Diagnosis of feline lymphoma requires a series of blood tests, radiographic studies, bone marrow aspiration and biopsies. A complete blood profile will be done. Factors like complete blood count, blood platelet count and biochemistry profile will be recorded. This will help identify the presence of anemia or other illnesses like liver or kidney associated diseases which can interfere with treatment. 

A physical examination may reveal the present of tumors and abnormal growths in the abdomen or chest cavity. This can be confirmed by radiographic studies like X-rays, MRI scan and CT scan. These tests will show the size and extent of the tumors and also help identify the organs which have been affected. If radiographic studies show that organs like liver and spleen are enlarged, fine needle aspiration of the fluid in these organs may be recommended. This will help identify whether these organs are infiltrated with lymphoma cells.

 Fluid from the bone marrow will also be aspirated and analyzed. A biopsy of tissue sample from the affected regions will help confirm diagnosis. It is also essential to identify whether the lymphoma is associated with FeLV infection. In such cases, the treatment regimen will have to be altered accordingly.