The Rheumatoid Factor (RF) test is a blood test that can find a special protein called RF. This protein is often found in higher amounts in people who have certain diseases where the body’s immune system attacks its own parts, like in rheumatoid arthritis, SLE (a disease that can affect many parts of the body), scleroderma (a disease that makes the skin thick and hard), and Sjogren’s syndrome (a disease that makes the eyes and mouth dry). It can also be higher in people with some infections like leprosy, tuberculosis, and syphilis, and in some serious diseases like cancer and thyroid disease. Older people who are healthy might also have higher RF levels.