Diagnosis of FIP: an update series on available tests-part three

Tasker S. Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests. J Feline Med Surg. 2018 Mar;20(3):228-243. doi: 10.1177/1098612X18758592.

Routine Laboratory Tests

Hematology– Changes in hematologic results in cats with FIP are often non-specific yet can be incorporated in building an increasing index of suspicion for a diagnosis. Lymphopenia is most commonly found (55-77% of cases are lymphopenic), neutrophilia (39-57%), left shift, and a mild to moderate normocytic, normochromic anemia (37-54%). Microcytosis (with or without anemia) has recently been associated with FIP.

Serum biochemistry – hyperglobulinemia: Hyperglobinemia is reported in 89% of cases, frequently with hypoalbuminemia or low-normal serum albumin concentrations.  With lower albumin levels, hyperproteinemia may not always be present, especially in dry FIP cases. This combination of hyperglobulinemia and hypoalbuminemia also leads to a low albumin:globulin (A:G) ratio. Reports often suggest that an A:G ratio ratio of < 0.4 makes FIP very likely and an A:G ratio of 0.8 makes FIP very unlikely. The author states that she does not use a specific value but notes that the lower the A:G ratio value is, the greater the suspicion for a diagnosis of FIP, especially if other findings are consistent with FIP. An A:G ration of > 0.6 is useful to rule out  but lower ratios do not help to rule in FIP.

Serum biochemistry – hyperbilirubinemia: Hyperbilirubinemia occurs in 21-63% of FIP cases. There are often no to mild increases in alanine transferase (ALT), alkaline phosphatase (ALP) or γ-glutamyltransferase activity. The presence of hyperbilirubinemia in the absence of elevated hepatic enzyme activities or severe anemia should increase the index of suspicion for FIP. Hyperbilirubinemia appears from reports to be identified more commonly just prior to death or euthanasia in these cases.

Serum biochemistry – acute phase proteins: α1-acid-glycoprotein (AGP) is an acute phase protein (APP). Elevations of AGP is not specific for FIP, marked elevations (> 1.5 mg/ml) are often seen with FIP. The magnitude of the increase can aid in the index of suspicion and diagnosis of FIP.

Feline coronavirus serology (FCoV):These serum antibody tests for FCoV are primarily ELISAs, indirect immunofluorescence antibody tests or others. A positive test indicates infection with FCoV and seroconversion (this occurs 2-3 weeks after infection). There is overlap in FCoV titers in FIP cats and non-FIP cats, so this test is of limited use in distinguishing cats with FIP. A positive result indicates exposure to FCoV only.

Effusion Sample Analysis: Because analysis of effusion samples from a suspected case of FIP is quite helpful in obtaining a diagnosis, getting a sample of any effusion should be a priority. Radiography is not considered as sensitive an imaging technique in obtaining samples compared to ultrasonography which can detect small volumes of fluid in body cavities such as the abdomen and thorax. Effusions associated with FIP are usually clear, viscous or sticky with straw-yellow color and a protein rich background on cytology, though chylous effusions are occasionally described. A total protein concentration of >35g/l (>50% globulins) are typical and FIP effusions frequently have low A:G ratios and raised AGP concentrations like those found in serum. These effusions also usually have poor cellular numbers (<5 x10cells/l) which are pyogranulomatous in makeup with macrophages, non-degenerate neutrophils and few lymphocytes. Serology on effusions for FCoV antibodies is often not performed since results are often varied and non-diagnostic. Additional testing on effusion samples using immunostaining and RT-PCR will be discussed in the next part.

Rivalta test on effusion: 8 ml of distilled water is mixed with 1 drop of 98% acetic acid (or vinegar can be used). Then 1 drop of effusion is placed onto the surface of the liquid (usually in a test tube environment). A positive test will have the drop staying attached or connected with the surface or slowly floating to the bottom of the tube. A negative test is when the drop disappears and the solution remains clear. The test is subjective, therefore, the results are difficult to interpret properly as to whether a result is positive or negative.

Miscellaneous Tests: In cats with neurological signs of disease, imaging (MRI) can be performed and also procedures to collect cerebrospinal fluid (CSF) can be done with caution. Though some cases with FIP can demonstrate unremarkable CSF results, FIP cats can have marked elevations of protein (>2g/l) and an increased cell count (>8 x 10cells/l) with the cell type predominantly neutrophils, mononuclear or mixed. Immunostaining and RT-PCR can also be done on CSF. Part Three (VT)

See also:
Felten S, Weider K, et al. Detection of feline coronavirus spike gene mutations as a tool to diagnose feline infectious peritonitis. J Feline Med Surg. 2017 Apr;19(4):321-335.