Updated: Nov 14, 2021
Laboratory tests are importantly used by veterinary doctors to assist in the diagnosis of diseases in sick animals. Progressively, laboratory tests are also becoming part of routine health checks to detect hidden diseases before they develop obvious symptoms. This allows a sick animal to be treated earlier and more effectively. The other essential use of laboratory tests is the testing of patients' kidneys and liver to ascertain that they are working properly before a surgical operation.
Who carries out the test?
A number of limited tests are usually done locally by the veterinary surgeon within the practice. These tests provide quick results that allow rapid decisions on treatment. These quick tests in the practice are usually confirmed by sending a sample to a commercial laboratory to check that the results tally. For a broader range of tests, samples are usually sent to a commercial laboratory which usually relays the results of routine tests within 24 hours either by telephone or e-mail (although some tests may take 10 days or longer to complete).
What is being tested and why?
There is a whole battery of tests that can be done on different types of samples, although not all are used to investigate every disease. Some samples are easier to obtain than others with varying effects on the animal.
The tests are:
1) Blood tests:
These are the most commonly performed laboratory tests. Suitable samples are easily obtained and a great deal of information can be obtained about a cat's or dog's health and these assist in the diagnosis of disease. The information is deduced from the various concentration of different chemicals in the blood. Blood tests also help to determine the state of a pet's health during regular check-ups.
Usually, a complete blood count (CBC) and a biochemistry profile are performed. Even though these tests can be performed separately, these tests are frequently done together. When the results are interpreted together, a good overview of the body's functions is obtained. This information is usually combined with physical examination findings, medical history, and other information to assess a pet's health status and determine if additional testing is needed.
a) Complete Blood Count (CBC)
Complete blood count (CBC) involves counting the numbers of different types of cells in the blood. This helps in identifying if the pet is dehydrated, anaemic (having inadequate numbers of red blood cells), or dealing with an infection. Additionally, an examination of a blood smear provides additional information about the form of the red blood cells, white blood cells, and platelets. The CBC results indicate how many red blood cells are present (Haematocrit - HCT), and the quantity of haemoglobin (Hgb) within each red cell. A low HCT might indicate anaemia, and a high HCT could indicate dehydration. Haemoglobin determines how well the red blood cells will carry oxygen to the body's tissues.
White Blood Cell (WBC) count is the total number of white blood cells counted. Certain types of white blood cells increase in number when there is infection or inflammation in the body. If the total number is low, it could mean several things, including a severe infection that has overwhelmed the body, or a bone marrow problem that is limiting the production of white blood cells.
There are several different types of white blood cells, neutrophils, eosinophils, basophils, lymphocytes and monocytes. These cells respond to different events in the body.
Platelet count (PLT) refers to the number of platelets (also called the platelet count) in the circulating blood. Platelets are involved in the body's blood clotting process.
b) Chemistry Profile
The chemistry profile measures a variety of chemicals and enzymes (proteins that are involved in the body's chemical reactions) in the blood to provide very general information about the status of organ health and function, especially of the liver, kidneys, and pancreas.
The chemistry profile also shows the patient's blood sugar level and the quantities of important electrolytes (molecules like sodium, calcium, and potassium) in the blood.
Chemistry values that help provide information about the liver include ALKP (alkaline phosphatase), ALT (alanine aminotransferase), AST (aspartate aminotransferase), and TBIL (total bilirubin).
Chemistry values that help evaluate the kidneys include the BUN (blood urea nitrogen) and CREA (creatinine). Of these two values, creatinine is a more sensitive indicator of kidney damage. There should be a concern even if it's only slightly elevated.
AMYL (amylase) and LIP (lipase) are enzymes produced by the pancreas
The proportion of different types of blood cells and the presence of proteins called antibodies (which are produced as part of the body's defence against disease) tell how well a pet is fighting a disease.
Blood samples are usually drawn from a vein in the leg or neck using a hypodermic needle and syringe. A patch of fur over the vein is shaved and the skin disinfected with surgical spirit to clean the skin and allow good visibility of the vein. A few millilitres - about a teaspoon - of blood are put into special containers to prevent it from clotting. Blood sampling is not painful although some animals don't like being held whilst the sample is taken. In animals with delicate skin or animals that struggle when the sample is being taken, some bruising may occur. The puncture hole heals quickly unless the animal has a blood disorder that prevents the blood from clotting.
Blood tests can reveal if an animal is anaemic or whether its liver and kidneys are working properly.
2) Urine tests:
Urine tests are carried out to check for diseases such as diabetes or cystitis (inflammation of the urinary bladder). Urine is checked to see if it contains proteins, sugar or signs of infection. Urine samples are collected by catching a few drops of urine in a thoroughly cleaned container under the animal as it empties its bladder. The sample is kept in a sealed bottle inside a refrigerator and tested as soon as possible. When it is not possible to wait for a naturally produced urine sample, the veterinary surgeon may collect one using a catheter (a special tube), passed directly into the bladder through the urethra, or using a needle inserted into the bladder through the skin over the belly. It is possible to collect samples in this way without sedating the animal and these techniques are no more complicated or dangerous than taking a blood sample.
3) Faeces (stool, droppings):
Small samples of faeces are often used to help identify diseases of the digestive system.
The sample may be tested to either see if any unusual bacteria are growing, indicating an infection in the intestines or if the animal is unable to digest certain foods or if its faeces contain eggs from parasitic worms.
An animal's eyes, ears and nose or skin can often become infected with disease-causing bacteria, viruses or fungi. Swabs are taken by gently rubbing the affected area with a small piece of cotton wool. The swab is then either transferred onto a glass slide for examination under a microscope or cultured and tested to see if bacteria can be grown. The results of a culture test may take a few weeks or longer, in some cases of slow-growing bugs.
5) Skin scrapings:
A cat's or dog's skin disease can be tested to see if they are infected with parasitic mites. The skin is scraped gently with the edge of a scalpel blade until bleeding occurs. This may cause minor discomfort to some dogs although others tolerate it fairly well.
There are usually only small numbers of mites and a large number of scrapings may have to be taken from several areas before finding them. The skin sample is transferred onto a glass slide and examined under a microscope.
6) Tissue biopsies:
If a dog has a growth or a lump on its body, it is normal to take a tissue sample, a biopsy, which involves removing a small part of the lump which is then examined under a microscope to see what sort of cells it contains.
Cell samples may also be collected by putting a needle into the lump and sucking out some cells.
Fluid samples may be taken from the airways via a tube placed in the throat, or the digestive system via an endoscope passed into the stomach. In this way, the veterinary surgeon obtains more information without performing a full operation on your dog.
How many tests are needed to diagnose a disease?
An instant diagnosis is not possible because of the many diseases and disease-causing organisms. Several tests may have to be performed for a diagnosis to be reached. Whereas in some instances a disease can be confirmed with one test, in others several tests have to be done on one or more tissues or body fluids. In other instances, repeat tests may need to be performed over a period of time before a confirmed diagnosis is reached, e.g., monitoring changes in antibody levels in the blood over a period of time.
NB: Diagnostic tests are performed on companion animals, or samples from them, to help in the diagnosis and provision of the best possible care. If unsure of what a test involves or why the veterinarian needs to do it, please always ask for a more detailed explanation.
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