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

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.

More Advanced Diagnostic Testing for FIP

RT-PCR: The author states that RT-PCR assays are available for the detection of FCoV, but they are not specific for FIP-associated FCoVs. It is advised that laboratories should be able to report the sensitivity and specificity of the RT-PCRs they use for detection of FCoV RNA. Immunostaining offers a definitive diagnosis where RT-PCR does not.

The samples from suspected cases of FIP that can be tested by RT-PCR for FCoV RNA are tissue, effusion, blood, CSF and aqueous humour. Tissue samples should not be formalin fixed. RT-PCR can be performed on fecal samples, yet this is used more to detect cats that are shedding virus in managing environments such as a multi-cat household. Cats with FIP are more likely to shed FCoV and have higher amounts of FCoV RNA in their feces than cats without FIP. An additional question is raised if further mutation analysis is useful, such as analyzing for fusion peptide sequence mutations. The authors found in one study that S gene mutations present in most of the FIP tissues they analyzed were also present in most tissues of non-FIP cats that had systemic FCoV infection. Another recent study confirmed these findings and found that including identification of the S gene mutated FCoVs as an additional confirmatory step only slight increased specificity and moderately decreased sensitivity of this test. Therefore, additional S-gene mutation analysis in FCoVs does not significantly improve the ability to diagnose FIP in fluid or effusion samples compared to RT-PCR alone.

Histopathology:Tissue samples are usually evaluated for the characteristic histopathological changes of FIP and when present, are regarded as reliable for diagnosis. Immunostaining for FCoV antigen is recommended to confirm the diagnosis. One study was mentioned that 5 of 8 FIP cases did not exhibit histopathology changes consistent with FIP even though large samples were taken. Diagnosis was then confirmed through positive FCoV antigen immunostaining.

Immunostaining of FCoV antigen: Immunostaining is used on formalin-fixed tissues using IHC, or cytology samples (usually effusions) using immunocytochemistry (ICC) or immunofluorescence (IF). Positive FCoV antigen immunostaining is very specific and confirms diagnosis. A negative result though does not exclude FIP as a diagnosis. Immunostaining of effusion samples has demonstrated variable sensitivity (due to the effusion is often cell or macrophage poor) so a false negative result may be obtained. Specificity of immunostaining of effusions can be higher though a recent study has raised questions about the specificity of ICC. More study needs to be done on application of ICC to CSF and aqueous humour samples to evaluate how useful this technique is for diagnosis in cases with neurological or ocular involvement.
Part Four (VT)

See also:

Barker EN, Helps CR, et al. Limitations of using feline coronavirus spike protein gene mutation to diagnose feline infectious peritonitis. Vet Rec. 2017 Oct 5;48(1):60.