The diagnosis of feline infectious peritonitis is based on a combination of historical details, clinical signs and lab tests. A blood test may be conducted to determine whether the cat has been exposed to feline coronavirus.

This test detects t presence of antibodies against the feline coronavirus. These antibodies are produced when the cat is exposed to the virus. Mere exposure however does not mean that the cat has feline infectious peritonitis. To make a definitive diagnosis, a lab technician will conduct a series of tests. The biochemistry profile of the cat will be analyzed. Low red blood cell count, elevation in the levels of proteins and globulins are indicative of FIP. In case of wet peritonitis, the fluid from the chest or abdomen will also be analyzed. X-rays will also help identify the accumulation of fluid in the chest or abdominal cavity of the cat.

 An ultrasound will also give an idea about the condition of the internal organs and help identify whether the organs are surrounded by fluid. Another test used to diagnose feline infectious peritonitis is the DNA test or PCR. This analyses DNA samples from the blood to detect the presence of viral genetic material.