context: Both rheumatoid arthritis ( RA) and psoriatic arthritis ( Ps A) are conditions that damage your joints, causing swelling, stiffness, and pain. Both are autoimmune diseases, which means your immune system attacks healthy parts of your body by mistake. They can also damage organs such as skin, eyes, and blood vessels. But RA and Ps A differ in key ways. You most likely will have one condition or the other but not both. The same drugs tend to work on both RA and Ps A. When you have RA, your immune system attacks the lining of the tissues around your joints. They swell up and become painful. Over time, they can become damaged and deformed. With Ps A, your immune system attacks and damages not just your joints, but your skin, as well. It causes your body to make too many skin cells, which leads to psoriasis, a skin condition that often affects people with Ps A. RA runs in families. If you have a close relative with the disease, your chances of having it are higher. Women are more likely than men to get RA. Usually the symptoms start between ages 40 and 60. Ps A also runs in families. Certain genes may be linked to the condition. Compared to RA, Ps A often starts between ages 30 and 50. Researchers don't know exactly what triggers either type of arthritis. But they think it probably comes from a mix of genes and other factors, including hormones and bacterial or viral infections that may send the immune system into overdrive. Both RA and Ps A cause joint swelling, stiffness, and pain. Although both conditions affect joints in the fingers and toes, they do it in slightly different ways. And each one can cause other symptoms, too. Rheumatoid arthritis: Often starts in the smaller joints, like the ones in your fingers and toes; over time, it may affect other joints, too, like your wrists, knees, hips, and ankles. Usually shows up on the same joints on both sides of your body (such as both the left and right index fingers); that means it's symmetric. Often makes joints feel stiffer in the morning Can lead to fatigue, low-grade fever, and weight loss Psoriatic arthritis: Can affect joints in the back and pelvis in addition to the ones in fingers and toes Often affects only one side of your body; that means it's asymmetric. Sometimes causes foot pain, especially on the sole of your foot or the back of your heel May make your fingers swell up like sausages May make your nails pit and flake Tends to affect entheses, areas where tendons or ligaments attach to bones With both conditions, you'll probably have times when your symptoms get worse. These are called flares. In between these flares are times without symptoms called remissions. Because these two conditions share similar symptoms, it's important to get an accurate diagnosis from a rheumatologist. A rheumatoid factor ( RF) blood test is one way your doctor can tell which condition you have. RF is a protein found in people with rheumatoid arthritis. People with Ps A usually don't have it. Blood tests looking for other antibodies such as anti- CCP 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 Ps A. 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 Ps A together, but it's rare. If you do have both, many of the treatments, including some medications, will work for both conditions.