Bigstock Feline infectious peritonitis (FIP) was once seen as an almost uniformly fatal diagnosis, but it can now be treated with antiviral medication. Still, veterinarians lack a single, dependable antemortem test that can confirm or exclude the disease in every patient. That uncertainty is a big deal when an answer can lead directly to treatment. “FIP is extremely diagnostically challenging, primarily in the noneffusive form,” says Samantha J.M. Evans, DVM, PhD, DACVP, DACVM, associate professor of clinical pathology at Colorado State University (CSU) and lead author of a new diagnostic study looking at FIP. “It’s much more important now to get a diagnosis, because we actually have treatment available.” In the study, Dr. Evans and collaborators evaluated multiplex fluorescent immunocytochemistry, or MF-ICC, in cats with signs compatible with FIP.1 The assay detects feline coronavirus antigen within macrophages—the cells central to systemic disease—without requiring a surgical biopsy. The test was the most accurate of the four diagnostic approaches compared in the study, but Evans is emphatic about what that finding does and does not mean. “It is not a panacea,” she says. “We don’t have a silver bullet. It’s really an amalgamation of clinical signs and diagnostic tests that all come together to build a case for FIP, rather than a single test that we’re relying on.” Looking for the virus in the right cell Feline coronavirus is common, particularly in multi-cat environments, and most exposed cats never develop FIP. That prevalence complicates tests that detect exposure without showing whether the virus is causing systemic disease. RT-PCR looks for viral RNA. Serology detects the cat’s antibody response to coronavirus. The serum albumin-to-globulin ratio can support suspicion based on biochemical changes. Histopathology paired with immunohistochemistry can provide a definitive diagnosis, but obtaining tissue may require an invasive procedure in an unstable patient. MF-ICC approaches the problem differently. It uses fluorescent antibodies to detect both feline coronavirus antigen and vimentin, a cellular marker that helps identify macrophages. Images taken at different wavelengths are superimposed to determine whether both markers are present in the same cell. “The innovation we added was multiplexing [the viral marker] with another marker for cellular antigen called vimentin,” Evans says. “That cellular marker allows us to look specifically at macrophages, which are the hallmark cell type of FIP.” Detecting coronavirus alone may reflect the widespread enteric form of the virus rather than FIP. Detecting viral antigen inside macrophage-like cells provides evidence more consistent with systemic disease. Benjamin Curtis, DVM, DACVP, clinical assistant professor with the University of Michigan’s Unit for Laboratory Animal Medicine Pathology Core and a study co-author, says the assay combines strong features of established methods. “It’s minimally invasive,” Dr. Curtis says. “Pulling fluid out of the belly or doing a needle aspirate is much less invasive than needing to do a tissue biopsy. But then it brings in the gold-standard concept of immunohistochemistry, in that we can confirm there is viral antigen inside the cells that matches the presentation of FIP.” Samples can include effusion fluid or needle aspirates from affected tissues. Collection often can be performed without general anesthesia. What the study found The clinical trial included 84 cats: 58 with FIP and 26 controls with other diseases. Researchers compared MF-ICC with RT-PCR, serology, and the serum albumin-to-globulin ratio. Final case determinations were based on necropsy with histopathology and immunohistochemistry, response to antiviral treatment, and/or clinical follow-up. Using a threshold of at least one cell positive for both markers, MF-ICC demonstrated 77 percent sensitivity, 81 percent specificity, a 92 percent positive predictive value, and a 53 percent negative predictive value. Its overall accuracy was 78 percent, compared with 76 percent for the albumin-to-globulin ratio, 75 percent for serology, and 69 percent for RT-PCR in this cohort. One finding surprised Evans: The presence of a single dual-positive cell produced the best overall accuracy. “Literally a single virus-infected cell was enough information to discriminate FIP versus non-FIP disease,” she says. “It was surprising to me that little material was clinically significant.” The results varied by sample and disease form. MF-ICC was more sensitive but less specific when performed on effusion fluid. In tissue samples from cats with non-effusive FIP, a positive result carried stronger diagnostic weight—potentially useful because these cases are often the hardest to resolve. Even so, the 78 percent figure should not be interpreted as proof MF-ICC is categorically superior to PCR, Evans says. The cohort was substantial for a veterinary trial, but not large enough to make small numerical differences definitive. The more meaningful finding may be that the assays did not always identify the same cats. “MF-ICC picked up some FIP cases that PCR did not, and PCR picked up some FIP cases that MF-ICC did not,” Evans says. “There is value to ordering both. You’ll have a greater chance of picking up an FIP case by submitting both, and the same sample type can be used for these different assays in a complementary way.” A positive MF-ICC result can strongly support an FIP diagnosis, but a negative result cannot reliably rule it out. Sample selection, disease distribution, and the patient’s phenotype remain critical. A cat with neurologic or ocular disease, for example, may have virus concentrated in sites that are difficult to sample. Treatment raises the stakes Before effective antivirals, an uncertain FIP diagnosis often led to a prognosis and euthanasia discussion. Today, uncertainty can delay lifesaving therapy, commit an owner to costly treatment for the wrong disease, or obscure another condition. Curtis says speed is especially important because severely affected cats may have days—not weeks—for the clinical team to act. “Having something with a rapid turnaround is going to be life and death for a lot of cats,” he says. “Being able to get a rapid answer lets owners and veterinarians know whether we need to start moving on treatment or make other decisions.” Petra Černá, PhD, DACVIM (SAIM), Dipl. ECVIM-CA, DABVP (Feline), MANZCVS, an assistant professor of small animal internal medicine at the University of Georgia and a study co-author, has seen cats treated empirically for weeks despite not having FIP. “It is very important to have access to fast and accurate diagnostic tests so we can confidently treat these cats,” Dr. Černá says. “Very often, I see cases that have been treated for several weeks with antiviral therapy, and they do not have FIP. That is not good antiviral stewardship, but we are also missing the actual diagnosis and are not able to help these patients.” For general practitioners, MF-ICC may be useful when cytology samples have already been collected, when no material was retained for RT-PCR, or when PCR is negative despite a clinical picture that continues to point toward FIP. “I think this test is another helpful tool in our toolbox to feel more confident about FIP diagnosis,” Černá says. “No single test is perfect, and the more possibilities we have, the better we can diagnose these cats and treat them appropriately.” From a laboratory test to a diagnostic system MF-ICC is not an in-clinic test. It requires specialized equipment to concentrate cells on slides, fluorescent reagents and pathologist expertise to interpret the staining. CSU intends to make the assay available through its Veterinary Diagnostic Laboratory and evaluate its performance as more field cases accumulate. Evans expects the assay to become part of a broader FIP diagnostic service that incorporates MF-ICC, PCR, the albumin-to-globulin ratio, and other patient data. Her group is also developing a machine-learning tool based on routine CBC and serum biochemistry findings, while CSU’s FIP biobank and digital repository could support retrospective evaluation as outcomes become available. “We’re going to offer not only MF-ICC and PCR, but a panel of tests as a package to veterinarians who are really hunting down this disease,” Evans says. “We can continually evaluate it in larger cohorts to see how it bears out, especially compared to PCR and other tests.” The published method is not proprietary, Curtis adds. Veterinary diagnostic laboratories with the appropriate equipment and expertise can obtain the antibodies and reagents described in the paper and establish the assay. Broader adoption could reduce shipping distances and improve turnaround times. The long-term goal is a rapid, minimally invasive test that delivers an answer within hours. MF-ICC is not there yet, but it moves the diagnosis toward a more usable set of antemortem evidence. The next FIP questions Improved diagnosis is only one part of a rapidly changing clinical landscape. Černá says research must also focus on cats that fail to respond as expected, particularly critically ill patients with severe systemic inflammation, suspected viral sepsis, immune-mediated hemolytic anemia, myocarditis, coinfections, and other comorbidities. “We still lose some cats,” she says. “Learning how to best manage these comorbidities to improve prognosis is crucial.” Long-term follow-up will also be needed as more treated cats survive. Černá noted some recovered patients later develop gastrointestinal disease and, in reported cases, large-cell lymphoma, raising questions about post-treatment health that could not be studied when FIP was nearly always fatal. For practitioners, the immediate message is practical: FIP is treatable, diagnosis remains evidence-based rather than test-based, and MF-ICC adds another useful piece. “There’s no perfect test for FIP,” Evans says. “One piece is PCR, one is MF-ICC, and this new test is one part of putting together that puzzle.” A graduate of the University of Miami, Keith Loria is a D.C.-based, award-winning journalist who has been writing for major publications for close to 20 years on topics as diverse as veterinary medicine, travel, and entertainment. Loria started his career with the Associated Press and has held senior editorial positions at publications focused on healthcare, sports, and technology. Reference Evans SJM. The Shifting Paradigm of Feline Infectious Peritonitis: New Diagnostics and Therapeutics. Vet Clin North Am Small Anim Pract. 2026 Jul 30:S0195-5616(26)00082-3. doi: 10.1016/j.cvsm.2026.06.003. Epub ahead of print. PMID: 42532773.
