There are several categories of diagnostic tests that may be performed to help your veterinarian determine the cause of your pet's disorder.
Histology is the study of the structure, function, and chemical composition of cells within the body. Experts in histology (called pathologists) examine small tissue samples to determine if they are normal or diseased. These experts can often point to a cause for the abnormal cells.
Small tissue samples will often be sent to a pathologist if your veterinarian suspects conditions such as cancer or other diseases that cause tissue changes.
Microbiology is the study of small organisms such as bacteria, viruses, fungi, and other single-celled life forms. In a veterinary laboratory, specialists in microbiology can perform any of hundreds of tests looking for signs of infection. Common tests include growing and identifying bacteria, viruses, and fungi. Bacteria can also be tested to see which antibiotics should be effective for eliminating them from the body. Other tests use antibodies to detect the presence of microorganisms in a sample from an animal. Samples commonly used to culture microorganisms include blood, urine, feces, secretions from the nose or lungs, and swabs taken from a wound or abscess.
Toxicology is the branch of science that studies how poisons affect animals and how animals respond to poisons. If your veterinarian suspects that your pet has been poisoned, samples will be collected for toxicologic tests to identify the poison and the amount of damage it may have caused. For common poisons, the tests may be performed at the clinic to quickly identify the poison. Rapid identification of a poison can be critical for your pet's survival. In other cases, samples may be sent to an outside laboratory that can accurately test for a much wider range of poisons. If your pet has eaten something toxic, your veterinarian may ask you to bring a sample of it with you for testing.
Hematology is the study of blood, its chemistry, and components. The most common blood test is a complete blood count (CBC). This test determines the number and type of white and red blood cells circulating in the bloodstream. It is most often used to check for signs of inflammation or infection (revealed by changes in the number and appearance of different kinds of white blood cells). Determining the number, size, and hemoglobin content of red blood cells helps identify disorders such as anemia. Platelets are also examined during a CBC; changes in platelets can help identify blood clotting disorders (see Diagnostic Tests and Imaging: Blood Samples).
Clinical chemistry is the study of the chemical composition of a sample. Usually, the sample is the liquid portion of blood (serum or plasma), although other body fluids may also be studied. Clinical chemistry tests are important for determining how well different organs are working. They can also help identify particular disorders, such as diabetes or pancreatitis.
Specialists in serology study the clear portions of body fluids. Most serologic tests determine the level of antibodies (called the titer) against a particular infectious microorganism. A high level of antibodies, or an increase in their level from one sample to another taken a few weeks later, shows that an animal has been exposed to the microorganism and its immune system has produced specific antibodies against the infectious agent. It used to be very cumbersome to test for specific antibodies in many different kinds of pets. Today, there are commercially available test kits for a wide range of serologic tests. The test kits are used on a regular basis by both in-house and outside laboratories to test for diseases such as heartworm disease, feline leukemia virus infection, Lyme disease, equine infectious anemia, and many others.
The study of individual cells, their structure and origin, function(s), and death is known as cytology. Specialists in this field can provide a veterinarian with information about the cells in your pet's body. In particular, specialists in cytology are often called upon to identify cancerous cells or determine whether or not a tumor is benign or cancerous (malignant). Samples of tissue or fluid are collected, then slides are prepared and stained for microscopic examination to determine the type or types of cells present.
Fluid analysis is the study of bodily fluids other than blood. Specialists in analyzing body fluids work closely with other specialists to help provide information about the health of an animal. Typically, fluid analysis includes checking the sample for cells and proteins.
Last full review/revision July 2011 by Morag G. Kerr, BVMS, BSc, PhD, Cbiol, FIBiol, MRCVS; Jimmy C. Lattimer, DVM, MS, DACVR, DACVRO; John B. Malone, DVM, PhD; Karen W. Post, DVM, MS, DACVM; Susan J. Tornquist, DVM, PhD, DACVP; Trevor J. Whitbread, BSc, BVSc, MRCVS, DECVP