Veterinary laboratory tests, part 1

Our pets and farm animals get checked by the vet every once in a while. Often the vet needs a sample to test. For us owners the samples are usually classified by the substance needed: there are blood test, tests from urine samples, test from stool samples etc. It's easy to understand and definitely clear enough for everyone involved.

Veterinarians and laboratories, however, use a bit different kind of classification. For their purposes it provides more clarity. Some common types of tests are 

• Hematology
• Chemistry
• Urinalysis
• Cytology and histology

Tests are often run in "panels", meaning a group of tests which are often requested together. Instead of separately requesting for a test for all liver-related values, for example, the vet can just order a "liver panel". The panels available differ per laboratory and testing company.  

Hematology

Hematology tests the different cells in the blood. It studies the life cycle, form and size (morphological features) and the amounts of these cells.  The blood to be sampled is submitted either in a test tube or as a smear on a glass. The sample should be as fresh as possible to avoid lysis (cells breaking down) and clotting. If the sample is shipped to an external laboratory correct packaging and shipping are vital to preserve the sample in good quality. 

The tests are further classified by what cells are being studied. Red blood cells, white blood cells and platelets all have their own functions in the body. For red blood cells we can for example test for hematocrit (HCT), which is calculated from the amount of red blood cells and the mean cell volume. We can also calculate reticulocyte percentage to see how much of the red blood cells are immature. A high number of immature cells could indicate blood loss by bleeding or cell death. 

For white blood cells we calculate the total number, morphology and percentage of different types of white blood cells. The changes in these cells form a leukogram. 

Platelets help the blood clot, and are the third type of cells used in hematological tests. Similarly to red and white cells, we count the number and percentage of platelets. We can also determine their morphology. Too many platelets is called thrombocytosis (increased clotting risk), and not enough platelets is called thrombocytopenia (blood is not clotting properly). 

Chemistry

Chemistry tests urine or blood, but in a different way than hematology. These tests can be called clinical chemisty, clinical pathology of blood chemistry. Unlike hematology, chemistry is tested most often from serum or plasma. Serum and plasma are acquired by removing the cells from the blood sample. If the blood is left to clot,  serum remains. If the clotting is prevented with anticoagulant, the result is plasma. 

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Some examples of chemistry tests are electrolytes, minerals and kidney function.

Electrolytes help cells to function properly. They are carefully regulated, and any deviations can point to a health concern. Sodium concentration can be measured to estimate the the amount of fluids in the body (dehydration). Potassium is crucial to the correct working of muscles. Minerals like calcium and magnesium can reveal organ failure (the body will get lacking minerals from internal organs) or muscle weakness due to lack of calcium (milk fever in cows being a well-known example).   

To estimate kidney function a chemistry test measuring creatinine or urea nitrogen can be done. 

Urinalysis

Urinalysis is the analysis of urine. It focuses on describing the urine (color, cloudiness) and looking at the bacteria and biochemical properties (such as protein, blood and pH). All of these values describe the functioning of the kidneys and urinary tract. 

Much of basic urinalysis can be done at a clinic using a simple dipstick. It's a stick with several pads, which is dipped into the urine. Each pad reacts to a specific substance in the urine. Looking at the colors of the pads the nurse or vet can determine the basic properties of urine. 

 

Anatomically correct directions

An old Internet story claims that a 10-year old boy described a cow thus:

"A cow has seven sides: upside, downside, upper side, below side, right side, left side and inside. The head is on the upper side."

He is right, in a way, but the description could be more specific. We could say that the cow has head, body and legs.. but what about the neck? Is that a separate part, or does it belong to the head or the body? 

Also, a cow is easy since we all know how they look like. It's relatively easy to say where the head is (in the front, attached to the neck) or where the hind legs are (under the pelvic area, attached to the body). Alright, but where is the liver in relation to the spleen?  Where's the lump you felt, the one kinda close to the ribs but more to the top, if you look directly at the cow and you're about the same height as the cow? There's a wound in the intestines - where is it? 1".5 meters from the start"?

We need anatomical directions. Words that are unambiguous and describe things in relation to the animal itself. With these words we can describe precisely where something is.

Let's start with the main regions. We can divide the body to regions and cavities (hollow areas). A vertebrate has usually

• A head, with three cavities: 
• nasal cavity or rhinarium (the hollow space in the nose)
• oral cavity (mouth)
• the orbit (which contains the eyeballs)
• The neck, attached to the head
• The trunk, with three cavities: 
• the thorax or thoracic cavity (lung area)
• abdomen (stomach(s) and intestines)
• pelvis (urinary tract and reproductive organs)
• Forelegs
• Hind legs

To get more into detail several specific words are used to describe location. Note that they are not related to the animal's position. Think of the phrase "above the dog's hind legs". If the dog is standing, above the legs is the pelvis. If the dog is sitting, above the legs is his stomach!   

Caudal - Cranial

Caudal means to the direction of the tail (or rump in animals without a tail), and cranial means to the direction of the head. For example, an important blood vessel called caudal vena cava , which goes from the heart to the direction of the tail. They can also be used in the meaning "related to or located in", like cranial nerve are nerves emerging from the brain (cranial region). 

Dorsal - Ventral

Dorsal is to the direction of the back, and ventral to the direction of the stomach. Easy to remember when you think about people who "speak from their stomach" - ventriloquists! 

Distal - Proximal

Distal means moving away from the trunk. This could be to any direction, as long as it's away from the trunk. Proximal is to the direction of the trunk. The hooves of an animal are distal to their body, and the shoulders are proximal to the hooves. 

These words have no meaning on some organisms, such as starfish. For these we could use "peripheral" instead of distal - something away from the center.

Medial - Lateral

Let's again use a hind leg as the example. How do we describe something which is on one side of the leg? Right/left is difficult. Are you facing the animal, is it the animal's right/left, and and what if the animal is on it's side? 

Medial refers to "closer to the midline", i.e. inside of the leg. Lateral is away from the midline, i.e. the outside of the leg. Note that the when you face the animal, the right side of the right foreleg is lateral, but the right side of the left foreleg is medial... Hence, medial and lateral rather than right and left.

With these terms we can now say that the lump is 5 cm caudal and 10 cm proximal to the last rib. Or that the horse has soreness on the left medial forelimb, distal to the knee. Much better than "let me show you, if you squeeze here then OOW it kicked me!"

Sources:

Wikipedia https://en.wikipedia.org/wiki/Anatomical_terms_of_location

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