Clinical observations

• There is not one scientific test to diagnose psoriatic arthritis in a patient. A physician makes this diagnosis based on clinical observations, such as psoriasis (patchiness, sores) of the skin or arthritis of the joints. The physician will check for acne, ridges in the fingernails, joint stiffness and inflammation of the Achilles tendon.
  
  

Patient and family history

• Determining a patient's full history can help a physician make an accurate diagnosis of psoriatic arthritis. For instance, individuals with a family history of arthritis are more susceptible to the disease. Also, patients with AIDS or other immunodeficiency disorders are at increased risk.
  
  

Tests

• Laboratory test results may suggest, but not guarantee, that a patient has psoriatic arthritis. For instance, increased sedimentation rate in the blood can occur in affected patients. This procedure tests how long it takes for red blood cells to settle in a test tube, and rapid sedimentation can be indicative of an inflammatory disease. Also, a physician may perform X-rays to look for weakening cartilage and bone tissue, another warning sign of psoriatic arthritis.
  
  

Genetic predisposition

• Doctors may also test for the presence of HLA-B27, a gene that predisposes an individual to psoriatic arthritis. This gene is often, but not always, a factor in the inflammation of the joints and skin of psoriatic arthritic patients.
  
  

Ruling out other conditions

• Other tests may be performed to rule out other similar conditions. For instance, a patient's rheumatoid factor can be measured in the blood to check for rheumatoid arthritis. An arthrocentesis may also be performed, collecting fluid from an inflamed knee to test for gout or synovial infections (infection of the fluid around the joint).