Histology and cytology

 In the last post we looked at typical tests done from blood and urine. The list was by no means comprehensive, and in fact we only scratched the surface. So continue, let's look at two more sample types which are routinely examined from all kinds of animals (farm, wild and companion).

Histology / histopathology (tissues)

Histology is the study of the processes in, and the structure of, tissues. A typical sample is a piece of tissue removed during a surgery. It could be from the surface, like a lump on a paw, or from the inside of the animal, like a piece of the liver. The tissue is examined under a microscope.

Similar term is pathology or histopathology, which essentially means the same thing. There is a small nuance difference between pathology and histology, though. Histology is the study of tissue, while pathology is the study of disease process in the tissues.  Histological examination can answer questions like:

• is this tissue healthy? If not, what is the likely disease or injury causing the changes?
• describe this tissue and why is it abnormal? (for example on a malformed fetus)
• what is this unknown tissue? (typical question for those removed lumps and bumps)
• for this cancerous mass, what is the expected behavior and malignancy?
• are there signs of damage caused by parasites inside this tissue?

Once a piece of tissue is removed, it starts to change very quickly. It can dry out, change shape, cells can dry or break etc. To stop these changes the tissue is fixated. The fixating agent creates bindings which stabilize the tissue in it's current form. One possibility is to soak the tissue in formalin (formaldehyde mixed with water) for several hours. 

Because the sample must be fixated and prepared more in the laboratory, getting histological results can take several days. Animal owners should also note that the results are often highly detailed and therefore incomprehensible to others than the veterinarian. 

There are plenty of educational videos available. One I like, about the general principles, is available on https://www.youtube.com/watch?v=Xu0QjQ4fbQo. A beautiful look inside a laboratory is provided by IDEXX: https://www.youtube.com/watch?v=wjbmPBQy2F4.

Cytology

Like histology, cytology also looks at cells under a microscope. However, the cytological samples are much smaller. The main difference is then the scale and samples: histology looks at tissues and organs, describing a larges mass and process, while cytology looks at tiny samples, describing the individual cells. Also, cytology can be done on Cytology can answer to questions like

• Are there abnormal cells visible in this sample? (for example abnormal red blood cells)
• Are there parasites, bacteria or other changes visible in the sample?
• Does a fluid show signs of infection, or could it be caused by physical trauma?

A typical sample is a fine needle aspirate (FNA). A thick needle is inserted into the tissue to be analyzed, causing cells to enter the hollow needle. These cells are then ejected on a small slide (object slide). A slide can also be made with blood or any body cavity fluid, such as synovial fluid or fluid from the abdomen. 

Similarly to histology, the report will be very detailed and complicated. A veterinarian is required to understand the implications of the findings.

Comparison

	Histology	Cytology
Sample	Hard and soft tissue	Fluids, cells, smears
Can analyze entire organs?	Yes	No
Invasiveness of sampling	Very invasive	Not invasive
Analysis speed	Slow, due to preparation	Fast
Main tool	Microscope	Microscope
Complexity of preparatoion	Complex	Simple

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.