Abnormal liver function tests are commonly used to help in diagnosis as the spleen and liver are strongly affected by one another. Splenomegaly
If rheumatoid arthritis is present and other symptoms occur that are not commonly found within RA itself, such as a palpable spleen, further testing should be done. A palpable spleen is not always a clinical significance, therefore CT scan, MRI, or ultrasound can be administered in order to help diagnose the condition. According to Poulin et al., dimensional guidelines for diagnosing splenomegaly are as follows:
Moderate if the largest dimension is 11–20 cm
Severe if the largest dimension is greater than 20 cm

Rheumatoid arthritis
RA in patients with Feltys syndrome is chronic (after 10–15 years), and presents with increased severity along with extra articular manifestations. RA can be mistaken for other conditions such as gout if not clinically diagnosed. Diagnosis can be confirmed by use of X-rays or synovial fluid analysis. Treatment
There is no real treatment for Feltys syndrome, rather the best method in management of the disease is to control the underlying rheumatoid arthritis. Immunosuppressive therapy for RA often improves granulocytopenia and splenomegaly; this finding reflects the fact that Feltys syndrome is an immune-mediated disease. A major challenge in treating FS is recurring infection caused by neutropenia. Therefore, in order to decide upon and begin treatment, the cause and relationship of neutropenia with the overall condition must be well understood. Most of the traditional medications used to treat RA have been used in the treatment of Feltys syndrome.
The spleen is an important lymphatic organ that is involved in filtration of the blood by discarding old and damaged red blood cells as well as maintaining platelet levels. The spleen is a lymphatic organ, which means it is largely involved in the immune system and immune responses. When the spleen becomes enlarged, it is a strong sign of infection somewhere in the body and can be caused by inflammatory conditions such as rheumatoid arthritis. The increased need for production assistance of white blood cells to affected areas causes hyperfunction of the spleen. This increase in defense activities ultimately causes hypertrophy of the spleen, leading to splenomegaly. The spleen is found in the left upper quadrant (LUQ) of the peritoneal cavity and due to its enlargement, can cause stress on neighboring organs. Diagnosis
This condition affects less than 1% of patients with rheumatoid arthritis. The presence of three conditions: rheumatoid arthritis, an enlarged spleen (splenomegaly), and an abnormally low white blood cell count are indications that Feltys syndrome is possibly occurring. This condition as a whole is difficult to diagnose due to its complexity given a combination of disorders. It is commonly overlooked or misdiagnosed as other conditions (e.g., leukemia, systemic lupus erythrematosus) because of the rarity and lack of good understanding about it. An acronym can be used to make recognizing this disease somewhat easier:S: SplenomegalyA: AnemiaN: NeutropeniaT: ThrombocytopeniaA: Arthritis (rheumatoid)

Conditions of the blood
A complete blood count (CBC) can be done to diagnose anemia (normochromic, normocytic), thrombocytopenia, and neutropenia.