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Glossary

Rheumatoid factor

An antibody aimed at part of your own antibodies. It's named for the disease it was first found in, and it turns up in people who don't have that disease.

Rheumatoid factor was named for rheumatoid arthritis, and it isn't specific to it. The test that replaced it in weight finds no more cases than it does. It just gives fewer false alarms.

Quick answerRheumatoid factor is an antibody aimed at part of your own antibodies. One review pooled the studies, where the test caught 69 percent of the people who had rheumatoid arthritis and cleared 85 percent of the people who didn't. Anti-CCP caught 67 percent and cleared 95 percent, and that last figure is why it counts for more.
Neither test finds about a third of the cases, which is the number behind one term you'll hear, seronegative rheumatoid arthritis, meaning the disease with negative blood tests. Two databases recorded ordinary care in consecutive patients, where all three tests read normal in 15 and 14 percent, those three being the sedimentation rate, CRP, and rheumatoid factor. All three read abnormal in only 28 and 23 percent. So a negative result is common in people who really have the disease, and treating it as reassurance is the most common misreading of this test.

What it is, and why the name misleads

Rheumatoid factor is an antibody that targets part of your own antibodies. That sounds odd on first reading, and it's what the blood test looks for. It was named for rheumatoid arthritis, the disease it was first found in, and the naming happened decades before anybody knew how unspecific it would turn out to be.

The name has outlived its accuracy, and it does real damage. A test called rheumatoid factor sounds like a test for rheumatoid arthritis, which invites both of the misreadings this page exists to correct. A positive result gets taken as a diagnosis and a negative result gets taken as an all-clear, and neither one follows from what the test measures.

One review pooled the studies of this test, where it caught 69 percent of the people who had rheumatoid arthritis, between 65 and 73, and cleared 85 percent of the people who didn't, between 82 and 88. Doctors call the first number sensitivity and the second one specificity. Those two numbers explain most of what goes wrong with this test.

What to ask instead

Given all of that, the useful questions aren't about the rheumatoid factor number. They're about what the whole picture supports, which is how a rheumatologist reads these results anyway. The blood tests contribute rather than decide.

If your result is positive and you have no joint symptoms, ask what else would need to be true before this counted for anything. If your result is negative and your joints are swollen and stiff in the mornings, the bigger question is what the plan will be to explain that. Neither conversation is helped by treating one antibody as the answer.

This page holds no source on whether a higher level counts for more, so it doesn't address the level, which is sometimes called the titer. The review we hold reports one pooled catch rate and one clearing rate and doesn't report performance by level. If you've been told a specific number means something specific, asking where that came from is reasonable.

Common misconceptions.

Myth. A positive rheumatoid factor means I have rheumatoid arthritis.

Reality. It doesn't, and the name is most of why people think it does. One review pooled the studies, where the test cleared 85 percent of the people who didn't have the disease, so about 15 percent of them test positive anyway. It also appears in other autoimmune conditions, in some long infections, and in healthy people. So a positive result gets read alongside symptoms and an exam rather than on its own.

Myth. A negative rheumatoid factor rules it out.

Reality. This is the more serious error of the two. The test caught 69 percent of the people who had the disease, so about three in ten of them test negative, and two databases recording consecutive patients found all three blood tests normal in about one in seven. Those three are the sedimentation rate, CRP, and rheumatoid factor. Seronegative rheumatoid arthritis is a recognized diagnosis rather than an oddity to explain away.

Myth. Anti-CCP is the better test because it finds more cases.

Reality. It finds slightly fewer, because anti-CCP caught 67 percent of cases while rheumatoid factor caught 69 percent, so the two of them miss a similar share of the people who have the disease. What separates them is the false alarm rate. Anti-CCP cleared 95 percent of healthy people against 85 percent for rheumatoid factor, so a positive anti-CCP counts for more, and a negative one settles no more than a negative rheumatoid factor does.

Myth. The published accuracy figures apply to my situation.

Reality. They probably overstate what the test can do for you, and that holds most for new joint symptoms with no diagnosis yet. One review covered 151 studies, where studies that started with people already diagnosed overstated the catch rate by about 8 percentage points. In early disease, studies that followed a group forward gave 54 percent while the other designs gave 66 percent, and across all 151 studies the catch rate ranged from 12 to 93 percent.

Related terms.

  • Anti-CCP, the test that carries more diagnostic weight and finds no more cases.
  • Rheumatoid arthritis and blood tests, for how these results are read together.
  • Autoantibody, for the general point that an antibody against yourself isn't a diagnosis.
  • CRP and ESR, the swelling markers that read normal alongside it more often than people expect.

References.

  1. Nishimura K; Sugiyama D; Kogata Y et al. Meta-analysis: diagnostic accuracy of anti-cyclic citrullinated peptide antibody and rheumatoid factor for rheumatoid arthritis. Annals of internal medicine. 2007;146:797-808. 10.7326/0003-4819-146-11-200706050-00008Diagnostic accuracy meta-analysis
  2. Whiting PF; Smidt N; Sterne JA et al. Systematic review: accuracy of anti-citrullinated Peptide antibodies for diagnosing rheumatoid arthritis. Annals of internal medicine. 2010;152:456-64; W155-66. 10.7326/0003-4819-152-7-201004060-00010Diagnostic accuracy systematic review and meta-analysis of 151 studies
  3. Sokka T; Pincus T. Erythrocyte sedimentation rate, C-reactive protein, or rheumatoid factor are normal at presentation in 35%-45% of patients with rheumatoid arthritis seen between 1980 and 2004: analyses from Finland and the United States. The Journal of rheumatology. 2009;36:1387-90. 10.3899/jrheum.080770Retrospective analysis of two consecutive usual-care RA databases

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This page gathers the published research on this subject into one place. The studies behind it were published between 1989 and 2026, and every figure links to the paper it came from. Those studies were peer reviewed. This summary of them was not. Dr. Sarah Luebker is reviewing these pages one at a time and has not reached this one yet, so it carries no medical review date and nothing here is her opinion or her advice to you. Each page gets updated as she reaches it. It is here in the meantime because the science is worth having in one organized place that is easy to find and easy to read. Talk to your own clinician before acting on any of it.