Beyond Modified Transudates: Diagnostic Performance of the Feline Infectious Peritonitis Effusion Index on the Sysmex XN-1000V

Abstract

The antemortem clinical and clinicopathological diagnosis of feline infectious peritonitis (FIP) remains challenging, particularly when effusions deviate from the classically protein-rich, low-to-moderate-cellularity phenotype. The FIP Effusion Index, defined as the ratio of delta total nucleated cells (ΔTNC) to the albumin-to-globulin (A/G) ratio, was developed and initially evaluated in 50 effusions. Subsequent European Advisory Board on Cat Diseases (ABCD) guidelines recommended evaluation in larger cohorts and in cats with confirmed FIP. This study retrospectively evaluated the prespecified threshold (≥7.54) in 176 independent feline effusion samples analysed on the Sysmex XN-1000V and classified by effusion FCoV RT-qPCR, without independent confirmation of FIP status. Relative to this molecular reference, the FIP Effusion Index yielded a sensitivity of 97.8% (95% confidence interval [CI], 92.4–99.7%), a specificity of 88.1% (95% CI, 79.2–94.1%) and an area under the receiver operating characteristic curve (AUC) of 0.962 (95% CI, 0.928–0.995). Discrimination exceeded that of ΔTNC alone (AUC, 0.947; ΔAUC, 0.014; p = 0.012) and A/G alone (AUC, 0.887; ΔAUC, 0.074; p = 0.012). In exudates, sensitivity remained 100% (95% CI, 80.5–100%) and specificity was 82.8% (95% CI, 64.2–94.2%). Among reference-negative effusions, false-positive classification was more frequent in septic than in non-septic samples (6/19 vs. 4/65; odds ratio, 7.04; 95% CI, 1.74–28.53; p < 0.01). These findings demonstrate that the prespecified FIP Effusion Index threshold retained very high sensitivity and strong discrimination for molecular-reference positivity across a broader spectrum of feline effusions. Future studies should evaluate the FIP Effusion Index within a sequential in vivo diagnostic algorithm, using negative results to support rule-out and positive results to prompt confirmation by FCoV antigen detection using immunocytochemistry and/or immunohistochemistry.

The authors' abstract, as published at the source. Pathogens, 2026 · DOI ↗