RA and PsA are both autoimmune diseases. They have similar symptoms, so it’s important to get an accurate diagnosis from a rheumatologist. A rheumatoid factor (RF) blood test can check for a protein found in people with rheumatoid arthritis. People with PsA usually don't have it. Blood tests looking for other antibodies can also help differentiate the two.
Another way to tell is to look at your skin and nails. If you have scaly patches on your skin, pitting and flaking on your nails, or both, you have PsA.
Once you've had the disease for a while, X-rays might also be able to distinguish the two conditions.
It is possible to have RA and PsA together, but it's rare.