A cat stool sample tests primarily for intestinal parasites, including roundworms, hookworms, tapeworms, and single-celled organisms like Giardia and coccidia. Depending on the type of test your vet runs, it can also screen for bacterial infections and, in some cases, specific protozoal parasites that cause chronic diarrhea. The basic fecal exam is one of the most routine tests in veterinary medicine, but there are several tiers of testing, each catching different things.
Parasites: The Primary Target
The standard fecal exam is built to find intestinal parasites, and roundworms top the list. They’re the most common intestinal parasite in cats, affecting 25% to 75% of the feline population, with kittens being hit hardest. Adult roundworms are three to five inches long and live freely in the intestine, eating whatever your cat eats. A fecal test detects them by finding their microscopic eggs in the stool, since the worms themselves often stay inside the gut.
Hookworms are the second major target. These thread-like worms are less than half an inch long and attach to the intestinal wall, feeding on your cat’s blood. They’re too small to see in stool with the naked eye, so microscopic testing is the only reliable way to catch them. Their prevalence varies widely depending on where you live in North America.
Tapeworms are trickier. They live in the small intestine and shed mature body segments filled with eggs, which pass out in feces. You might spot these rice-like segments near your cat’s rear end before a lab does, because standard fecal flotation tests are notoriously poor at detecting tapeworm eggs. The eggs are shed intermittently and often stay locked inside those segments rather than floating freely in the sample.
Single-Celled Organisms
Beyond worms, fecal tests screen for protozoal parasites, which are microscopic single-celled organisms. Coccidia is one of the most common. Virtually all cats become infected with coccidia at some point in their lives, typically by swallowing cysts from contaminated soil. Most healthy adult cats clear the infection without symptoms, but kittens and immunocompromised cats can develop significant diarrhea.
Giardia is another protozoan your vet may test for. It’s often detected using an antigen test rather than standard microscopy, because the organisms are small and easy to miss under a microscope. Antigen-based tests work by detecting proteins the parasite sheds, which means they can identify an infection even when the organism isn’t actively visible in that particular stool sample.
Toxoplasma gondii, the parasite behind toxoplasmosis, can also be identified through fecal testing. Cats are the only animals that shed Toxoplasma eggs in their feces, which is why it gets so much attention from pregnant cat owners. Infection is fairly common but rarely causes illness in cats themselves.
Bacterial and Viral Screening
If your cat has diarrhea that doesn’t resolve or your vet suspects something beyond parasites, they may run a broader diarrhea panel. These panels add bacterial cultures to the standard fecal float, screening for pathogens like Salmonella, Campylobacter, and Yersinia. A panel from Washington State University’s diagnostic lab, for example, combines an enteric bacterial culture, a Campylobacter culture, a Giardia antigen test, and a standard fecal float into one submission.
Not all diarrhea panels include viral testing. Some specifically exclude viruses like feline enteric coronavirus, so if your vet suspects a viral cause, they may need to order separate tests.
Specialized Tests for Chronic Diarrhea
One parasite that standard fecal exams frequently miss is Tritrichomonas foetus, a protozoan that causes persistent large-bowel diarrhea, especially in young cats from multi-cat environments like shelters and catteries. A direct fecal smear catches it only about 14% of the time, and even when it does show up under the microscope, it’s easily confused with Giardia.
For this reason, PCR testing (a DNA-based method) is the preferred diagnostic tool for Tritrichomonas. Your vet sends a fresh stool sample to a reference lab, where the parasite’s genetic material is identified directly. The sample needs to be free of cat litter, since certain litter components can interfere with the test. If your cat has chronic diarrhea that hasn’t responded to Giardia treatment, Tritrichomonas is one of the first things a vet will want to rule out.
How the Testing Methods Differ
The most common fecal test is flotation. A small amount of stool is mixed with a dense salt or sugar solution, which causes parasite eggs to float to the surface where they can be collected on a glass slide and examined under a microscope. Centrifugal flotation, where the sample is spun in a machine, is more sensitive than passive flotation, which relies on gravity alone. The flotation solution needs to be maintained at a specific density (between 1.18 and 1.20 grams per milliliter) to reliably float the range of eggs found in cats. If it’s too dilute, heavier eggs sink and get missed.
Antigen testing uses a different approach. Instead of looking for eggs, it detects proteins shed by the parasite itself. This is especially useful for catching infections during periods when the parasite isn’t actively releasing eggs, a common cause of false negatives with standard flotation. Antigen tests can sometimes detect infection before eggs even appear in the stool.
PCR testing is the most sensitive option, identifying parasite DNA directly from the fecal sample. For tapeworms specifically, PCR has at least double the sensitivity of centrifugal flotation. It’s also the gold standard for Tritrichomonas. PCR costs more and requires a reference lab, so vets typically reserve it for cases where standard tests come back negative but symptoms persist.
Why a Negative Result Doesn’t Always Mean Clear
False negatives are a real limitation of fecal testing. Many parasites shed eggs intermittently rather than continuously, so a single sample might simply catch a gap between shedding periods. Tapeworms are especially prone to this because their eggs pass out in discrete body segments rather than being distributed evenly through the stool.
Sample quality matters too. Watery or diarrheic stool dilutes parasite eggs, which means a larger sample is needed to compensate. If the sample is too small, the chance of missing an infection rises substantially. For the best results, provide at least a teaspoon of feces, keep it refrigerated (not frozen), and get it to the vet within 24 hours of collection.
Young parasites also create a diagnostic blind spot. There’s a window between when a cat first becomes infected and when the parasites mature enough to produce eggs. During this pre-patent period, the infection is real but invisible to any egg-based test.
How Often Cats Should Be Tested
The Companion Animal Parasite Council recommends at least four fecal exams during a cat’s first year of life, then at least twice a year for adult cats. Cats that go outdoors, hunt, or live in multi-cat households may need more frequent screening. Even indoor cats can pick up parasites from contaminated soil tracked in on shoes or from insects they catch inside the house, so routine testing applies across the board.