Veterinarians perform laboratory tests on animals to evaluate their overall health and to identify potential health issues.
A urinalysis (UA)  is a chemical and visual analysis of the urine and can be done from a "free-catch" sample or an expressed sample; both of these involve collecting the urine outside of the bladder. Urinalysis samples can also be obtained by inserting a hollow tube into the bladder via the uretha. This is called catheterization.  Cystocentesis  means inserting a thin, sterile needle into the wall of the bladder and withdrawing the urine to be tested. 
Both "free-catch" and expressed urine samples may contain contaminants such as bacteria, epithelial cells or white blood cells  which might influence the results of the tests. The potential for such contaminants is greater using either of these "outside the bladder" methods than by using either catheterization or cystocentesis. A urine culture (UC)  is done by getting a sterile sample (with a needle, directly from the animal's bladder, a procedure called a cystocentesis or cysto) and plating it on a growth medium to see if any bacteria grow. If there is growth, the bacteria are tested for susceptibility to different antibiotics. 
Because diabetes affects the immune system, those with the disease are often more prone to infections of all types.  Some of them produce little to no clincal signs of being present. Doing a culture often identifies them and the antibiotic needed to cure them. 
A urinalysis can only give you limited information on an animal's infection status. If you do a free-catch sample at home and take it to the vet, and there are some bacteria are found in the UA, what does it mean? It could be from the container, or from the animal cleaning him- or herself shortly before you caught the sample, and so forth. It doesn't tell you whether or not there is an infection because bacteria could be found with or without an infection being present. You may be able to use a UA to confirm a very likely urinary tract infection (UTI) but not to diagnose an occult (hidden) one. Free-catch urine samples are not preferred for culture purposes because of the bacterial possibilities and the inability to identify whether they originated from the actual urinary system or not.
Animals on steroids (like prednisone) and diabetic animals should have periodic UCs to catch possible UTIs. For the diabetics, it is strongly recommended for those animals who are not regulated and have prolonged glycosuria. Animals on steroids are immunosuppressed and more susceptible to a UTI. Those animals should get a cysto and UC several times a year just to make sure. Bladder infections can travel to the kidneys and damage them irreparably, so for an animal at high risk for a UTI, being more aggressive is recommended. 
For diabetic animals that are proving difficult to regulate, a vet check for a physiologic reason for insulin resistance should include a UC. Your vet may also recommend a UC when the animal seems "off" but there is no clear reason. If you brought in a urine sample and it showed an enormous amount of bacteria and a ton of white blood cells (WBCs), there's a strong likelihood that your animal has a UTI and your veterinarian may put him or her on antibiotics in that case. But what if there isn't? What if there is just a trace of bacteria? It doesn't tell you whether there is an infection. You'd still need to do a UC to rule out an infection so all you've done by submitting a UA and waiting for results is just waste time. It’s possible that UA results are dismissed as not probative for a UTI but the animal does indeed have a UTI and treatment is delayed. Another benefit of the UC is that it tests a bacteria's susceptibility to antibiotics. For an infection that has the potential to go to the kidneys, it's great to know for sure that you're eliminating it as quickly and as effectively as possible.
The UC never replaces the UA -- there is information gained from the UA like other types of cells present, crystals, amount of glucose, and so forth -- but it is a valuable laboratory diagnostic tool.
Lastly, it’s true that there is more expense and stress on the animal when you must in bring them to the vet for a cysto. But the situations in which you're likely to order a UC are biannual checkups or full-on physicals to investigate insulin resistance or possible illness anyway, so the animal would need to come in for an office visit to begin with.
Vets can test feces in-house or send the sample to a lab where it is done by another method. There is hard evidence that the lab screening is more likely to find parasites than the in-house float and direct smear, so if parasites are suspected, have the sample sent out. In some cases, your vet may want to do a direct smear to see what types of bacteria are present in the stool. Another type of fecal test is a snap test  for giardia.  This can be done in-house if your vet has the special kit for it, otherwise it is sent out. Giardia is very difficult to find in a fecal so the snap kit has been really useful. 
Typical abnormal findings in a fecal include parasites such as roundworms,  whipworms,  hookworms,  coccidia,  and giardia. Coccidia and giardia are hard to find using in-house methods and tapeworm  is extremely rarely seen. The significance of this is that if you think your animal may have had exposure to these--coccidia from cats or going outdoors, giardia from pond or lake water, tapeworm from fleas--your vet may decide to act based on symptoms rather than testing. The exception is giardia--you should try the snap test. But yes, you should still test. Dogs who have contact with other dogs, and especially those who frequent areas high in “dog-traffic” like training rings and dog runs, should have a yearly fecal test.
Many gastrointestinal disorders which are severe can be diagnosed by measuring the protein loss in stool. The definitive way to measure the loss is a complex test that involves exposing the patient to radiation. A less-complicated test that is able to perform a servicable estimate of the possible protein loss is called Fecal α1-Proteinase Inhibitor Concentration. At the present time, the only provider of this test is Texas A & M University.
Blood is often send to an outside lab for testing. A veterinary lab commonly used in the United States is Antech Diagnostics. 
When you are going to have your pet's blood tested, unless otherwise directed, withhold its food (not water) for 8-12 hours before the blood is taken. If your pet has any special needs, consult your vet about how much time s/he should fast. Please note that many lab tests are not diagnostic when seen alone, but together with other lab work and evaluation of symptoms, a diagnosis is reached.
Blood Chemistry Panel/Complete Blood CountEdit
The general health screen using blood work is a blood chemistry panel (Chem) and a complete blood count (CBC).  The CBC has standard components; the chemistry panel will vary in the number of things it screens for. For young healthy animals, you may only check liver and kidney values to make sure they are normal. The regular chem panel is called a chem; the extended one is called a superchem. For older animals, (7 and up is considered “senior”) a chem/CBC should be run at least once a year. This may detect problems before an animal becomes symptomatic. Cats are especially good at seeming ok until things reach a dire stage. You should also consider doing a Chem/CBC on your young, healthy animals before they are 5 years old to get a baseline. For example, your baseline may show that one of your animal’s liver values in his general screen tests abnormally low. That's fine, high liver values are the bad guys. But the benefit of having done this screen is that if this value jumps to the high-normal end, you and your vet will know that is meaningful given that your animal consistently tested quite low on that particular value while s/he was healthy.
Starting with the CBC: The CBC is an evaluation of the cellular components of the blood. There are standard normal ranges for both red blood cells (RBCs) and white blood cells (WBCs). Even when we aren't fighting an infection, we have a "standing army" of WBCs available to respond to invaders. The army is divided into groups based on specialties (like neutrophils and eosinophils); these groups are normally found in certain proportions. Looking at the WBC count gives us information on infectious and inflammatory processes that are occurring in the patient; in addition, autoimmune disease and chronic disease can both affect these numbers.
The CBC also measures RBCs and evaluates components of RBCs like hemoglobin. This multi-faceted look at the RBCs can help diagnose different types of anemia and other disorders. After a machine counts the blood cells in the sample, a lab worker makes a slide of the blood and looks at it to see if there are any abnormalities. Abnormalities might include too many immature RBCs or WBCs (depending on type) or parasites. 
The chemistry panel:  This is a screening test for chemical components normally found in the blood. Again, there is a set range of normal values and there will be a certain normal range for every individual (see the value in having a baseline done?). This panel checks for lots of things including metabolic and endocrine abnormalities (like diabetes), organ damage, inflammation, efficiency, and levels of electrolytes. Metabolic processes are those things which a body has to do to sustain life like gain energy, excrete waste, replace old/dying cells. Endocrine has to do with the hormonal system. Electrolytes are extremely important. They are charged bits of chemical elements having interactions with cells (through guiding the flow of water in and out of cells) that are essential for the muscles (including the heart, which is a muscle) and nervous system to work. Chem results are reported by the lab with actual value, and the range for normal values. Normal range can vary among the labs. Normal ranges also vary depending on the animal being tested, as shown on link below. 
This is a test that is done when anemia (low hematocrit value) is seen in the CBC. Reticulocytes are immature red blood cells. When there is acute anemia, the animal's body tries to maintain the oxygen-carrying capacity of the blood by releasing immature blood cells. A high reticulocyte count means that the animal is trying to get back to a normal RBC count. A low/normal reticulocyte count means that the anemic state may be chronic; the body has adjusted to the presence of fewer RBCs and isn't in emergency mode.
TLI, B12, FolateEdit
This test is actually for three separate things but labs often group them together as part of an intestinal profile. These tests are recommended for animals with a history of intestinal disease. The trypsin-like immunoreactivity (TLI) test gives info about the state of pancreatic function and the severity small intestine disease. An elevated TLI indicates (but is not diagnostic for) either or both pancreatitis and chronic small intestine disease. A decreased TLI indicates something called "Exocrine Pancreatic Insufficiency" (EPI) which is extremely rare in cats but sometimes seen in dogs, especially German Shepherds. See the further discussion of the TLI test at the Antech link below. 
Vitamin B12 and folate are both vitamins that must be adsorbed in the intestines. (Cobalamin is another name for B12.) In cases of chronic intestinal disease, the intestinal walls may not be able to adsorb these two vitamins properly. They are both essential vitamins and deficiencies in either one can cause problems. A low result of either one warrants supplementation. On its own, though, it is also not diagnostic for intestinal disease.
This test group is usually run in cases of suspected intestinal problems like Irritable Bowel Disease (IBD) or small intestine bacterial overgrowth (SIBO) and when pancreatitis is suspected. The pancreas is easily affected by inflammation in the gut and the liver and this may shed some light on the origin of the inflammation.
Antech has more information  about this test group.
The pancreatic lipase immunoreactivity (PLI) test is the definitive test for pancreatitis in cats and dogs. There are separate tests for canines (cPLI)  and felines (fPLI). The test looks for lipase that is specifically from the pancreas; in normal chemistry panels the lipase recorded may be from organs other than the pancreas. Remember what pancreatitis means, though: inflammation of the pancreas. In dogs, there is a strong case between diet (especially dietary indiscretions) and acute pancreatitis.
See the futher discussion at the article on pancreatitis.
The serum fructosamine test measures a protein complex found in the blood that's a reflection of blood glucose (BG) levels of the past 2 weeks (or so). This is a useful diagnostic test in cats as they are extremely prone to "stress hyperglycemia"; many non-diabetic cats will show BGs up to and over 200 when their blood is taken at the vet. 
Whether or not fructosamine has any value in pets whose BG levels are tested at home is debatable. It does have a place if you are not doing or are unable to do curves. The fructosamine gives you an average: let's say your pet is a bit fractious and you struggle to get your pre-shot tests done twice a day. You're getting great numbers and you think all is well. You should have a nice, low-end fructosamine, right? You might, but if your pet is skyrocketing in between shots the fructosamine will show you that all is not well. That's why spot-checks at home are important, too.
See also the separate article on fructosamine.
Toxoplasmosis IgG, IgMEdit
Toxoplasmosis is a protozoa most famous for being found in cat excrement. This is the reason that pregnant women are told not to clean the litterbox; toxoplasmosis can be transmitted to humans through the feces and cause birth defects. Note that you have to have direct (hand to mouth) contact with toxoplasmosis to contract it. Humans are much more likely to contract it from poorly cooked meat. It is a titer test --the IgG and IgM refer to the antibodies screened for in the titer.
Toxoplasmosis rarely affects dogs.
Cryptococcus is a fungus that may be shed in bird feces. In dogs, the symptoms are usually neurological and the prognosis may be guarded.
Though not seen very frequently on the east coast of the US, cryptococcus does appear throughout the country. If you have an animal exhibiting a cold that won't go away or a sudden onset of neurological symptoms, consider testing for cryptococcus. Many vets aren't used to hearing about it or testing for it so you may have to suggest this. It's thought that the fungus is more difficult to get rid of once it has moved onto the CNS so if you have reason to suspect it, go ahead and test. You needn’t do it as a general test on every stray you find with an upper respiratory infection (URI), but should would consider it if the URI was unresponsive to treatment or if there were other respiratory symptoms. See Merck  for an overview.
Please note that there is an intestinal protozoan parasite called cryptosporidium. This is related to coccidia, mentioned above. It is not related to cryptococcus.
Heartworm  is transmitted by mosquitoes. Dogs seem to be the best host of heartworm and are the most-frequently affected animal. Many vets are now testing cats for heartworm, a traditional disease of dogs. Research has shown that cats are more resistant to infection due to an extremely vigorous immune response but many vets think that it may be more prevalent than previously thought. In dogs, the screening test is available as an ELISA snap test and can be done in house.
Vaccine Titers for Canine Distemper and Canine ParvovirusEdit
A titer test  measures how much disease antibody is found in the bloodstream. The tests are specific per type of disease. Antibodies in the blood are evidence that the body "remembers" a pathogen and can quickly spring to action if the body is exposed to it again. This is the idea behind vaccines. Some people choose to check titers rather than re-vaccinate. Many with diabetic dogs choose titers for all vaccinations, including rabies, because vaccinating affects the immune system, which in turn can play a role in the management of the diabetes.
This combination test is a measure of how well your animal's blood clots. It is recommended if there is a suspected bleeding issue and often required by surgeons before any surgery and by ultrasonographers before doing an endoscopy-guided biopsy. Doberman Pinschers and certain other breeds of dogs have a higher incidence of a clotting disorder known as Von Willebrand’s disease. Screening for this disease is a good idea even if there is no immediate reason to.
The PT/PTT test can also tell the vet if a blood transfusion is needed (you can transfuse just the part of the blood that has clotting factors) in animals with internal bleeding.
Pre- and Post-Bile AcidsEdit
This is a set of tests used to measure liver function. The animal is boarded at the vet for the day and blood is taken from the animal after fasting and after eating. These tests look for a hepatic (liver) response to digestion. This test is called for if the regular chem panel has evidence of a decline in liver function.
PCR stands for Polymerase Chain Reaction. 
PCR testing is non-invasive - often only requiring the saliva of the animal. And it can be achieved directly [no vet required] though companies like Zoologix. PCR testing can be a quick, non-invasive, affordable and accurate way to test for the presence of many unique pathogens. 
False Test ResultsEdit
- ↑ Urinalysis-Getting the Most From a Wee Sample. NAVC (2005).
- ↑ Interactive urinalysis information. Cornell University.
- ↑ Catheterization. Washington State University.
- ↑ Cystocentesis-. Washington State University.
- ↑ Brooks, Wendy C.. Urinary Tract Infections. Veterinary Partner.
- ↑ 6.0 6.1 6.2 6.3 Daniels, Joshua B., Chew, Dennis J. (2011). Diagnosis and Treatment of Routine and Difficult Urinary Infections in Dogs. Western Veterinary Conference.
- ↑ Sine, Cheryl S. et. al. (2003). Urinalysis Dipstick Interpretation. University of Georgia.
- ↑ Wallace, Melissa A. (2001). Those Stubborn UTIs: Diagnostic and Therapeutic Techniques. Atlantic Coast Veterinary Conference.
- ↑ Bacteria in Urine. Cornell University.
- ↑ Introduction to Urinary System. Merck Veterinary Manual.
- ↑ Brooks, Wendy C.. Urinary Tract Infactions-Urine Culture. Veterinary Partner.
- ↑ Cultures and Sensitivity Testing for Infection. Pet Education.
- ↑ Cultures and Sensitivity Tests-What, When, Why. Pet Education.
- ↑ Understanding Susceptibility. Lab Tests Online.
- ↑ Diabetes Mellitus-Clinical Findings. Merck Veterinary Manual.
- ↑ 16.0 16.1 Bartges, Joe (2011). Urine Heaven: Of Bugs and Drugs-Urinary Tract Infections. Western Veterinary Conference.
- ↑ Understanding Urine Cultures. Lab Tests Online.
- ↑ Hormonal Therapy. Merck Veterinary Manual.
- ↑ ELISA Snap Test-Giardia. IDEXX.
- ↑ What Is Giardia?. Pet Center.
- ↑ Tams, Todd R. (2001). Diarrhea Caused by Giardia and Clostridium Perfringens Enterotoxicosis. Atlantic Coast Veterinary Conference.
- ↑ Roundworms. Marvistavet.
- ↑ Whipworms. Marvistavet.
- ↑ Hookworms. Marvistavet.
- ↑ Coccidia. Marvistavet.
- ↑ Tapeworms. Marvistavet.
- ↑ 27.0 27.1 27.2 Steiner, Jörg M. (2010). Helpful Laboratory Tests in the Diagnosis of Gastrointestinal Disorders. WSAVA.
- ↑ Alpha1-Proteinase Inhibitor (α1-PI) test information. Texas A & M University.
- ↑ Antech Diagnostics Website. Antech Diagnostics.
- ↑ Interactive CBC Information. Cornell University.
- ↑ Red Blood Cell Count-CBC. Cornell University.
- ↑ White Blood Cell Count. Cornell University.
- ↑ Interactive Blood Chemistry information. Cornell University.
- ↑ Wortinger, Ann (February 2001). Electrolytes, Fluids and the Acid-Base Balance. Veterinary Technician.
- ↑ Blood Test Ranges. Thepetcenter.com.
- ↑ TLI Test Information. Texas A & M University-Gastrointestinal Laboratory.
- ↑ TLI Test Information. ProVet UK.
- ↑ 38.0 38.1 Steiner, Jörg M. Pancreatitis in Dogs and Cats. Texas A & M University.
- ↑ Trypsin-Like Immunoreactivity (TLI) Test. Antech Diagnostics.
- ↑ Testing for Gastrointestinal/Liver/Pancreas Problems. Antech.
- ↑ Canine Pancreas-Specific Lipase Spec cPL™ Test. IDEXX Laboratories.
- ↑ PLI Test Information. Texas A & M University-Gastrointestinal Laboratory.
- ↑ 43.0 43.1 Variables affecting chemistry results. Cornell University.
- ↑ titer test. CaberFeidh.com.
- ↑ Cryptococcus. Merck Veterinary Manual.
- ↑ Heartworm. Marvistavet.
- ↑ Titer Test Information.
- ↑ Ford, Richard B.. Antibody Titers vs. Annual Vaccination. North Carolina State University.
- ↑ Hypothyroidism. Long Beach Animal Hospital.
- ↑ Webster, Cynthia R. L., Cooper, Johnanna C., (August 2008). Diagnostic Utility of Bile Acid Determination in Dogs. NAVC.
- ↑ PCR. Wikipedia.
- ↑ Zoologix-PCR Testing. Zoologix.
- Pet Owners Guide to Laboratory Testing Antech Diagnostics
- Laboratory Data in Geriatric Dogs and Cats Antech Diagnostics
- Blood Cells, and CBC Information in Animals Drs. Foster & Smith
- Chemistry Panels and Other Tests for Pets Drs. Foster & Smith
- What Do Those Words Mean-Diagnostics, Tests, & Procedures A-D Petplace.com
- What Do Those Words Mean-Diagnostics, Tests, & Procedures E-L Petplace.com
- What Do Those Words Mean-Diagnostics, Tests, & Procedures M-Z Petplace.com
- What Do Those Abbreviations Mean-Medical Terms A-H Petplace.com
- What Do Those Abbreviations Mean-Medical Terms I-Z Petplace.com
- Urinalysis Dipstick Interpretation Sine, Cheryl S., et. al., 2003, University of Georgia
- Clinical Lab Medicine-Harvesting The Gold: Interpretation and Techniques of Urinalysis Taylor, Judith A., November 2006, District of Columbia Academy of Veterinary Medicine
- Urine Chemistries Merck Veterinary Manual
- Urine strips:Maximizing the Diagnostic Value Osborne, Carl A., April 2009, DVMNews
- Preanalytical Variables in Urine Testing BD Diabetes 2006
- Serum Biochemical Reference Ranges Merck Veterinary Manual
- Hematological (Blood) Reference Ranges Merck Veterinary Manual
- Urine Volume and Specific Gravity Merck Veterinary Manual
- Laboratory Tests-Understanding Bloodwork PetsHealth
- Gastrointestinal Testing Antech Diagnostics
- Lab Tests Which Require Fasting Antech Diagnostics
- Routine Urinalysis Cornell University
- CBC (Complete Blood Count) Basics Cornell University
- Blood Chemistry Panel Basics Cornell University
- Infectious Agents in Urine Cornell University
- Laboratory Tests-Understanding Your Pet's Bloodwork PetsHealth
- Common Hematology Terms and Abnormalities Carmichael Torrance Labs
- Urine Sample Information Carmichael Torrance Labs
- Helpful Laboratory Tests in the Diagnosis of Gastrointestinal Disorders Steiner, Jörg M. 2010, WSAVA
- Diagnostic Approach to Hepatobiliary Diseases in Dogs and Cats Verbergh-Hoffman, Gaby, 2010, WSAVA
- Why Should We Care About Proteinuria? Syme, Harriet M., 2010, WSAVA
- Infectious Diseases and the Kidney Goldstein, Richard E., 2010, WSAVA
- Canine Hyperadrenocorticism, Diabetes Mellitus, or Both? Zwicker, Kirsten, et. al., 2003, University of Georgia
- Diagnosis and Management of Large Intestinal Disorders in Dogs Tams, Todd R. 2001, Atlantic Coast Veterinary Conference