Making the Diagnosis
With careful analyses of history, physical exams, X-rays, and the course of the symptoms over time, a doctor is able to diagnose psoriatic arthritis and eliminate other possible conditions. Often, the skin symptoms of psoriasis (present or past) are clear indicators. Pitting of the fingernails is one sign that joint pain is probably caused by psoriatic arthritis and not some other form of arthritis.
The first step is to take an X-ray of the joints to see what changes are taking place. Next, a doctor may do blood tests and other evaluations to look for other possible causes. Tests of fluid in the joints can also rule out gout, which may have arthritic symptoms very similar to psoriatic arthritis. This process of elimination is necessary because there's no test that confirms a diagnosis of psoriasis or psoriatic arthritis. That's why diagnosis of psoriatic arthritis is usually based on the history and the characteristic presence of both skin and joint symptoms.
Blood tests are not helpful in diagnosis, but in 40% of cases the ESR (erythrocyte sedimentation rate, which is a measure of inflammation) is elevated and can be used to help determine how the disease is progressing.