QUARTERLY BEAT
APRIL 2026
real life, however, IHC isn’t always feasible in a timely manner, so clinicians commonly rely on a layered approach of stacking supportive findings until the overall clinical picture is compelling enough to justify treatment. When FIP is on the differential list, it’s best to start with baseline diagnostics: a complete blood count (CBC), serum biochemistry, urinalysis (to assess for possible renal involvement), and FeLV/FIV testing.
Viscus, “stringy” fluid sampled from a kitten’s abdomen with suspected FIP based on clinicopathologic support. (Image courtesy of Dr. Amy Kaplan- Zattler, cVMA, DACVECC, MRCVS). RT-qPCR on effusion is often treated as a “heavy hitter” because published performance can be excellent, and it’s fast enough to fit into clinical workflows, but it’s limited by cost, equipment, and technical expertise, which can make routine access variable. Rivalta’s test is a simple, low-cost bedside assay performed on effusion fluid that can be a helpful piece of the diagnostic puzzle for FIP. It evaluates whether the effusion contains a high concentration of protein and inflammatory macromolecules (classically including fibrin/fibrinogen), which will precipitate when exposed to dilute acetic acid. To perform the test, prepare a dilute acetic acid solution by adding 20–30 uL of 98% acetic acid to 7–8 mL of distilled water in a plain, sterile test tube (no additives), then gently place a single drop of effusion onto the surface of the solution in the tube.
The albumin-to-globulin (A:G) ratio is calculated from measured albumin and globulin concentrations in serum/plasma or effusion. In cats with FIP, a low A:G ratio reflects the classic combination of polyclonal hyperglobulinemia (chronic immune stimulation/inflammation) and hypoalbuminemia (negative acute-phase response, decreased production during inflammation, and/or redistribution). Clinically, this is supportive but not diagnostic , because other inflammatory, infectious, and neoplastic diseases can produce a similar pattern.
Next, if an effusion is identified, the diagnostic workup should “pivot” toward effusion characterization and targeted effusion tests, because FIP effusions often carry higher-yield clues. While variability of the clinical features exists (and concurrent disease can muddy the waters), FIP-associated effusions commonly share these features:
• Color: yellow/amber/straw; transparent to cloudy • Texture: viscous (“sticky”) • Protein: typically >3.5 g/dL (>35 g/L)
• Cellularity: often low, but variable (approximately 2–6 × 10³/uL, sometimes higher); commonly non- degenerate neutrophils and macrophages with fewer lymphocytes and a granular proteinaceous background • Effusion A:G ratio: often <0.4
Video courtesy of Dr. Dave Gordon.
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