Concordance between Histology, Immunohistochemistry, and RT-PCR in the Diagnosis of Feline Infectious Peritonitis

Abstract

Histology, immunohistochemistry (IHC), and reverse transcription polymerase chain reaction (RT-PCR) have been used to diagnose feline infectious peritonitis (FIP), but no information regarding the comparison of their diagnostic performances on the same organ is available. The aims of this study were to determine the concordance among these tests and to evaluate which combination of tests and organs can be used in vivo. Histology, IHC, and nested RT-PCR (RT-nPCR) for feline coronavirus (FCoV) were performed on spleen, liver, mesenteric lymph node, kidney, large and small intestine, and lung from 14 FIP and 12 non-FIP cats. Sensitivity, specificity, predictive values, likelihood ratios, and concordance were calculated. IHC and RT-nPCR had the highest concordance in lung and liver, histology and IHC in the other organs. The sensitivity of histology, IHC, and RT-nPCR on the different organs ranged from 41.7 to 76.9%, 46.2 to 76.9%, and 64.3 to 85.7%, respectively, and their specificity ranged from 83.3 to 100.0%, 100% and 83.3 to 100.0%. Therefore, IHC is recommended when histology is consistent with FIP. If RT-nPCR is performed as the first diagnostic approach, results should always be confirmed with IHC. Lung or liver provide accurate information regardless of the method, while IHC is preferred to RT-nPCR to confirm FIP in the kidney or intestine.

2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines

“…Pyogranulomatous lesions can be identified around the vasculature, especially on serosal surfaces. 137,138 Immunohisto chemistry (IHC; see later discussion on 'Detection of viral antigen') will reveal FCoV antigen associated with macrophages within the lesions; thus IHC is more specific than histopathology alone 136 and is considered the gold standard for diagnosis. 80 A cat with suspected FIP is often too sick or debilitated to undergo surgical sample collection for histopathology and IHC.…”

“…However, negative IHC results do not exclude FIP because FCoV antigens can be variably distributed within lesions. 58,88,136,141 Ideally, samples should be taken from areas with histopathological changes.…”

“…The only way to truly confirm FIP is to detect the viral antigen associated with macrophages within lesions using IHC. 80,136 However, if the cat’s history and presenting clinical signs are consistent with FIP and the FCoV RNA is detected in fluid or tissue using real-time RT-PCR, then there is a significant chance that the cat has FIP. 80 Ultimately, the diagnostic approach to diagnosing FIP must be tailored to the specific presentation of the individual cat.…”

Clinical and Molecular Relationships between COVID-19 and Feline Infectious Peritonitis (FIP)

“…Signs tend to be mild and self-limiting, and only a small subset of animals will progress to FIP. Fibrinous serositis and pyogranulomatous lesions with vasculitis are the classic lesions of FIP and can be found in both the small and large intestines of affected cats [ 102 ]. FIP can cause solitary mass lesions in the intestinal wall, although this is considered an uncommon presentation (26/156 cats in one study) [ 103 ].…”

Role of paraoxonase-1 as a diagnostic marker for feline infectious peritonitis

“…Moreover, the sick group was divided in FIP and NON FIP groups. Specifically, the diagnosis of FIP (FIP group) was achieved by positive IHC revealing FCoV antigen within intra-lesional macrophages on post mortem biopsies, using the IHC protocol employed in previous studies (Stranieri et al, 2020). When the post mortem examination was not authorized by the owners, the diagnosis of FIP was achieved based on signalment, clinical signs and at least five of the following laboratory changes consistent with FIP (lymphopenia; hyperproteinemia; hypoalbuminemia; low albumin/globulin ratio; electrophoretic peaks in -2 and region; serum AGP higher than 1.5 mg/mL; positive RT-PCR on blood, tissues or effusions and a delta total nucleated cells measured by Sysmex XT-2000iV [ TNC] higher than 2.5 on effusions along with consistent cytology) along with the worsening of the clinical conditions that always led to spontaneous death or to humane euthanasia (Paltrinieri et al, 2007;Giordano et al, 2013).…”