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Rheumatoid factor
An antibody supporting the diagnosis of rheumatoid arthritis — informative together with anti-CCP, CRP, and ESR.
Other names: RF, rheumatoid arthritis factorSection: Immunology
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortRheumatoid factor is an IgM antibody against the body's own IgG. A typical normal is below 15 IU/mL (15 kIU/L) or a titer below 1:80. It supports the diagnosis of rheumatoid arthritis but also appears in infections and in healthy older adults.
Reference ranges
Group
Reference range
Concentration — typical range (international reference, not a national standard)
< 15 IU/mL (< 15 kIU/L)
Titer — typical range (international reference, not a national standard)
< 1:80
Values from the MedlinePlus Medical Encyclopedia (international reference, not a national standard). Methods differ — the report's reference applies. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
RF is found in about 70–80% of people with rheumatoid arthritis. High titers correlate with a more aggressive course and with extra-articular disease.
Because low positive results occur in chronic infections, other autoimmune diseases, and in up to 10–20% of healthy elderly people, RF alone never establishes the diagnosis — the joint picture and anti-CCP decide.
When it is high
Rheumatoid arthritisEspecially with symmetric small-joint pain, swelling, and morning stiffness over 6 weeks.
Other autoimmune diseasesSjögren's syndrome, lupus, and systemic sclerosis can be RF-positive.
Chronic infections and ageHepatitis C, endocarditis, and older age raise the rate of false positives.
When it is low
Expected findingMost healthy people are negative; up to a fifth of rheumatoid arthritis patients stay RF-negative.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
AgeChronic infectionsOther autoimmune diseases
How to prepare
Blood from a vein; usually no special preparation. Most informative together with anti-CCP, CRP, and ESR.
When a retest makes sense
A borderline result is repeated together with anti-CCP; the rheumatologist interprets the trend.
Who usually reviews this result
A result outside the reference range is usually reviewed by: a rheumatologist; your GP or primary care doctor for the first step. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: Rheumatoid factorRF
Units and conversion
Measured in: IU/mL
IU/mL=kIU/L
1 IU/mL = 1 kIU/L — the number stays the same. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
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