• Feline infectious peritonitis (FIP) is the name of the disease which is caused by feline coronavirus (FCoV). Feline enteric coronavirus (FECV) is sometimes used interchangeably with FCoV, but the proper scientific name, as determined by the International Committee on Taxonomy of Viruses, is FCoV. However, in databases such as Genbank, early strains of FCoV may be named “FIPV-etc” or “FECV-etc” as indicators of whether they originated from a cat with or without FIP.

RT-PCR 

• Feline infectious peritonitis is the disease which occurs in a minority of FCoV-infected cats: there is no such thing as a “FIP test,” tests claiming to be FIP tests are actually tests for feline coronavirus RNA or antibodies. 
• The advent of sensitive real time (quantitative) reverse-transcription polymerase chain reaction (RT-qPCR) test PCR (Polymerase chain reaction) for FCoV Feline corona virus: FIP have made in vivo feline infectious peritonitis (FIP) diagnosis much simpler and less invasive than previously. 
• Positive RT-qPCR tests on effusions are 96% specific for effusive (wet) FIP. 
• Positive RT-qPCR tests on mesenteric lymph node (MLN) fine needle aspirates (FNA) (Veterinary Diagnostic Services, University of Glasgow Veterinary School) are 96% specific for non-effusive (dry) FIP.  
• RT-qPCR tests on MLN FNA have reduced the need for biopsy diagnosis in non-effusive (dry) FIP cases, but at post mortem, organ sections for histopathology should still be taken, especially in cases where a pedigree cat purchaser intends to sue the breeder. 
• The usefulness of negative RT-qPCR tests depends on the sensitivity of the laboratory test used and that the correct sample has been submitted (eg not blood or feces for FIP diagnosis). 
• RT-PCR testing to detect viral messenger RNA (mRNA) should, in theory, demonstrate replicating virus, however the primers occasionally detect human DNA, resulting in false positive results. 
• Mutation M1058L and S1060A tests (Idexx Laboratories) have poor sensitivity but positive results on effusions are 95.8% specific for FIP.  (Specificity on FNAs has not been determined.) 
• Quantitative RT-PCR yields a threshold cycle (C_T) result which is an indirect indicator of viral RNA quantity: the lower the C_T, the more viral RNA was present in the sample. Some laboratories translate C_T results into RNA quantities, others report C_T, and others simply report positive or negative: the latter is inadequate - ideally C_T or viral quantity should be given on the report. 
• Blood should not be sent for FCoV RT-qPCR testing since most cats with FIP are not viremic and will test negative; in addition around 5% of cats without FIP test positive. 
• Fecal samples are useful for monitoring FCoV shedding, but positive RT-PCR results are not predictive of a cat having FIP, negative tests do not rule out FIP, but mostly rule out FCoV as a cause of diarrhea. 

FCoV antibody tests 

• Infection with coronavirus results in the production of circulating antibodies detectable by serological tests Serology. 
• FCoV antibody tests have a poor positive predictive value for FIP and FCoV-associated diarrhea, but negative antibody tests are excellent for ruling out a diagnosis of FIP or FCoV-associated diarrhea, provided the test is sufficiently sensitive. 

FIP profile 

• Various laboratories offer an FIP profile which usually consists of FCoV antibody titer; hematology (checking for non-regenerative anemia, lymphopenia); biochemistry (checking for hypergammaglobulinemia leading to decreased albumin to globulin ratio, raised bilirubin, raised alpha-1 acid glycoprotein acute phase protein) Acute phase proteins, cytology (if effusion); plus possibly FCoV RT-PCR test. 

Immune staining 

• Immune staining of post-mortem and biopsy samples is often regarded as the gold standard of FIP diagnosis Indirect immunofluorescence. However, due to the property of feline tissue, especially macrophages, to non-specifically bind antibodies, false positive results are a problem, unless the laboratory uses a control antibody (ie one not against FCoV) on every tissue sample to control for non-specific signaling. 
• Immune staining of FNA or effusion cells also have problems of both sensitivity and specificity and are no longer widely used, having been supplanted by RT-PCR.

Uses

FCoV RT-PCR tests 

• FCoV RT-PCR tests on effusions and fine needle aspirates can be used to diagnose FIP, provided history, clinical signs, hematology and biochemistry are consistent with a diagnosis of FIP.  Specificity is around 96%, sensitivity depends on the laboratory used. 
• FCoV RT-PCR tests on feces are useful for detecting FCoV shedding. 
• A positive fecal FCoV RT-PCR test in a diarrheic cat does not mean that the diarrhea is due to FCoV, other infections must be ruled out: FCoV-associated diarrhea is a diagnosis of exclusion. 
• However, a negative FCoV RT-PCR test on feces generally rules out a diagnosis of FCoV-associated diarrhea (with the rare exception of when the diarrhea is due to the colonic form of FIP where occasionally no virus is released into the lumen). 
• FCoV RT-PCR on blood has poor predictive value (either negative or positive) and serum /plasma should never be submitted for RT-PCR testing because the virus is mostly cell-associated. 
• Mutation M1058L and S1060A tests have poor sensitivity (68.6%) but positive results on effusions were 95.8% specific for FIP. 

FCoV antibody tests 

• A negative FCoV antibody test, provided adequately sensitive, can be used to rule out a diagnosis of FIP or FCoV-associated diarrhea. 
• A negative FCoV antibody test establishes that a cat in contact with a cat with FIP is not at risk of developing FIP. 
• Screening for FCoV antibodies should be performed after 21 days post-exposure, since seroconversion takes 18-21 days. 
• A positive FCoV antibody test only tells you that a cat has been exposed to FCoV: it cannot be used to diagnose FIP or FCoV-associated diarrhea, only to indicate that those conditions could be on your list of differential diagnoses. 
• FCoV antibody tests are effective for screening cats prior to introduction into a FCoV-free multicat environment. 
• FCoV antibody tests can be used to screen stud and queen cats prior to mating. 
• FCoV antibody tests should be routinely used on at-risk animals (eg pedigree or ex-shelter kittens) prior to stressful procedures, such as neutering or even vaccine boosters, to establish if the stress could trigger FIP development. 
• FCoV antibody tests can be used to screen blood donors since passive transfer of FCoV antibody could (theoretically) trigger antibody dependent enhanced disease should the cat be exposed to virus. 
• Monitoring FIP treatment: declining FCoV antibody titers are useful for monitoring recovery from FIP, serving as an indicator that it is safe to discontinue treatment (provided the antibody titers are not being artificially depressed, eg by corticosteroids - and that all other indicators of recovery from FIP are met: for example weight gain, reducing alpha-1 acid glycoprotein and globulin levels, reversal of anemia and lymphopenia). 
• Determination of cessation of FCoV shedding: where accurate RT-PCR testing is not available, FCoV antibody titers reducing to below 1:25 is an indicator of recovery from infection (ie that the cat is no longer shedding virus in feces).