There is no single test that definitively confirms feline infectious peritonitis (FIP) in a living cat. Instead, veterinarians piece together blood work, fluid analysis, imaging, and clinical signs to build a case for or against the diagnosis. The process differs depending on whether a cat has the “wet” form (with fluid buildup) or the “dry” form (without fluid), and understanding what each test can and cannot tell you makes the diagnostic journey less confusing.
Why FIP Is So Hard to Diagnose
FIP develops when a common and usually harmless virus, feline coronavirus, mutates inside an individual cat and triggers a destructive immune response. The challenge is that many healthy cats carry feline coronavirus without ever developing FIP. A positive coronavirus antibody test only confirms exposure to the virus, not that the virus has mutated into the form that causes FIP. Plenty of cats in multi-cat households will test positive for coronavirus antibodies and never get sick.
Because of this overlap, vets rely on a combination of findings rather than any one result. A cat with a high coronavirus antibody titer, a fever, and weight loss might have FIP, or it might have something else entirely. The diagnostic approach works by layering evidence: each test narrows the possibilities until FIP becomes the most likely explanation.
Blood Work: The First Clues
Routine blood tests are typically the starting point. Two patterns raise suspicion for FIP. The first is a drop in albumin (a blood protein made by the liver) combined with a rise in globulins (immune-related proteins). The ratio between these two values, called the albumin-to-globulin ratio, is one of the most useful screening numbers. A ratio below 0.4 suggests FIP, while a ratio above 0.6 to 0.8 makes the diagnosis highly unlikely.
The second pattern involves markers of inflammation. Cats with FIP often have elevated white blood cell counts (particularly neutrophils), anemia, and high levels of certain inflammatory proteins. None of these results are unique to FIP, but when several appear together in a young cat with a persistent fever that doesn’t respond to antibiotics, the picture starts to come into focus.
Testing Fluid in Wet FIP
The wet form of FIP causes sticky, straw-colored fluid to accumulate in the abdomen, chest, or both. When this fluid is present, it becomes the single most valuable diagnostic material available. A vet can collect a sample with a needle and run several tests on it.
FIP effusion has distinctive properties. The protein concentration is high (above 35 g/L), yet the cell count is surprisingly low. Under a microscope, the fluid shows a thick, protein-rich background with a mix of immune cells, primarily macrophages and neutrophils, but very few lymphocytes. This pattern of inflammation, called pyogranulomatous, is characteristic of FIP and different from what you’d see with bacterial infections or cancer.
The Rivalta Test
One simple, low-cost test that can be performed right in the clinic is the Rivalta test. A drop of the cat’s fluid is added to a tube of diluted acetic acid. If the drop forms a visible clump or strand that holds together as it sinks, the test is positive. If it dissipates, it’s negative.
The Rivalta test is better at ruling FIP out than confirming it. A negative result means FIP is unlikely as the cause of the effusion, with a negative predictive value of about 93%. A positive result is less conclusive on its own, since other inflammatory conditions can also produce a positive reaction. The test works best on clear fluid samples and is most accurate in cats under two years of age.
PCR Testing on Fluid
A more advanced option is sending the fluid for PCR (polymerase chain reaction) testing, which detects genetic material from feline coronavirus. A positive PCR result from effusion fluid is strong evidence for FIP, especially when combined with the typical fluid characteristics described above. A negative result doesn’t completely rule it out, though, because the virus isn’t always present in high enough quantities to detect.
Diagnosing Dry FIP
The dry form of FIP is considerably harder to diagnose because there’s no fluid to analyze. Instead, it causes granulomatous lesions (clusters of inflammatory tissue) in organs like the kidneys, liver, brain, and eyes. Vets have to rely more heavily on clinical signs, imaging, and sometimes biopsy.
Eye and Neurological Signs
Ocular involvement is one of the more recognizable features of dry FIP. The earliest sign is typically uveitis, an inflammation inside the eye that can cause a change in iris color, cloudiness, debris floating in the front of the eye, and pupils that become unequal in size. Some cats also develop retinitis, visible as areas of inflammation and tiny hemorrhages in the back of the eye. In severe cases, glaucoma or full-blown eye inflammation can develop, sometimes requiring removal of the eye.
Neurological FIP produces signs that depend on where in the brain or spinal cord the inflammation settles. Seizures, difficulty walking, head tilting, and behavioral changes are all possible. MRI scans in cats with neurological FIP consistently show abnormalities: inflammation along the membranes covering the brain, enlarged fluid-filled spaces in the brain (hydrocephalus), and in some cases herniation of the cerebellum, where brain tissue pushes through the opening at the base of the skull. These imaging findings, combined with bloodwork patterns and clinical signs, can make a strong case for FIP even without fluid to test.
Tissue Biopsy
The most definitive way to confirm dry FIP in a living cat is through biopsy of affected tissue, typically from the kidney, liver, or lymph nodes. A pathologist can identify the characteristic inflammatory pattern and use a staining technique called immunohistochemistry to detect viral protein inside the damaged tissue. This is as close to a gold-standard diagnosis as currently exists, but it requires a surgical or ultrasound-guided biopsy, which isn’t always practical depending on the cat’s condition.
Coronavirus Antibody Titers: What They Actually Mean
Many cat owners encounter coronavirus antibody testing early in the process and find the results confusing. A high titer means the cat’s immune system has produced a strong antibody response to feline coronavirus. It does not mean the cat has FIP. Up to 40% of cats in single-cat households and up to 90% in multi-cat environments carry feline coronavirus, and the vast majority never develop FIP.
A very high titer in a sick cat with other suggestive findings adds another piece to the puzzle. A high titer in an otherwise healthy cat is essentially meaningless for FIP diagnosis. Conversely, a cat with FIP can occasionally have a low or even undetectable titer, because the immune system is so overwhelmed that it stops producing antibodies effectively. This is why titer results should never be interpreted in isolation.
Putting It All Together
In practice, diagnosing FIP works like building a legal case with circumstantial evidence. A young cat (most FIP cases occur in cats under two) with a persistent fever, weight loss, and a swollen abdomen gets blood work showing a low albumin-to-globulin ratio. Fluid is drawn from the abdomen and comes back with high protein, low cell counts, and a positive Rivalta test. PCR testing on the fluid detects coronavirus. At that point, the cumulative evidence is strong enough for most veterinarians to make a confident diagnosis and begin treatment.
For dry FIP, the path is longer. Blood work, imaging, eye exams, and possibly biopsy all contribute. The diagnosis often comes together over days or weeks rather than in a single visit. If your vet suspects FIP but wants to run additional tests before committing to the diagnosis, that careful approach is appropriate given how much the tests overlap with other diseases. The goal is always to be as certain as possible, because FIP treatment is intensive and the consequences of misdiagnosis go in both directions.