tried very hard to ensure that the information provided in this website is accurate, but I am NOT a vet, just an ordinary person who has lived through CKD with three cats. This website is for educational purposes only, and is not intended to be used to diagnose or treat any cat. Before trying any of the treatments described herein, you MUST consult a qualified veterinarian and obtain professional advice on the correct regimen for your cat and his or her particular requirements; and you should only use any treatments described here with the full knowledge and approval of your vet. No responsibility can be accepted.


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