Tests to Confirm the Diagnosis
A person with joint aches and pains should talk to a doctor about diagnosis and treatment. Primary-care doctors or dermatologists can treat psoriatic arthritis, but psoriatic arthritis patients should consider seeing a rheumatologist, a doctor who specializes in arthritis.
There is no definitive test for psoriatic arthritis. The diagnosis is made mostly on a clinical basis and by a process of elimination. Medical history, physical examination, blood tests, MRIs and X-rays of the joints that have symptoms may be used to diagnose psoriatic arthritis. It is important to communicate your history of psoriasis to your doctor.
The symptoms of psoriatic arthritis are similar to those of three other arthritic diseases: rheumatoid arthritis, gout and reactive arthritis. Rheumatoid arthritis generally involves joints symmetrically distributed on both sides of the body, and it may produce bumps under the skin that are not present in psoriatic arthritis. However, some forms of psoriatic arthritis look very similar. The simultaneous presence of psoriasis on the skin and nail changes supports a diagnosis of psoriatic arthritis.
A certain antibody, called a rheumatoid factor, is normally present in rheumatoid arthritis. The rheumatoid factor is not usually found in the blood of psoriatic arthritis patients. A blood test for that antibody may help distinguish between the two diseases. A person can have rheumatoid arthritis and psoriatic arthritis, but that is rare. Many of the treatments for psoriatic arthritis and rheumatoid arthritis overlap.
Likewise, it is possible to have gout along with psoriasis and psoriatic arthritis. If you have an excruciatingly painful attack in a joint, particularly in the big toe, you may want to have a test for gout. Fluid drawn from the affected joint is examined to resolve the diagnosis of gout or psoriatic arthritis. Psoriatic arthritis patients are commonly misdiagnosed as having gout, because they often have elevated serum uric acid levels, which also can be caused by taking low-dose aspirin or by increased skin cell turnover. It is important to distinguish between the two forms of arthritis, because they may be treated with different medications.
In the very early stages of the disease, X-rays usually do not reveal signs of arthritis and may not help in making a diagnosis. In the later stages, X-rays may show changes that are characteristic of psoriatic arthritis but not found with other types of arthritis, such as the "pencil in cup" phenomenon where the end of the bone gets whittled down to a sharp point. Changes in the peripheral joints and in the spine support the diagnosis of psoriatic arthritis. However, most of the changes occur in the later stages of the disease.