Rheumatoid factor

Rheumatoid factor is an antibody directed against the tail piece of your own IgG antibodies. That sounds odd, but it is an ordinary reaction of the immune system to lasting stimulation: with a persistent infection, a chronic inflammation and also with getting older, some people make this antibody without anything being wrong with their joints. In rheumatoid arthritis the stimulation is the disease itself, and there rheumatoid factor is present in about seven in ten people. That makes the test sensitive but not specific. A positive rheumatoid factor means the immune system is stimulated, not by what. Five percent of healthy people have it, rising to one in five over seventy; smokers have it more often, and hepatitis C, long-lasting infections, Sjögren's syndrome and lupus produce it too. Anti-CCP is the younger test that solves that problem by being positive almost only in rheumatoid arthritis. Your report shows rheumatoid factor in IU/ml with a limit that differs per lab: one lab calls the result negative below 14 IU/ml, another already below 3.5 IU/ml. A value between those two is therefore called negative at one lab and positive at another. The higher the value above your own lab's limit, the more it says.

Rheumatoid factor is worth measuring with joint complaints, and then above all together with anti-CCP. With painful, swollen joints that are stiff in the morning and affected on both sides, a positive rheumatoid factor makes rheumatoid arthritis more likely, and positive on both antibodies is the strongest pattern; a high rheumatoid factor also goes with a form of the disease that runs more stubbornly. Because early treatment makes the difference between joints that stay intact and joints that deform, that is the situation in which this number counts. Without joint complaints a positive rheumatoid factor says little. Most people with a positive result do not have rheumatoid arthritis and will not get it; the antibody then belongs to age, to smoking, to a past infection or to nothing in particular. That is why this is not a test for a health check, and why in general practice it is no longer used as a first step when rheumatoid arthritis is suspected: the joints themselves say more than the blood. What you influence yourself is small but real: smoking raises the chance of rheumatoid factor and of rheumatoid arthritis, and healthy gums and a healthy weight lower that chance. That is the same prevention as with anti-CCP.

A positive rheumatoid factor is, in order of how often it occurs: rheumatoid arthritis in people with joint complaints, an ordinary finding in older people and smokers, a reaction to a lasting or past infection such as hepatitis C, and another autoimmune disease such as Sjögren's syndrome or lupus. The higher the value, the more likely a real cause; a low-positive value in someone without complaints is usually noise. A negative rheumatoid factor is the ordinary outcome, and does not rule rheumatoid arthritis out: three in ten people with the disease do not have it, especially at the start. With persistent complaints it is repeated after a while, together with anti-CCP. What you do about it depends on the context. With complaints and a positive result, seeing a rheumatologist quickly is the step that protects the joints. Without complaints there is nothing to treat and much to gain from not smoking, healthy gums and staying alert to swollen joints. Read rheumatoid factor beside anti-CCP, which is more specific, beside CRP and ESR, which show whether there is inflammation, and beside the joints themselves. Compare over time at the same lab, because the limit differs per lab by a factor of four.