Feline infectious peritonitis (FIP) is a fatal disease of cats, and a sequela of systemic feline coronavirus (FCoV) infection. Mutations in the viral spike (S) gene have been associated with FCoVs found in tissues from cats with FIP, but not FCoVs found in faeces from healthy cats, and are implicated in monocyte/macrophage tropism and systemic spread. This study was designed to determine whether S gene mutation analysis can reliably diagnose FIP. Cats were categorised as with FIP (n = 57) or without FIP (n = 45) based on gross post-mortem and histopathological examination including immunohistochemistry for FCoV antigen. RNA was purified from available tissue, fluid and faeces. Reverse-transcriptase quantitative-PCR (RT-qPCR) was performed on all samples using FCoV-specific primers, followed by sequencing of a section of the S gene on RT-qPCR positive samples. Samples were available from a total of 102 cats. Tissue, fluid, and faecal samples from cats with FIP were more likely to be FCoV RT-qPCR-positive (90.4, 78.4 and 64.6% respectively) than those from cats without FIP (7.8, 2.1 and 20% respectively). Identification of S gene mutated FCoVs as an additional step to the detection of FCoV alone, only moderately increased specificity for tissue samples (from 92.6 to 94.6%) but specificity was unchanged for fluid samples (97.9%) for FIP diagnosis; however, sensitivity was markedly decreased for tissue (from 89.8 to 80.9%) and fluid samples (from 78.4 to 60%) for FIP diagnosis. These findings demonstrate that S gene mutation analysis in FCoVs does not substantially improve the ability to diagnose FIP as compared to detection of FCoV alone.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-017-0467-9) contains supplementary material, which is available to authorized users.

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Background: Assays based upon PCR technology are non‐culture‐based molecular diagnostic tools that have the potential to detect pathogens, characterise the presence of both desirable and deleterious genetic traits, and facilitate the diagnosis of neoplasia. The availability of PCR assays to investigate disease and to guide treatment choices is ever increasing. They have the advantages of being highly sensitive and specific, can be performed on a wide variety of sample types, and have the potential to be an incredibly useful clinical tool. However, these assays are not without limitations, which must be considered when using them in practice.Aim of the article: This article aims to provide an overview of the technology behind PCR assays, highlighting where this has been exploited for clinical use. Limitations to its application (ie, production of false positive and false negative results) are also reviewed.