Psoriatic arthritis (PA) can be difficult to diagnose because in adults, psoriasis and arthritis rarely occur at the same time. In addition, PA also mimics other conditions, such as:

• Rheumatoid arthritis (RA). Occurs when an autoimmune response causes inflammation in the lining of the joints, especially those in the hands and feet.
  
  
• Gout. Arthritis characterized by sudden, severe attacks of pain, redness and tenderness in a single joint, usually at the base of the big toe.
  
  
• Reiter's syndrome. Form of reactive arthritis that inflames the joints, eyes, genitals, and urinary or digestive tract.
  
  
• Infection. An infection of the joint can mimic arthritis as it causes a red, swollen and inflamed joint. Psoriasis patients can have an episode of infectious joint without any evidence of arthritis related to the psoriasis.

In attempting to diagnose PA, a physician will review the patient’s medical history and perform a physical examination. Additional tests that may be used to diagnose PA include:

• X-rays. Can reveal changes in the joints that occur in PA. In severe disease, the distal interphalangeal (DIP) joints of the fingers or toes give a distinctive "pencil-in–cup" appearance on an x-ray.
  
  
• Arthrocentesis. A small sample of fluid is removed from a joint, typically in the knee, for laboratory analysis. A physician can use this test to rule out gout, which is indicated by the presence of uric acid crystals.
  
  
• Erythrocyte sedimentation rate (ESR). Blood test that checks ESR, or "sed rate," by measuring how far from the top of a glass tube red blood cells (erythrocytes) fall in a given time. Generally, blood cells fall when inflammation is present. However, this test alone cannot confirm the presence of PA as inflammation can be caused by many factors.
  
  
• Rheumatoid factor (RF) test. RF is an antibody (protein made by the immune system) present in the blood of people with RA, lupus or certain other diseases. This test can help determine if a patient's symptoms are due to PA or another condition.
  
  
• Skin biopsy. This may be needed to rule out other conditions, such as fungal infection.