CSU study adds a new diagnostic tool for suspected FIP

A new diagnostic study from Colorado State University is making the case for MF-ICC, a multiplex fluorescent immunocytochemistry assay, as another practical tool for working up suspected feline infectious peritonitis. The assay is designed to detect feline coronavirus antigen specifically within macrophages, the cell type most associated with systemic FIP, using samples such as effusion fluid or fine-needle aspirates rather than surgical biopsy. In a field-based clinical trial published in the Journal of Feline Medicine and Surgery on April 14, 2026, investigators reported that MF-ICC was the most accurate of the four approaches compared in that study cohort, though they were careful not to frame it as a standalone answer. (pmc.ncbi.nlm.nih.gov)

That caution reflects the long history of FIP diagnostics. The disease has been notoriously hard to confirm ante-mortem, particularly in non-effusive cases, because feline coronavirus exposure is common and many cats never develop FIP. The 2022 AAFP/EveryCat FIP diagnosis guidelines stress that clinicians should build a case using signalment, clinical findings, clinicopathologic changes, imaging, and targeted testing, rather than relying on any single assay. Histopathology with immunohistochemistry remains the reference standard in many cases, but it can require invasive tissue sampling that isn't always feasible in unstable patients. (pmc.ncbi.nlm.nih.gov)

In the new trial, researchers enrolled 84 cats, including 58 with FIP and 26 controls with other diseases. Final case classification was based on necropsy with histopathology and immunohistochemistry, response to antiviral treatment, and/or clinical follow-up. Using a threshold of at least one dual-positive cell, MF-ICC showed 77% sensitivity and 81% specificity, with a 92% positive predictive value and 53% negative predictive value. Overall accuracy was reported at 78%, compared with 76% for the albumin-to-globulin ratio, 75% for serology, and 69% for RT-PCR in this cohort. The authors described MF-ICC as a complementary test, not a replacement, and Veterinary Practice News reported that MF-ICC and PCR did not always identify the same cats, suggesting combined use may improve case detection. (pmc.ncbi.nlm.nih.gov)

The technical idea is straightforward but clinically relevant: instead of only detecting coronavirus material, MF-ICC looks for viral antigen inside macrophages by pairing coronavirus staining with vimentin, a cellular marker used to identify those cells. That matters because coronavirus positivity alone can reflect common enteric infection rather than FIP. According to the study coverage, the assay was more sensitive but less specific in effusion samples, while positive tissue aspirates from non-effusive cases may carry stronger diagnostic weight, an important point given how often dry FIP creates the biggest uncertainty in practice. (veterinarypracticenews.com)

Investigators and outside-facing commentary both underscored the clinical stakes. Samantha Evans of CSU said FIP is especially challenging to diagnose in noneffusive disease, and co-author Benjamin Curtis said the appeal is that the method is minimally invasive while still borrowing the logic of immunohistochemistry by confirming viral antigen in the relevant cells. Petra Černá of the University of Georgia added that she sees cats treated empirically for weeks despite not having FIP, raising concerns about missed alternative diagnoses and poor antiviral stewardship. Those comments align with broader 2026 clinical education on FIP, which has increasingly emphasized that timely diagnosis now matters because treatment decisions can no longer be deferred as they often were when prognosis was uniformly poor. (veterinarypracticenews.com)

Why it matters: For veterinary professionals, this is less about a breakthrough single test and more about a better diagnostic workflow. A positive MF-ICC result appears to offer strong rule-in value in the right clinical context, especially when biopsy is impractical, but its modest negative predictive value means clinicians still can't use it to confidently exclude FIP. In real-world terms, that supports a layered approach: combine history, effusion analysis or aspirates, routine lab changes, PCR, and now potentially MF-ICC when sample type and turnaround make sense. As more pet parents pursue antiviral treatment, the pressure to distinguish true FIP from lookalike inflammatory, infectious, or neoplastic disease is only increasing. (pmc.ncbi.nlm.nih.gov)

There are also operational considerations. CSU said MF-ICC is not an in-clinic assay and requires specialized slide preparation, fluorescent reagents, and pathologist interpretation. The university plans to make the assay available through its Veterinary Diagnostic Laboratory and is exploring a broader FIP diagnostic package that could combine MF-ICC, PCR, albumin-to-globulin ratio, and other patient data. Veterinary Practice News also reported that the published method is not proprietary, meaning other diagnostic laboratories with the right expertise could potentially establish the assay, which could improve access and turnaround over time. (veterinarypracticenews.com)

What to watch: The next important signals will be whether larger field use confirms the assay's performance, whether outside laboratories adopt the method, and whether MF-ICC ultimately becomes part of a standardized FIP panel rather than a niche add-on test. (veterinarypracticenews.com)