Abstract / Summary
Feline infectious peritonitis (FIP) remains diagnostically challenging, and rapid antemortem con-firmation is increasingly important in the era of effective antiviral therapy. This pilot diagnostic accuracy study assessed agreement between effusion RT-qPCR and cell tube block immunohisto-chemistry (CTB-IHC) in 40 cats with suspected FIP and developed an exploratory effusion-based predictive score using CTB-IHC as the reference standard. RT-qPCR and CTB-IHC showed almost perfect agreement (κ = 0.86): 30 cats were positive by both methods, eight were negative by both, and two were CTB-IHC-positive/RT-qPCR-negative. RT-qPCR showed 93.8% sensitivity (95% CI: 79.2–99.2%), 100% specificity (63.1–100%), and 95.0% accuracy (83.1–99.4%). The three-parameter FIP Predictive Score, combining globulin concentration, FIP Effusion Index, and A:G ratio, achieved an apparent AUC of 0.916 (95% CI: 0.842–0.990), with 78.1% sensitivity (60.0–90.7%) and 100% speci-ficity (63.1–100%) at ≥ 2 points. Given that the score was derived and evaluated within the same small cohort, independent external validation is essential before clinical implementation. Neverthe-less, neither a low score nor a negative molecular result should independently close investigation when FIP remains clinically suspected. These findings provide a foundation for a structured, mul-timodal effusion-based diagnostic approach integrating molecular, antigen, biochemical, and cy-tometric evidence.