QUARTERLY BEAT
APRIL 2026
Conclusion FIP workups are less about chasing certainty and more about making high-quality decisions with the data you can realistically get - think pattern recognition + smart sampling. Baseline labs can raise (or lower) suspicion, but when effusion is present it becomes your diagnostic MVP (“most valuable player”). Your endgame is clinical confidence: enough evidence to move forward quickly, explain your reasoning clearly to clients, and start therapy without unnecessary delays. Next up: FIP treatment updates! Part 2 hits the main VETgirl website on April 3, 2026 with the latest 2025 treatment options, protocols, and real- world monitoring tips – check it out HERE. And while you’re treating FIP and wondering “Should this lab value be normal by now?”, here’s a sneak peek of our FIP monitoring timeline, to help keep FIP treatments on track!
The catch? This visible reaction isn’t exclusive to FIP; a positive
Rivalta’s test can also be seen with bacterial peritonitis, pleuritis, or lymphoma. A negative
Rivalta’s test, however, may help to rule out FIP. Bottom
line: don’t rely on a positive Rivalta’s test alone. Instead, interpret it alongside cytologic findings and the patient’s overall clinical picture.
More recently, delta total nucleated cell count ( TNC) has been investigated as an objective, analyzer-based method to support
a diagnosis of FIP in cats with effusion. Lopes et al., describe the underlying principle that many FIP effusions are markedly protein-rich, and when they are processed through certain automated hematology analyzers, the BASO reagent channel can promote fibrin clot formation. These micro clots then physically entrap nucleated cells from the sample, resulting in an artificially low nucleated cell count within the BASO channel, as compared to the other channels. The magnitude of this discrepancy in nucleated cell counts between channels is reported as the TNC, and larger values (i.e., larger discrepancies in cell counts) supports a diagnosis of FIP.
ABBREVIATIONS: • A:G: albumin-to-globulin ratio; also abbreviated as A/G, AG, AGR
Conceptually, the TNC is similar to Rivalta’s test (both are indirectly demonstrating that an effusion is unusually high in protein). However, TNC is typically considered a more objective diagnostic because it’s machine-quantified and less prone to inter-observer variability. It has also shown improved differentiation of FIP effusions from some septic and neoplastic effusions compared with Rivalta’s and A:G alone. If you want to push diagnostic confidence even further, the FIP Effusion Index combines two complementary signals, capturing both the inflammatory ‘protein signature’ and the effusion’s ‘cellular behavior’ in one value:
• ADR: “ain’t doin right” • CBC: cell blood count • FCoV: feline coronavirus • FeCV: feline enteric coronavirus • FeLV: feline leukemia virus • FIP: feline infectious peritonitis • FIPV: feline infectious peritonitis virus • FIV: feline immunodeficiency virus
• IHC: immunohistochemistry • MVP: most valuable player • RT-qPCR: quantitative reverse transcription PCR • TNC: delta total nucleated cell count
• Protein pattern (A:G) and • Cellular behavior ( TNC)
The FIP Effusion Index is calculated by dividing the TNC by the A:G, meaning it increases when TNC is higher and/or the A:G ratio is lower, with a higher result being supportive of FIP.
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