This document summarizes a study that evaluated the performance of different diagnostic tests for feline infectious peritonitis (FIP) in challenging clinical cases where the diagnosis was difficult. Twelve cats were included that were later confirmed or excluded from having FIP based on immunohistochemistry (IHC) or follow up. The study assessed tests performed in vivo and postmortem, including analysis of effusions, serum protein electrophoresis, anti-feline coronavirus serology, serum α1-acid glycoprotein concentration, and histopathology. Sensitivity, specificity, and concordance with the final diagnosis were calculated for each test. The test with the highest sensitivity and specificity was serum α1-acid glycoprotein concentration, while histopathology had