A new antemortem test for feline infectious peritonitis correctly classified 78% of cases in an 84-cat clinical trial — the highest accuracy of four assays compared by Colorado State University researchers.

The multiplex fluorescent immunocytochemistry (MF-ICC) assay detected feline coronavirus antigen inside macrophages, the cells central to systemic FIP. Across 58 cats with confirmed FIP and 26 controls with other diseases, MF-ICC produced 77% sensitivity, 81% specificity, a 92% positive predictive value, and a 53% negative predictive value. RT-PCR reached 69% accuracy, serology 75%, and the serum albumin-to-globulin ratio 76%.

The findings appear in Veterinary Clinics of North America: Small Animal Practice (Epub ahead of print, July 30, 2026; PMID 42532773).

Why did a single positive cell matter?

Lead author Samantha J.M. Evans, DVM, PhD, DACVP, DACVM, associate professor of clinical pathology at CSU, said the smallest clinically meaningful finding was one dual-positive cell.

"Literally a single virus-infected cell was enough information to discriminate FIP versus non-FIP disease," Evans said. "It was surprising to me that little material was clinically significant."

What makes the assay different?

MF-ICC uses fluorescent antibodies to detect both feline coronavirus antigen and vimentin, a cellular marker that identifies macrophages. Images from separate wavelengths are superimposed to confirm both markers sit inside the same cell. Antigen inside macrophage-like cells points to systemic disease rather than routine enteric coronavirus exposure.

• Sample types: effusion fluid or needle aspirates from affected tissue
• Anesthesia: frequently avoidable
• Comparison targets: RT-PCR (viral RNA), serology (antibody response), albumin-to-globulin ratio (biochemical shift)
• Tissue confirmation: histopathology with immunohistochemistry, often requiring biopsy

Why does diagnostic speed matter now?

Before effective antivirals, an uncertain FIP diagnosis often ended in a euthanasia discussion. Today, a wrong or delayed answer can start costly treatment in cats that do not have FIP.

Co-author Petra Černá, PhD, DACVIM (SAIM), Dipl. ECVIM-CA, DABVP (Feline), MANZCVS, an assistant professor of small animal internal medicine at the University of Georgia, said she regularly sees empirically treated cats that never had the disease.

"It is very important to have access to fast and accurate diagnostic tests so we can confidently treat these cats," Černá said. "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."

Co-author Benjamin Curtis, DVM, DACVP, clinical assistant professor with the University of Michigan's Unit for Laboratory Animal Medicine Pathology Core, added that severely affected cats may have days — not weeks — for clinicians to act.

"Having something with a rapid turnaround is going to be life and death for a lot of cats," Curtis said.

How should practitioners use the result?

Evans said a positive MF-ICC result strongly supports FIP, but a negative result does not rule it out. Sample type, disease distribution, and patient phenotype remain critical — neurologic or ocular disease may concentrate virus in hard-to-sample sites.

The assays did not always identify the same cats. MF-ICC picked up some FIP cases PCR missed, and vice versa.

"There is value to ordering both," Evans said. "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."

For general practice, Evans suggested MF-ICC when:

• Cytology samples have already been collected
• No material was retained for PCR
• PCR is negative despite continuing clinical suspicion of FIP

Where can practitioners access the test?

MF-ICC is not an in-clinic assay. It requires specialized equipment to concentrate cells, fluorescent reagents, and pathologist expertise. CSU plans to offer it through its Veterinary Diagnostic Laboratory and evaluate field performance as cases accumulate. The published method is not proprietary, so other academic or commercial labs with the right equipment and antibodies can establish it.

Evans expects MF-ICC to anchor a broader FIP diagnostic package that also includes PCR and the albumin-to-globulin ratio. Her group is also building a machine-learning tool based on routine CBC and serum biochemistry findings.

"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 said.

What FIP questions remain?

Černá said research must still address cats that fail to respond to antiviral therapy, including those with severe systemic inflammation, suspected viral sepsis, immune-mediated hemolytic anemia, myocarditis, or coinfections. Long-term follow-up will also be needed, as some recovered cats later develop gastrointestinal disease and, in reported cases, large-cell lymphoma — outcomes that could not be studied when FIP was almost always fatal.

"There's no perfect test for FIP," Evans said. "One piece is PCR, one is MF-ICC, and this new test is one part of putting together that puzzle."