By Bethany Afshar WebMD Feature

If you are one of the 7 million people in the United States who have the skin condition psoriasis, you should know the signs and symptoms of psoriatic arthritis. As many as 40 percent of people with psoriasis also have psoriatic arthritis.

The causes of psoriasis and psoriatic arthritis are not completely understood. Genetics, the environment, and your immune system are all thought to play roles. Some scientist believe that your immune system attacks your skin when you have psoriasis. When you have psoriatic arthritis, it attacks the joints, causing inflammation.

It can be hard to find out you have psoriatic arthritis. There are no significant tests that can be done to support diagnosis. Doctors must exclude other types of arthritis, and patients must have a history of psoriasis or have active psoriasis to get the diagnosis, says Erin Boh, MD, chairman of dermatology at Tulane University Health Sciences Center in New Orleans.

Your joint symptoms may be mild, so your doctor may suspect other typical causes of pain, says Eric Matteson, MD, rheumatology chair at Mayo Clinic in Rochester, MN.

The first symptoms may not seem related to your skin condition, Matteson says. But that stiffness in your back may be caused by spine inflammation. Its important to get a correct diagnosis so that you can be treated before your joints have permanent damage.

The condition can develop very slowly. In most cases, a person will develop or have psoriasis before he has signs of psoriatic arthritis.

In the less common cases where joint problems show up before the skin symptoms, it can be even more difficult to diagnose. When it does affect the joints of the arms and legs, it can cause swelling that looks like rheumatoid arthritis and be confused with this disease, Matteson says.

Your doctor will ask you questions about your medical and family histories, as well as examine you for swollen and tender joints. You may have an X-ray to see if you have joint damage, and blood tests can help rule out other diseases.

Skin specialists, known as dermatologists, are often the first to suspect psoriatic arthritis in people theyre treating for psoriasis.

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Tests That Diagnose Psoriatic Arthritis