Routine Blood Tests - Haematology and Biochemistry

Routine blood tests frequently form part of the full clinical examination of any animal presented to the veterinarian. These tests are extremely helpful when arriving at an accurate diagnosis.

All that is required is a small quantity of blood usually less than half a teaspoonful (2–3 ml). This is obtained from either a vein in the neck or the leg. It is a quick and relatively painless procedure well tolerated by most pets.

What information can be obtained from such a small quantity of blood?

The first and most important test carried out is the complete blood bount (CBC).

A complete blood count gives an indication of the quantity and quality of all the various cells found in the blood. It provides information about

• Red blood cells (erythrocytes)
• White blood cells (leucocytes)
• Blood platelets

The information usually contains three separate parameters, (1) the number of cells, (2) their size and shape and (3) an indication regarding any abnormalities that may have been noted.

Red blood cells (erythrocytes)

These are the most numerous cells in the blood and responsible for its red colour. Erythrocytes carry oxygen via their contained haemoglobin from the lungs to the tissues of the body where it is released. CBC gives an indication of the numbers of red blood cells (RBCs) and also how much haemoglobin is present.

These values are very important to the veterinarian.  For example:-   

Numbers of cells  

1. A decrease in the number of red cells and/or a reduction in the amount of haemoglobin indicates anaemia. This is not a disease in itself but a sign of an underlying problem that needs investigation. 

2. An increase in the number of red cells is usually due to dehydration or excitement or can occur simply due to the stress of blood sampling. In very rare cases it can be a sign of a serious disease.

3.  Size. Young red blood cells are normally larger than mature red blood cells. Thus in an anaemic animal the presence of larger than normal cells indicates that the bone marrow is responding to the anaemic state and producing more erythrocytes. This is called regenerative (responsive) anaemia.

"Young red blood cells are normally larger than mature red blood cells. Thus in an anaemic animal the presence of larger than normal cells indicates that the bone marrow is responding to the anaemic state and producing more erythrocytes."

If on the other hand the red cells are smaller than normal, it is an indication that new cells are not being produced in sufficient quantities. This may be due to chronic problems such as iron deficiency or liver disease. This is termed non-regenerative (non-responsive) anaemia.

If the cells are the correct size but fewer in number, it may indicate a chronic problem which may be due to inflammation or cancer. This is termed  anaemia of inflammatory disease (AID) or anaemia of chronic disease (ACD) and is also a non-responsive anaemia. A non-regenerative anaemia is also seen with kidney disease.

The appearance and numbers of young red cells is therefore important. Special dyes are used to differentiate these. They are known as the reticulocytes and are recorded in the CBC as the reticulyte count.

White blood cells

White blood cells or leucocytes help to protect the body against infections, e.g. bacteria, viruses and any foreign material that may enter the body through various routes. There are five types of leucocytes. These are neutrophils, monocytes, eosinophils, basophils and lymphocytes.

The different types can act independently or in combination depending upon the disease or problem that threatens the body. 

As with erythrocytes, the CBC usually measures three parameters:

1.  Numbers. This is the white blood cell count (WBC). It is a useful indicator of the pet's level of protection against infection. This total WBC is also broken down and the differential white blood cell count is listed. Any change from the accepted normal differential for the species, (e.g. dog, cat, rabbit, etc) can give an indication of underlying problems, e.g. lung cancer, leukaemia, etc.

2.  Appearance. Microscopic appearance of the leucocytes gives information regarding the   response to infection, inflammation, toxins, etc depending on the cells involved. It can indicate if the immune system has been activated or if cancer cells are present in the circulation.

3.  Abnormal cells, e.g. the presence of cancer calls.

Blood platelets

Platelets are much smaller than leucocytes or erythrocytes. They are present in large numbers in the blood. They play a vital role in blood clotting especially with small wounds. Platelets are the first line of defence against bleeding and it is vital to maintain adequate numbers in the circulation. As with the other groups of cells the CBC will give an indication of:-

• Numbers. This will give a general indication of the blood's clotting ability. Below a certain critical level lack of platelets may result in spontaneous bleeding. A low platelet count may indicate problems with platelet production in the bone marrow, a result of platelets being removed from the circulation or indicate increased destruction of platelets.

An increased platelet count can simply be due to excitement or exertion or an active bone marrow. Occasionally it can be due to bone marrow tumours.

• Size. Platelets shrink with age so that a high proportion of large platelets indicates a very active bone marrow producing ample new platelets. This is useful when interpreted in relation to numbers. If platelet numbers are low it indicates poor response on the part of the bone marrow.
• Appearance. Rarely immature platelets or bizarre giant platelets are seen. These may be an indication of the presence of an underlying bone marrow problem.

 Serum biochemistry

What is blood serum?

Serum is a major component of blood. It is the liquid part of the blood without any of the cells listed above and also without fibrinogen which is involved with clotting. Serum also contains many products produced by the body. The analysis of the various biochemical components can provide information about the functioning of liver, kidneys and other organs in the body. An abnormal result may indicate the presence of disease which can then be further investigated.

What is a biochemistry profile?

There are many different serum biochemistry tests that can be performed. Not all of these are needed in every case and thus the most commonly tested biochemical parameters are usually listed on the serum biochemistry profile. This is designed to assess the function of various organs such as the liver and kidneys but biochemistry profiles can be altered according to the needs of the patient.

Various liver enzymes and bilirubin are routinely tested for together with albumin and globulin, two main proteins found in the blood. The results are useful in assessing liver function. Urea (blood urea nitrogen – BUN) and creatinine are measured in order to assess kidney function. Certain pancreatic enzymes, particularly amylase and lipase are also measured to give an indication of exocrine pancreatic function (EPI) as is blood sugar for possible diabetes. Minerals, particularly calcium and phosphorus, are routinely measured. Certain muscle enzymes are also routinely measured together with cholesterol which is produced in the liver as part of fat metabolism.

"Various liver enzymes and bilirubin are routinely tested for together with albumin and globulin, two main proteins found in the blood. The results are useful in assessing liver function."

Electrolytes, particularly potassium, chloride, sodium and bicarbonate, are assessed. These all have important roles to play in the body. Collectively, they help to maintain the blood and tissue fluids in a stable and balanced state. Disturbances in electrolytes are often caused by vomiting and/or diarrhoea so serum biochemistry can be of immense value in diagnosis, particularly if combined with the results of a simple urine analysis. (See below)

Urine analysis is a simple in-house test which measures parameters such as specific gravity (USG). This gives a measure of urine concentration, urine analysis and also records the colour and clarity or otherwise of the urine. This sometimes can be an indication of underlying disease. The pH of the urine measures how acidic or alkaline it is. This is useful since extremes of pH are often associated with disease. For example animals with highly acidic urine are at risk of developing calcium oxalate stones whereas those with highly alkaline urine are more susceptible to bladder infections and may develop entirely different bladder stones.

Substances such as protein, glucose (sugar), ketones, blood and other substances can be assessed by a simple `dipstick' technique which helps to corroborate or refute some of the biochemistry results. (For more details please see Handout – Urine analysis.)