From Effusion to Diagnosis: Integrating the FIP Effusion Index and Cell Tube Block Immunohistochemistry in a Geriatric Cat

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Abstract
Feline infectious peritonitis (FIP) can be overlooked in older cats with competing causes of effusion. An older female domestic shorthair cat with hyperthyroidism and a hypertrophic cardiomyopathy phenotype developed abdominal effusion in December 2023. Initial effusion total protein was 5.46 g/dL, the albumin-to-globulin ratio was 0.48 and delta total nucleated cell ratio (ΔTNC) was 60.58, yielding a retrospectively calculated FIP Effusion Index of 126.2. Coronavirus RT-PCR detected FCoV RNA, with M1058L mutation positivity, and cell tube block immunohistochemistry (CTB-IHC) demonstrated viral antigen within macrophages. A second sample collected from scant residual effusion 9 days after the initiation of GS-441524 had a ΔTNC of 2.15 and a FIP Effusion Index of 7.17, despite a higher total protein concentration and a lower albumin-to-globulin ratio. GS-441524 treatment began on 20 December 2023 and ended on 22 March 2024. Abdominal fluid was no longer detected at the assessment on 25 January. No confirmed FIP relapse was documented before death during cardiopulmonary decompensation in January 2026. This case illustrates complementary contributions of the FIP Effusion Index and CTB-IHC, with an exploratory observation of serial FIP Effusion Index change. Threshold comparison is retrospective. The lower index was observed during treatment alongside markedly reduced effusion, but this observation does not establish a treatment effect or validate treatment monitoring.

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