Glossary
Rheumatoid factor
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.
What the research found.
One review pooled the studies of IgM rheumatoid factor, where the test 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. Anti-CCP caught 67 percent, between 62 and 72. It cleared 95 percent, between 94 and 97.
Two databases recorded consecutive rheumatoid arthritis patients, where three tests read normal in 15 percent of one group and 14 percent of the other. Those three are the sedimentation rate, CRP, and rheumatoid factor. All three read abnormal in only 28 percent and 23 percent, so the textbook picture was the minority in both places.
One review covered 151 studies of anti-CCP, where the share of cases caught ran from 12 to 93 percent and the share of healthy people cleared ran from 63 to 100 percent. Study design drove part of that spread. Studies that started with people already diagnosed overstated the catch rate by about 8 percentage points.
In early rheumatoid arthritis, study design changed the answer. Studies that followed a group forward gave an anti-CCP catch rate of 54 percent, between 48 and 60, and cleared 95 percent, between 93 and 97. Two other designs gave 66 percent, between 59 and 72, and cleared 94 percent, between 92 and 95.
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.
Why a positive result settles less than it sounds like
Clearing 85 percent leaves 15 percent, so roughly 15 percent of people without rheumatoid arthritis test positive anyway. That isn't a rare event. Roughly one in seven is a large enough share to change how a result gets read, because a positive result in somebody with no joint symptoms is one situation and a positive result with swollen finger joints and morning stiffness is another entirely.
Rheumatoid factor also appears in other autoimmune conditions, in some long infections, and in people with no disease at all. So the antibody being present says something about immune activity without saying which disease produced it. Read it alongside the symptoms and the exam. That's the only way it counts for anything.
The comparison that makes this concrete is anti-CCP, which cleared 95 percent of healthy people against 85 percent for rheumatoid factor. So anti-CCP gives far fewer false alarms, and that's the whole reason it now counts for more. What it doesn't do is find more cases.
Why a negative result settles even less
This is the more serious of the two errors. The test caught 69 percent of the people who had rheumatoid arthritis, so roughly three in ten of them test negative, and anti-CCP is no better on this count, having caught 67 percent. Both tests miss about a third of the people who have the disease.
The combined picture from ordinary care is more striking than either figure on its own. Two databases recorded consecutive rheumatoid arthritis patients, one in Finland and one in the United States, where three tests read normal in 15 percent of one group and 14 percent of the other. Those three are the sedimentation rate, CRP, and rheumatoid factor, and all three read abnormal in only 28 percent and 23 percent, so the textbook picture with everything raised was the minority in both places.
Seronegative rheumatoid arthritis is a recognized diagnosis, meaning the disease with negative blood tests, and it isn't an oddity to explain away. If your joints swell and stiffen and your rheumatoid factor is negative, that's a reason to keep asking why rather than a reason to stop. It's the single most useful thing to take away from this page.
The accuracy figures probably overstate what you'll get
There's a further problem with the published numbers, and it's about how the studies were built rather than about the tests. One review covered 151 studies of anti-CCP and found that design drove the results. Studies that started with people already diagnosed overstated the catch rate by about 8 percentage points.
Early rheumatoid arthritis is the situation a person with new symptoms is in. Studies that followed a group forward gave a catch rate of 54 percent, between 48 and 60, and cleared 95 percent, between 93 and 97. Two other designs, which looked at one moment or started with people already diagnosed, gave 66 percent, between 59 and 72, and cleared 94 percent, between 92 and 95. Across all 151 studies the catch rate ran from 12 to 93 percent and the clearing rate from 63 to 100 percent.
Our record for the rheumatoid factor review comes with a related warning. Only one of its included studies met all the quality criteria, and the reviewers didn't properly weight study quality when they pooled. So read the 69 and 85 percent figures as approximate rather than as settled properties of the 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 ran 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.
Questions patients ask.
What is rheumatoid factor?
An antibody your immune system makes against part of your own antibodies, which sounds strange, and it's what the blood test looks for. It was named for rheumatoid arthritis, the disease it was first found in. The name has comfortably outlived its accuracy since then, because the antibody turns up in other autoimmune conditions, in some long infections, and in plenty of people with no disease at all.
My rheumatoid factor is positive. What does that mean?
On its own, less than the name suggests. One review pooled the studies, where the test cleared 85 percent of the people who didn't have rheumatoid arthritis, so around 15 percent of them test positive. The result counts for a good deal next to joint symptoms, an exam, and other tests, and for very little alone. A positive result in somebody with no joint symptoms is a different situation again.
My rheumatoid factor is negative. Am I in the clear?
Not necessarily, and this is the reading that causes more harm. The test caught 69 percent of the people who had rheumatoid arthritis, so about three in ten of them test negative, and seronegative disease is a recognized diagnosis, meaning the disease with negative blood tests. If your joints swell and stiffen and won't settle, a negative result isn't a reason to stop asking why.
Why do I hear that anti-CCP is better?
Because of the false alarm rate. Anti-CCP cleared 95 percent of healthy people while rheumatoid factor cleared 85 percent, so anti-CCP gives far fewer false alarms and a positive result therefore means more. On finding cases the two are close, at 67 and 69 percent, and both miss about a third of the people who have the disease.
Are both tests usually sent together?
They commonly are, and they answer questions that overlap rather than one question twice. They measure different antibodies, so a person can be positive for one and negative for the other. What neither one does is rule the diagnosis out, because a large share of people with rheumatoid arthritis test negative on both, and many read normal on their inflammation markers too.
Do the published accuracy numbers apply to me?
Probably less than they look like they do, and the reason is study design. One review of 151 studies found that design drove the results, where studies starting 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 the studies the catch rate ran from 12 to 93 percent. So the tests don't all behave alike.
Does a higher rheumatoid factor level mean more?
This site holds no source on that, so this page can't answer it. The review we hold reports one pooled catch rate and one clearing rate, and it doesn't report how the test performs by level. If you've been told a number means something specific, asking where that came from is fair, and sending us the reference would let this page cover it.
References.
- 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
- 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
- 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
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.