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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteFeline infectious peritonitis (FIP) is diagnosed by combining the cat’s clinical picture with several complementary tests, not by relying on one perfect result. Antiviral treatment has made an accurate, timely diagnosis more consequential, but no antemortem test can confirm or exclude FIP in every cat. A newer method, multiplex fluorescent immunocytochemistry (MF-ICC), looks for coronavirus antigen inside macrophages and can add useful evidence, especially when interpreted alongside fluid or tissue findings, imaging, blood work and follow-up.
How is FIP diagnosed?
FIP develops when feline coronavirus infection is associated with a systemic inflammatory disease. Coronavirus exposure is common, particularly in multi-cat settings, while only a minority of exposed cats develop FIP. That distinction is why a positive coronavirus result by itself does not establish systemic FIP.
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Veterinarians generally build the diagnosis in stages:
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- Assess the pattern of illness. The history and examination establish whether the cat has features compatible with effusive (“wet”) FIP, non-effusive (“dry”) FIP, or neurologic or ocular involvement. Persistent fever, weight loss, poor appetite, fluid accumulation, inflammatory eye disease or neurologic abnormalities can all change which samples are most useful, but none is diagnostic alone.
- Characterize fluid, masses or affected organs. If fluid is present in the abdomen, chest or around other organs, it may be collected for cytology and laboratory testing. A mass or enlarged organ may be sampled with a needle aspirate. These procedures often avoid general anesthesia, although the safest approach depends on the cat’s condition and the site being sampled.
- Combine targeted tests. Depending on the presentation, the veterinarian may request MF-ICC, reverse-transcription polymerase chain reaction (RT-PCR), serology, serum albumin-to-globulin (A:G) ratio testing, cytology, imaging, or tissue examination.
- Reconcile the results with clinical progress. A result that supports FIP should fit the cat’s signs and sample findings. If results conflict, the clinician may repeat sampling, test a different site, investigate another disease, or monitor the response to treatment and subsequent clinical course.
The strongest possible confirmation is demonstration of characteristic lesions with histopathology and feline-coronavirus immunohistochemistry. That combination can require invasive tissue collection, and it is not available as a routine, risk-free test for every living cat.
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Is there a reliable test for FIP in cats?
There is no single reliable test that works as a universal rule-in or rule-out test before death. Different tests detect different parts of the disease process, and the amount and location of virus can vary between cats. A coronavirus-positive result may reflect common enteric infection or exposure rather than the macrophage-associated systemic disease of FIP.
Samantha J.M. Evans, DVM, PhD, DACVP, DACVM, a Colorado State University associate professor and study lead, summarized the limitation plainly: “There’s no perfect test for FIP.” She also described MF-ICC as “not a panacea” and said, “We don’t have a silver bullet.” Those statements are consistent with the way the assay should be used: as an additional piece of evidence, not a replacement for clinical judgment.
What does MF-ICC test for?
MF-ICC is designed to answer a more specific question than “Is feline coronavirus present?” It detects feline-coronavirus antigen inside macrophages, the immune cells associated with systemic FIP, while simultaneously identifying those cells with the marker vimentin.
In the reported method, fluorescent reagents label viral antigen and vimentin at different wavelengths. The images are then superimposed. A cell that carries both signals is counted as a macrophage containing coronavirus antigen. That cellular context matters because detecting coronavirus without showing its presence in macrophages can represent the common intestinal form of infection.
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What samples can be used?
The method can be applied to effusion fluid or to needle aspirates from affected tissue. Cell collection often does not require general anesthesia. Sample choice is still critical: a site that does not contain infected macrophages can produce a negative result even when the cat has FIP elsewhere.
What does a laboratory need?
MF-ICC is not an in-clinic consumer test. A laboratory needs equipment to concentrate cells onto slides, fluorescent reagents, appropriate microscopy and a pathologist who can interpret the dual-marker images. The method is described as non-proprietary, so a veterinary diagnostic laboratory with suitable equipment and expertise could establish it. Availability, submission rules, turnaround time and price must be confirmed directly with the laboratory; no universal service schedule has been established here.
What did the reported MF-ICC study find?
A clinical trial described by Veterinary Practice News in 2026 included 84 cats: 58 classified as having FIP and 26 control cats with other diseases. Final case determinations used necropsy with histopathology and immunohistochemistry, response to antiviral treatment, and/or clinical follow-up. Investigators compared MF-ICC with RT-PCR, serology and the serum A:G ratio.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11At the study’s threshold of at least one cell positive for both coronavirus antigen and vimentin, the reported MF-ICC results were:
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| Measure | MF-ICC result | How to read it |
|---|---|---|
| Sensitivity | 77% | Among cats classified as having FIP in this cohort, 77% had a positive MF-ICC result at the stated threshold. |
| Specificity | 81% | Among control cats with other diseases, 81% had a negative result. |
| Positive predictive value | 92% | Within this study population, 92% of positive results occurred in cats classified as having FIP. |
| Negative predictive value | 53% | Only 53% of negative results were from cats not classified as having FIP. |
| Overall accuracy | 78% | The proportion of all study cats correctly classified at that threshold. |
In the same cohort, reported overall accuracy was 76% for the A:G ratio, 75% for serology and 69% for RT-PCR. These are results from one relatively small study, not guaranteed performance in every country, laboratory, disease stage or sample type. The authors noted that 84 cats were not enough to treat small numerical differences as definitive, and the figures were reported by Veterinary Practice News from a paper described as ahead of print: Samantha J.M. Evans, “The Shifting Paradigm of Feline Infectious Peritonitis: New Diagnostics and Therapeutics,” Veterinary Clinics of North America: Small Animal Practice, published online July 30, 2026, DOI 10.1016/j.cvsm.2026.06.003.
Can a negative FIP test rule out the disease?
No. The reported MF-ICC negative predictive value was 53%, so a negative result did not reliably exclude FIP in that study. A negative result can occur when the sampled fluid or tissue contains too few infected macrophages, when disease is concentrated in another organ, or when the presentation is neurologic or ocular and the most informative site is difficult to reach.
RT-PCR can also miss cases, and MF-ICC and PCR did not always identify the same cats. That lack of perfect overlap is a reason to treat them as potentially complementary rather than interchangeable. If the clinical suspicion remains high after a negative test, the veterinarian may need to review the sample, test a different specimen, use another method or pursue follow-up rather than stopping the investigation.
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How MF-ICC compares with other FIP tests
| Approach | What it detects or measures | Typical material or procedure | Key strength | Important limitation |
|---|---|---|---|---|
| MF-ICC | Coronavirus antigen inside vimentin-positive macrophages | Effusion fluid or needle aspirate from affected tissue; specialized slide preparation and fluorescence microscopy | Provides cellular context rather than merely showing coronavirus exposure | Requires specialized laboratory expertise; a negative result cannot rule out FIP, and performance varies with sample site and disease form |
| RT-PCR | Feline-coronavirus RNA | Appropriate fluid, tissue or other submitted specimen selected by the veterinarian | Can detect viral genetic material and may add evidence not captured by MF-ICC | Virus may be present without systemic FIP; the study’s overall accuracy was 69% in this cohort, and PCR did not identify exactly the same cats as MF-ICC |
| Serology | The cat’s antibody response to coronavirus | Blood sample | Easy to obtain and useful as background evidence | Antibodies show exposure or immune response, not necessarily FIP; reported overall accuracy was 75% in the study cohort |
| Serum A:G ratio | A biochemical relationship between albumin and globulins | Blood sample | Accessible supportive information that can be interpreted with other findings | Not specific enough to establish FIP alone; reported overall accuracy was 76% in the cohort |
| Histopathology plus immunohistochemistry | Characteristic tissue lesions and coronavirus antigen in affected cells | Biopsy or necropsy tissue | Described as capable of definitive diagnosis when representative tissue is examined | May require invasive sampling or may only be available after death |
The percentages above should not be used to rank tests universally. A test’s value depends on the cat’s pre-test probability, the sample chosen, the disease distribution and the laboratory’s methods. A positive tissue result in a cat with non-effusive FIP may carry more diagnostic weight than a result from a less representative specimen, while effusion testing may be more sensitive but less specific in the reported experience.
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Why sample location and FIP form change the answer
Effusive disease
When abdominal or chest fluid is available, it is often the least invasive material to collect. The reported assay appeared more sensitive in effusion fluid, but it was also less specific there. Fluid can therefore be highly useful while still requiring confirmation against the complete clinical picture and other tests.
Non-effusive disease
Dry FIP can produce focal lesions in organs without a large volume of accessible fluid. In tissue samples from cats with non-effusive FIP, a positive MF-ICC result carried stronger diagnostic weight in the reported study. Obtaining representative tissue can be more demanding, so clinicians must balance diagnostic value against procedural risk.
Neurologic or ocular disease
Virus and inflammation may be concentrated in the nervous system or eyes, locations that are difficult or unsafe to sample. A negative result from an easier-to-collect specimen may therefore say little about an unsampled site. This is one reason a negative test should be interpreted in relation to the cat’s particular signs.
What should owners expect during a diagnostic work-up?
- Detailed history and examination: Tell the veterinarian about the duration of fever, appetite and weight changes, litter-mate health, abdominal enlargement, breathing changes, eye abnormalities, seizures, balance problems and previous treatments.
- Baseline testing and imaging: Blood work, serum proteins, imaging and fluid analysis help determine which organs are involved and which specimen is safest and most informative.
- Targeted laboratory submission: Ask which test is being ordered, what specimen it uses, where it will be analyzed and how the result will be interpreted with the other findings. MF-ICC requires a capable diagnostic laboratory rather than a home kit.
- Explicit review of uncertainty: Ask what a positive or negative result would change. If the result is negative but suspicion remains high, discuss whether another sample, another test or monitoring is appropriate.
- Reassessment: A changing clinical course can reveal that the original diagnosis was incomplete or that another disease is present. Follow-up is part of responsible diagnosis, not an admission that testing failed.
Why diagnosis matters more now that FIP is treatable
Antiviral treatment has changed FIP from an almost uniformly fatal diagnosis into a disease for which timely treatment decisions can matter. That makes uncertainty consequential in both directions. Delaying treatment in a cat that truly has FIP can forfeit valuable time, while treating a cat with another illness for weeks can be expensive, expose the patient to unnecessary medication and delay discovery of the real cause.
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For that reason, empiric antiviral treatment should remain under the treating veterinarian’s supervision, with a plan for monitoring and reconsidering the diagnosis when the response does not fit expectations. The diagnostic goal is not to obtain a reassuring single number; it is to reach the most defensible explanation while protecting the cat from avoidable procedures and inappropriate treatment.
Where is MF-ICC available?
Colorado State University has stated an intention to offer MF-ICC through its Veterinary Diagnostic Laboratory and to evaluate performance as additional field cases accumulate. A launch date, current submission instructions, turnaround time, price and service geography have not been confirmed here. Before arranging a shipment, the veterinarian should verify that the laboratory is accepting MF-ICC samples and ask about specimen requirements, preservation, paperwork and whether PCR or other testing should be submitted at the same time.
The practical answer
MF-ICC is a promising addition because it asks whether coronavirus is inside macrophages, not merely whether a cat has encountered coronavirus. Its reported 77% sensitivity, 81% specificity and 53% negative predictive value show both its potential and its boundary: a positive result can substantially support FIP, but a negative result cannot safely dismiss it. The most reliable diagnosis still comes from matching the cat’s presentation with representative samples, complementary tests and, when feasible, tissue-based confirmation.
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