AbstractBackgroundA number of categorization systems had been developed for the reporting of cytology specimens with the aim of providing uniform definitions, criteria, and diagnostic terminology. The intention of these systems is to improve reproducibility of diagnostic categorization with standardized estimates of malignancy risk. Required for the success of these systems is a high level of interobserver reproducibility for category assignment. Recently, the international system for serous fluid cytopathology (TIS) was proposed using the categories nondiagnostic, negative for malignancy, atypia of undetermined significance (AUS), suspicious for malignancy, and malignant. Little data exists documenting the interobserver agreement for these categories.DesignA search of the cytology records at the University of Missouri was performed for all pleural fluid specimens obtained between January 2014 and December 2019. A total of 200 specimens were reviewed independently by three board‐certified cytopathologists. Specimens were characterized as nondiagnostic, negative, AUS, suspicious for malignancy, and malignant. Interobserver agreement was analyzed using Cohen's kappa.ResultsOverall observer agreement was 68% and chance‐corrected weighted agreement (weighted kappa) was 0.63. Agreement was good for categories negative and malignant, but poor for categories atypia of uncertain significance, and suspicious for malignancy.ConclusionsThe TIS has performance characteristics similar to other cytologic classification schemes. Interobserver agreement is best for the negative (76%) and malignant (81%) categories. Interobserver agreement is poor for the category's AUS, and suspicious for malignancy. This is similar to interobserver agreement associated with other published categorization systems.