Specific Diagnostic testing for FIP
Firstly we will not list antibody tests or faecal FCoV tests in this section as they do not serve to help diagnose a case of FIP. They just indicate exposure to Feline Coronavirus, not the presence of the disease FIP.
Even the ideal tests have their problems and this is the case in many aspects of medicine. It becomes very emotional when it is your beloved young cat and there is the chance of successful treatment. It is certain that many cats without FIP have been treated with illegal antivirals. In some cases this treatment has appeared successful but we are yet to understand the consequences of this.
During the process of diagnosis and treatment of FIP, it can be very difficult to understand the changes in this complicated jigsaw of signs and test results. That is one of the reasons we are here – to support you and your vet. Before, during and after your consultation we will review your cat’s history, clinical findings and laboratory tests and aim to communicate these with you and your vet to help with any misunderstandings.
Immunohistochemistry
The ideal test to diagnose FIP infection is called immunohistochemistry. This is performed on a biopsy or post mortem sample of tissue from the cat. This can be a fine needle biopsy, or a trucut biopsy or a surgical biopsy. These tests are all to some degree invasive and carry some risks. The laboratory will look at the cells in the samples under the microscope and examine for signs of inflammation, cancer or infection. This is called histology. In some cases, the results of this will demonstrate a different cause such as lymphoma. If the histology is suggestive of FIP then the immunohistochemistry test will stain any particles of Feline Coronavirus present. If these are present inside the cells of the sample then this is practically certain to be due to FIP. Do remember that this is staining all forms of coronavirus and not specifically looking for mutated forms that cause FIP. Normal Coronavirus would not normally be inside the cells in these samples though. A negative immunohistochemistry test does not absolutely rule out FIP.
Immunocytochemistry
This is similar to the above but performed on cell samples from fluids. The sample collection is less invasive in many cases. Some experts take a positive test on a fluid sample as being confirmed FIP infection. Others rate this as FIP very likely. A negative test does not rule out FIP.
PCR testing – RT-qPCR
PCR stands for Polymerase Chain reaction. It is a technique for amplifying or copying the genetic material present in a sample. It can be so highly sensitive that false positives are possible. For Coronavirus tests we need to use an RT-PCR or Reverse Transcriptase PCR as we are looking for viral RNA and the RT converts it to DNA which is then amplified with the PCR.
To reduce false positive tests, many laboratories quantify or measure the amount of viral RNA present and the test is called RT-qPCR – Reverse Transcriptase – Quantitative Polymerase Chain Reaction. If this test is positive on fluid or tissue samples, FIP is considered very unlikely. A negative test does not rule out FIP. False positives do occur.
Testing on blood or faecal samples is not reliable as cats infected with FCoV will test positive.
Idexx FIP Virus RealPCR Test
This test aims to test for both mutated strains and enteric strains of coronavirus. The results can be viewed much like the RT-qPCR tests. Except a positive for the mutated strain demonstrates that viral mutation has occurred. This does not always imply FIP disease is present. Similar there will be some cases of FIP that have insufficient viral load in the samples to give a positive. Presence of only enteric coronavirus suggest there has been no mutation and makes FIP very unlikely. Idexx describe their test as an effective test to confirm the suspicion of FIP in cats with appropriate other findings.