Feline infectious peritonitis (FIP) is a fatal disease caused by feline infectious peritonitis virus (FIPV). At present, neither a licensed treatment nor an accurate ante-mortem diagnosis are available. In the present study, three available tests were evaluated for their diagnostic power on effusion samples. High feline coronavirus antibody titers, measured with an immunoperoxidase monolayer assay (IPMA), were correlated with FIP but its low specificity precluded a reliable diagnosis. The in-house 5’ reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) provided a much better specificity and high sensitivity. Given the low sensitivity of immunofluorescence staining (IF) of effusive cells, the RT-qPCR alone or in combination with IPMA represents a good alternative for IF. In the majority of the effusion samples from FIP positive animals, Sanger sequencing of the open reading frame encoding the spike protein (ORF S) revealed not only mutations that were previously associated with FIP (M1058L, S1060A, I1106T and D1108Y/E/G) but also two new, closely related mutations (T1112S/N).