CPD ACCREDITED ARTICLE
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Appearance: Usually straw-coloured and clear –
typical for FIP but NOT diagnostic
Cell count: Low (<2x109 /L) with the cell population consisting of a mixture of non-degenerate
neutrophils and macrophages with lower numbers of lymphocytes
Protein: High (>35 g/L) due to the presence of
gamma globulins. If possible, the albumin concentration of the fluid should be determined (this
can be performed on a bench-top analyser, do
not attempt if the fluid is very viscous and thick).
The globulin fraction is calculated by subtracting
the albumin from the total protein concentration.
The A/G ratio can then be calculated. An A/G ratio
<0.4 has a high predictive value for the presence
of FIP and a ratio of >0.8 a high predictive value
for the absence of FIP.3
Rivalta test: This is a simple, inexpensive test that
is useful to demonstrate a high content of inflammatory proteins in the effusion. A transparent tube
of 10-20 mL is filled with 7-8 mL of distilled water
and a drop of 98% acetic acid is added and mixed
well in order to acidify the solution. A drop of effusion is placed carefully onto the surface of the
mixture. If this drop disappears and the mixture
remains clear, the result is negative. If the drop
remains formed and slowly sinks to the bottom
of the mixture like a jellyfish, the test is positive. A
negative result has been found to have a high predictive value for the absence of FIP. A positive result could indicate FIP, but may also be seen in effusions due to lymphoma or bacterial infection.1,5
An effusion with a high cell count (<2x109 /L), predominance of cells other than neutrophils and macrophages, low protein concentration (<30 g/L), A/G
ratio >0.8 and negative Rivalta test is highly unlikely to
be from FIP and other causes should be investigated –
i.e. this piece belongs to another puzzle.
Serum albumin: globulin ratio: The serum A/G has
a higher diagnostic value than serum total protein or
gamma globulin concentrations. Cats with an A/G ratio >0.8 are highly unlikely to have FIP, cats with an
A/G ratio <0.6 are highly likely to have FIP.4
Histology and Immunostaining: Identification of the
FCoV virus in effusion macrophages or in tissue sections (dry form) using immunohistochemistry provides
definitive proof of FIP.
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Effusion: Immunohistochemical staining can be
used to demonstrate the presence of the virus
within macrophages in the fluid and should be
performed on all effusions fitting the criteria for
FIP. The sample required is at least 12 mL of effusion fluid in EDTA tubes (i.e. at least three filled 4
mL EDTA collection tubes), which should be submitted as soon as possible. (This test is performed
in the Pathology Laboratory at the Faculty of
Veterinary Science in South Africa, and commer-
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cial laboratories can forward samples to them.)
Positive results are 100% predictive for the presence of FIP, however a negative result does not
rule out FIP (the negative predictive value is only
57%). This is because false negatives can occur
due to insufficient numbers of macrophages in
the sample examined.6
Tissue sections: FIP lesions exhibit a typical histopathological pattern, which is considered to
be the gold standard test. The demonstration of
FCoV in organ biopsies using immunohistochemistry is also 100% predictive for FIP. These techniques involve invasive samp