Psoriatic arthritis differs from rheumatoid arthritis in several ways. First, psoriatic arthritis more frequently involves the last joint of the fingers and toes, whereas rheumatoid arthritis more commonly affects the hand, finger and toe joints closest to the palm. Psoriatic arthritis can affect joints on the fingers and toes in characteristic ways that can be revealed through x-ray at a certain stage.

Secondly, psoriatic arthritis is more likely to appear asymmetrically (affect one joint). Rheumatoid arthritis is more likely to appear symmetrically (affecting a pair of joints). The simultaneous presence of psoriasis and nail malformation are also markers of psoriatic arthritis. Nail malformation is rare in rheumatoid arthritis. Nodules under the skin do not appear in psoriatic arthritis as they do in rheumatoid arthritis. The rheumatoid factor is usually absent in the blood of psoriatic patients.

Choosing a treatment
There is no correlation between location of psoriatic skin lesions and the location of joints involved in the arthritis. In other words, the arthritis does not necessarily appear in the part of the body where the lesions are located. One exception to this might be when there is no substantial involvement of the fingers or toes. It is thought by some investigators that improvement of the psoriasis-involved skin is associated with improvement of the arthritis. There is no scientific evidence that there is a correlation, but investigators report that with some individuals, joint disease remits when the psoriasis clears. They, therefore, recommend that the skin be treated along with the arthritis.