Feline Infectious Peritonitis (FIP) Diagnostic Flowchart

Systematic exclusions, effusion analysis, supportive biomarker panel, and clinical scoring matrix

Feline Infectious Peritonitis (FIP) Diagnostic Flowchart
Clinical Presentation: Young feline evaluated for chronic pyrexia, lethargy, and progressive abdominal distension.

Clinical Presentation & Indicators

  • Chronic, progressive, antibiotic-refractory fluctuating pyrexia
  • Anorexia, progressive weight loss, muscle wasting, and lethargy
  • Body cavity effusion (ascites / pleural effusion), icterus, anterior uveitis, or neurological deficits

Immediate Triage Priorities

  • Priority 1: Immediate oxygenation and low-stress handling if dyspneic
  • Priority 2: Focused POCUS (AFAST / TFAST) to identify free fluid collections
  • Priority 3: Diagnostic and therapeutic paracentesis/thoracocentesis for effusion cytology

Diagnostic Algorithm Flowchart

Feline Infectious Peritonitis (FIP) Diagnostic Flowchart

Feline Infectious Peritonitis (FIP) is an immune-mediated systemic viral disease caused by systemic mutation of Feline Coronavirus (FCoV). Because definitive ante-mortem tissue biopsy is challenging, a stepwise clinical and laboratory exclusion algorithm is required.

Risk Factors & Key Categories
  • Young age (especially cats < 2 years old)
  • Multi-cat households, catteries, and rescue shelters
  • Recent history of physiological/environmental stress (adoption, neutering)
  • High background FCoV seroprevalence in environment
  • Purebred cats and overcrowded housing conditions
🩺1. CLINICAL SUSPICION
  • Chronic, progressive disease trajectory
  • Anorexia, weight loss, and antibiotic-refractory pyrexia
  • Lethargy, dull mentation, and poor body condition
  • Body cavity effusion (abdominal distension or tachypnea)
  • Icterus, neurological signs (ataxia, nystagmus, seizures), or ocular lesions (uveitis)
πŸ§ͺ2. BASELINE WORKUP & EXCLUSIONS
  • Comprehensive physical & ophthalmoscopic examination
  • Complete Blood Count (CBC) and serum biochemistry (TP, Alb, Glob, A:G ratio)
  • Urinalysis and urine protein:creatinine ratio (UPC)
  • FIV / FeLV point-of-care antigen/antibody testing
  • Evaluation for primary bacterial, fungal, or parasitic infections
  • Diagnostic imaging (Thoracic radiographs, abdominal ultrasound)
No alternative cause identified
Proceed to Step 3
Alternative cause identified
FIP Unlikely
Initiate targeted etiology-specific therapy and monitoring
πŸ’§ 3. EFFUSION PRESENT? (POCUS)
Point-of-Care Ultrasound (POCUS) Cavity Fluid Assessment
YES, EFFUSION PRESENT
3A. EFFUSION ANALYSISπŸ§ͺ
  • Gross appearance: Clear straw-yellow to amber, viscous, sticky, foaming on shaking
  • Total Protein > 3.5 g/dL (exudative protein content with modified-transudate cellularity)
  • Low to moderate total nucleated cell count (< 5,000 /ΞΌL; non-degenerate neutrophils & macrophages)
  • Positive Rivalta test (formation of a distinct jellyfish-like precipitate floating down the tube)
  • Fluid Albumin:Globulin (A:G) ratio < 0.45 (high positive predictive value for FIP)
Rivalta positive (+) AND Protein > 3.5 g/dL with macrophage-predominant cytology
βž” Highly Consistent with FIP (Effusive Phenotype)
Rivalta negative (-) AND low protein OR high cellularity (degenerate neutrophils / bacteria)
βž” Inconsistent with FIP β€” Investigate Septic Peritonitis, CHF, Lymphoma
NO, NON-EFFUSIVE / DRY FIP
3B. CLINICAL FINDINGS & ORGAN WORKUPπŸ”
  • Persistent antibiotic-refractory fever, progressive weight loss, and marked lethargy
  • Ocular Signs: Anterior uveitis, aqueous flare, chorioretinitis, perivascular cuffing, hyphema
  • Neurological Signs: Vestibular ataxia, paresis, nystagmus, cranial nerve deficits, seizures
  • Granulomatous Lesions: Irregular renomegaly, hepatomegaly, or mesenteric lymphadenopathy
  • Severe polyclonal hypergammaglobulinemia with serum A:G ratio < 0.6
πŸ§ͺ 4. SUPPORTIVE BIOMARKERS & TESTSβš—οΈ
β€’ Serum FCoV Antibody Titer
High titer indicates coronavirus exposure; does not differentiate enteric FCoV from mutated FIP virus.
 Β±
β€’ FCoV RT-PCR (Blood / Effusion / CSF)
Positive in effusion fluid or CSF is strongly supportive; negative blood PCR does not exclude FIP.
 Β±
β€’ FCoV Immunofluorescence Staining (Effusion / FNA)
Intra-macrophage FCoV antigen detection: > 98% specificity for definitive FIP confirmation.
 +
β€’ Alpha-1-Acid Glycoprotein (AGP)
> 1.5 g/L: Strong supportive acute-phase marker (especially critical in non-effusive 'dry' FIP).
 +
β€’ Serum Amyloid A (SAA)
Markedly elevated: Strongly supports active, marked systemic immune-mediated inflammation.
 +
β€’ Serum Albumin : Globulin (A:G) Ratio
< 0.6: Strongly supportive of FIP; > 0.8: High negative predictive value to rule out FIP.
 +
(+) Strong Supportive Marker / High Specificity (Β±) Moderate Supportive Marker
LOW PROBABILITY / UNLIKELYπŸ›‘οΈ
  • FIP diagnosis is unlikely.
  • Continue thorough investigation for alternative bacterial, parasitic, and neoplastic diseases.
  • Monitor clinical progression and re-evaluate in 1–2 weeks if constitutional signs persist.
MODERATE PROBABILITY / SUSPECTED❓
  • Additional confirmatory testing and close clinical monitoring recommended.
  • Repeat acute-phase proteins (AGP/SAA), perform targeted FNA/biopsy of affected organs for cytology and PCR.
  • Monitor clinical trajectory and organ function parameters frequently.
HIGH PROBABILITY / HIGHLY LIKELY⭐
  • Clinical presentation is strongly consistent with FIP.
  • Initiate targeted antiviral therapy (GS-441524, GC376, or oral remdesivir derivatives) without delay.
  • Monitor clinical response, hematocrit, and renal/hepatic biochemistry throughout the 12-week protocol.
πŸ’‘

Clinical Diagnostic Note: Definitive gold-standard diagnosis requires tissue biopsy and immunohistochemistry (IHC) or RT-PCR. However, ante-mortem clinical scoring combined with effusion cytology and biomarker analysis provides high diagnostic accuracy for prompt therapy initiation.

Differential Diagnosis Matrix

Condition / EtiologyKey Distinguishing SignsConfirmatory Diagnostic Test
Septic Peritonitis / Pleuritis Degenerate neutrophils with intracellular bacteria, fluid glucose < blood glucose by > 20 mg/dL, elevated fluid lactate. Effusion cytology, Gram stain, and aerobic/anaerobic microbiological culture.
Feline Lymphoma (Mediastinal / Alimentary) Monomorphic population of large immature lymphoblasts in effusion or FNA; discrete sonographic masses. Cytological/histopathological evaluation and PARR clonality assay.
Congestive Heart Failure (Hypertrophic Cardiomyopathy) Modified transudate or chylothorax, marked left atrial enlargement (LA:Ao > 1.6), gallop rhythm, elevated NT-proBNP. Transthoracic Echocardiography (TTE) and point-of-care lung ultrasound.
Systemic Toxoplasmosis / Mycobacteriosis Pyogranulomatous lymphadenopathy, uveitis, persistent pyrexia, marked hyperglobulinemia. Toxoplasma IgG/IgM serology, targeted PCR, and acid-fast (Ziehl-Neelsen) staining.

Clinical Pearls & Guidelines

In suspected effusive FIP, the combination of straw-yellow viscous fluid, Total Protein > 3.5 g/dL, A:G < 0.45, and a positive Rivalta test has a positive predictive value exceeding 98%. Do not delay antiviral therapy awaiting prolonged external laboratory results.

References & Sources

  • πŸ“šAddie DD, et al. (2009) Feline infectious peritonitis. ABCD guidelines on prevention and management. J Feline Med Surg 11:594–604.
  • πŸ“šTasker S. (2018) Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests. J Feline Med Surg 20:228–243.
  • πŸ“šISFM Guidelines on Feline Infectious Peritonitis (2022). Journal of Feline Medicine and Surgery.