Antemortem diagnosis of FIP can be difficult. Arriving at a diagnosis involves a combination of elements, with no single test
providing a definitive diagnosis, except for histologic examination or immunohistochemistry (Figure 1).
1. Metaphorically speaking, diagnosing FIP is like building a brick wall. The patient's history, signalment, and clinical
signs as well as findings from multiple tests must all be assessed to reach a diagnosis; only histologic examination or immunohistochemistry
provides a definitive diagnosis.
History and signalment
Diagnosing FIP starts with obtaining an animal's history and noting its signalment: most cases occur in young cats (usually
< 1 year of age), it occurs more frequently in purebred than it does in mixed-breed cats, and affected cats usually originate
from or are currently housed in multicat situations.29 In breeding catteries, examination of records may reveal a genetic connection among cases. A history of a stressful event
may precede the onset of signs by several weeks, such as surgery, adoption from a shelter, or trauma.
Clinical signs, physical examination findings, and effusion analysis
The cat may present with weight loss, fever, and inappetence. The fever may wax and wane and is not responsive to antibiotics.
Abdominal palpation of affected cats may reveal thickened bowel loops, mesenteric lymphadenopathy, or irregular serosal surfaces
of abdominal organs. Cats with the effusive form may not present as much of a diagnostic challenge as those with the noneffusive
form do. With the noneffusive form, signs may be referable to virtually any organ, singly or in combination. Granulomatous
lesions may occur in the eye (e.g. retinal changes, uveitis), central nervous system (multifocal lesions), or abdominal organs, including the intestines. In
addition, a combination of effusive and noneffusive forms may occur, and transition between the two can occur in any cat with
For cats with effusion, evaluation of this fluid can be informative. The fluid has been described as straw-colored and is
usually viscous because of the high-protein content. It usually has a relatively low cellular content that is pyogranulomatous
(macrophages and neutrophils; usually no toxic changes in the latter). Detection of feline coronavirus antigen by immunofluorescence
within inflammatory cells (macrophages) in effusive fluid correlates with a diagnosis of FIP.30 (Viral antigen detection by immunofluorescence is offered by many diagnostic laboratories and can be done on sediment from
submitted abdominal fluid.) A high protein concentration and a low albumin-globulin ratio in the fluid are also indicative
Serum chemistry profile, acute phase protein, CBC, and immunophenotyping findings
Serum chemistry profiles reveal that many cats with FIP have elevated serum total protein concentrations because of the high
globulin concentrations; however, even with normal total protein concentrations, a decreased albumin-globulin ratio may be
evident. As this ratio approaches 0.5, a diagnosis of FIP becomes more likely.30 Other abnormalities may be evident depending on the tissues involved (e.g. elevated hepatic enzyme activities or renal function values).
In addition to high globulin concentrations, elevation in acute phase proteins also occurs. Elevations in alpha-1 acid glycoprotein
in serum have been noted in cats with FIP and may aid diagnosis. In one study that evaluated the usefulness of measuring alpha-1
acid glycoprotein to diagnose FIP, it was found that high alpha-1 acid glycoprotein concentrations are a discriminating marker
for FIP.31 Measurement of this serum protein can be specifically requested from some commercial labs. But keep in mind that other inflammatory
conditions can lead to alpha-1 acid glycoprotein increase.
Cats with FIP may also have evidence of an anemia of chronic disease and a lymphopenia, despite elevated total white blood
cell counts.32 Immunophenotyping shows T lymphocyte depletion in particular; in fact, a normal T lymphocyte count has a significant negative
predictive value for FIP (immunophenotyping or flow cytometry is often offered by laboratories associated with academic institutions).12