A normal rheumatoid factor (RF) is less than 20 units per milliliter of blood. Results at or above that threshold are considered positive, meaning your blood contains higher-than-expected levels of RF, a type of antibody your immune system produces. But a positive result doesn’t automatically mean you have rheumatoid arthritis, and a negative result doesn’t rule it out.
What Rheumatoid Factor Actually Measures
Rheumatoid factor is an antibody that mistakenly targets your body’s own healthy proteins. In a well-functioning immune system, antibodies attach to bacteria, viruses, and other invaders. RF instead latches onto normal antibodies in your blood, forming clumps that can trigger inflammation, particularly in the joints. The RF blood test measures how much of this self-targeting antibody is circulating.
The test itself is a simple blood draw with no fasting or special preparation required. Results come back as a number in units per milliliter (U/mL). Below 20 U/mL is considered normal. Above that, the result is positive, and the higher the number climbs, the stronger the association with autoimmune disease.
What a Positive Result Means
A positive RF is most closely linked with rheumatoid arthritis, but the connection isn’t as tight as many people assume. A large meta-analysis found that only about 69% of people with confirmed rheumatoid arthritis test positive for RF. That means roughly 1 in 3 people with the disease have normal levels. On the flip side, about 4% of young, healthy adults test positive without having any autoimmune condition at all.
The level matters. In the classification system doctors use to evaluate rheumatoid arthritis, an RF result more than three times the upper limit of normal (above 60 U/mL in most labs) carries significantly more diagnostic weight than a mildly elevated result just over the cutoff. A borderline positive, say 22 or 25 U/mL, is far less meaningful on its own.
RF can also be elevated in several other conditions. It’s positive in 75 to 95% of people with Sjögren’s disease, an autoimmune condition that primarily affects moisture-producing glands. Lupus, chronic liver disease, and certain infections can raise RF levels too. Cigarette smokers may also test positive without having any underlying autoimmune disease.
Why Age Changes What “Normal” Looks Like
RF levels tend to rise naturally with age, even in people who are perfectly healthy. Among adults over 65, positive RF results are about 3.5 times more common than in younger people. In one study of apparently healthy individuals aged 67 to 95, the prevalence of elevated RF was as high as 14%. This doesn’t mean these individuals are developing rheumatoid arthritis. The reasons aren’t fully understood, but it’s a well-documented pattern that doctors factor into their interpretation.
If you’re over 65 and receive a mildly positive RF result with no joint symptoms, the likelihood of it being clinically meaningful is lower than the same result in a 35-year-old with swollen, stiff joints.
Why RF Alone Can’t Diagnose Rheumatoid Arthritis
Because RF is positive in so many conditions (and absent in a substantial percentage of RA patients), doctors rarely use it as a standalone diagnostic tool. A second blood test, called anti-CCP, is often ordered alongside it. Anti-CCP is 97% specific for rheumatoid arthritis, meaning a positive result is much more reliably tied to the disease than RF alone.
When both RF and anti-CCP come back positive, especially at high levels, the case for rheumatoid arthritis is strong. When both are negative, RA is still possible. About 20% of people with rheumatoid arthritis are “seronegative,” meaning neither RF nor anti-CCP ever turns positive. In those cases, doctors rely on physical examination, imaging, and the pattern of symptoms to make a diagnosis.
One important detail: once RF and anti-CCP have been used to help establish a diagnosis, they aren’t repeated to track how the disease is doing. Both tend to stay positive even when the disease is in remission, so they’re useful for diagnosis but not for monitoring.
What to Do With Your Results
If your RF came back under 20 U/mL, your result is normal. If you’re still experiencing joint pain, stiffness, or swelling, that normal result doesn’t exclude rheumatoid arthritis or other inflammatory conditions, but it does make them somewhat less likely.
If your RF is mildly elevated (just over 20 U/mL), context matters enormously. Your age, symptoms, smoking status, and results of other blood work all shape what that number actually means. A mildly positive RF in someone with no joint complaints may need nothing more than a note in the chart.
If your RF is strongly positive, particularly above 60 U/mL, the association with autoimmune disease is much more significant. This is typically when additional testing with anti-CCP, inflammatory markers, and imaging becomes most useful in building a clear picture of what’s happening.