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Blood Test Tracker

Rheumatoid Factor (RF)

A blood test for rheumatoid factor, an antibody often found in rheumatoid arthritis but also in other conditions and in some healthy people.

AI-verified against cited sources. Awaiting expert reviewAlso called RF, RA FactorLOINC 11572-5

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What it is

Rheumatoid factors (RF) are antibodies that attach to other antibodies, mostly a common type called IgG. Most are of the IgM type, and some are IgA or IgG. The immune system can make them in autoimmune conditions, during some long-lasting infections, and sometimes for no clear reason. Many healthy people have small amounts, especially older adults. Labs measure RF with several methods, so the cutoff on your own report is the one to use.

Why it matters

RF is one of the blood tests used when joint pain, swelling or stiffness might be rheumatoid arthritis (RA). More than 75% of people with RA have IgM RF, but the test is not specific. It also turns up in other conditions and in healthy people, and some people with RA test negative. For that reason, clinicians read it together with your symptoms, a joint exam and other tests such as anti-CCP antibodies. Higher levels are linked to more severe RA, but the number does not follow day-to-day disease activity well.

If it’s high

A result above your lab's cutoff means rheumatoid factor was found. This fits with RA when you also have the right symptoms, but it is also seen in other autoimmune conditions, some infections and some healthy people, so it cannot name the cause by itself. Higher levels make a related condition more likely, but people without RA can have high levels too.

Common causes

  • Older age, since RF is more common in healthy people aged 60 and older
  • A low-level positive in a healthy person with no related condition
  • Long-lasting infections such as hepatitis C, tuberculosis or infection of the heart valves (endocarditis)
  • Other autoimmune conditions, such as Sjögren syndrome, lupus, scleroderma or juvenile idiopathic arthritis
  • Rheumatoid arthritis
  • Certain cancers (uncommon)

What can help

  • Ask whether an anti-CCP antibody test would add information
  • Ask whether other blood tests, such as ESR or CRP, a joint exam or imaging make sense for your symptoms
  • Keep a simple record of which joints hurt, how long morning stiffness lasts and any swelling, and share it with your clinician

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–14 IU/mL. Source: ARUP Laboratories, Quest Diagnostics, Labcorp.

What can affect your result

  • Age: RF is more common in healthy people aged 60 and older
  • Method: latex tests, turbidimetry or nephelometry, and IgM-only assays have different cutoffs
  • Other conditions: long-lasting infections and other autoimmune diseases
  • Sample quality: markedly fatty (lipemic) or contaminated samples can give false-positive results
  • Markedly fatty (lipemic) or contaminated samples can give false-positive results (Labcorp), and hemolyzed samples can be rejected (ARUP, Labcorp).
  • ARUP lists a fasting specimen as preferred for this test. Mayo, Quest and Labcorp do not mention fasting.
  • Methods differ. Latex agglutination is more prone to false positives than other methods (Labcorp), and IgM-only or IgA-only assays use different cutoffs from total RF.
  • Older age, long-lasting infections and other autoimmune conditions can raise RF without rheumatoid arthritis.

RF levels track disease activity poorly (Mayo), so the number is not used to follow day-to-day changes in arthritis.

When to talk to a clinician

Talk with a clinician if you have joint pain, stiffness or swelling that keeps coming back or does not settle, especially if it is worse in the morning or after resting, if joints feel warm or look red, or if you also have fever, tiredness or hard bumps under the skin near your joints.

Questions you could ask

  • How does my RF result fit with my joint symptoms?
  • Should I have an anti-CCP antibody test or other tests as well?
  • Could something other than arthritis, such as an infection or another autoimmune condition, explain this result?
  • Should I see a rheumatologist?

Questions people ask

Does a positive RF mean I have rheumatoid arthritis?

Not by itself. Many healthy people, especially older adults, have some RF, and it also occurs in other autoimmune diseases and some infections. Your clinician looks at your symptoms, a joint exam and other tests.

Can I have rheumatoid arthritis with a negative RF?

Yes. MedlinePlus notes that people with RA can have little or no RF, and Mayo Clinic Laboratories reports that about 15% of people with RA test negative. Anti-CCP antibodies and other tests can add information.

Why is my lab range different from another lab's?

Labs use different methods and cutoffs. For total RF the cutoff is usually 14 or 15 IU/mL, and IgM-only assays can use a much lower one. Compare your result with the range printed on your own report.

Show the science and sources

Units

  • 1 IU/mL = 1 IU/mL
  • 1 IU/mL = 1 kIU/L

Is a change real?

RF naturally varies about 8.5% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 26% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).

For clinicians and researchers

Rheumatoid factors are autoantibodies directed against the Fc fragment of IgG. Most are IgM, but IgG and IgA isotypes occur (Labcorp). Mayo reports that more than 75% of people with RA have IgM RF, that IgM RF had a sensitivity of 69% and specificity of 85% in a meta-analysis (anti-CCP: 67% and 95%), and that 15% of RA patients are RF-negative while 8% of non-RA patients test positive. A 2023 meta-analysis of RF isotypes found a sensitivity of 68.6% for total RF by latex or nephelometry, and the highest specificity (91.4%) for the IgA isotype. Labcorp notes that latex agglutination is positive in 70% to 85% of RA with many false positives, that IgM RF has been reported in as much as 10% of the normal population, and that prevalence rises with age. In unselected hospital requests the positive predictive value was only 24% to 34% in one study (Shmerling and Delbanco 1991). In the 2010 ACR/EULAR RA classification criteria the serology domain scores 2 points for a low-positive RF or anti-CCP (above the upper limit of normal but no more than 3 times it) and 3 points for a high-positive result (more than 3 times the upper limit of normal). Total-RF cutoffs: Quest (immunoturbidimetric) below 14 IU/mL; Labcorp (latex immunoturbidimetry) below 14.0 IU/mL; ARUP (immunoturbidimetry) 0 to 14 IU/mL; Mayo (turbidimetry) below 15 IU/mL. Isotype-specific assays differ: Mayo's RF IgM chemiluminescent assay is negative below 5.0 IU/mL and its RF IgA below 20 CU. kIU/L is numerically identical to IU/mL.

LOINC 11572-5 (primary), 6928-6, 15205-8, 33910-1, 5297-7, 17857-4, 5299-3. Specimen: serum-or-plasma.

Sources

  1. Rheumatoid Factor (test 0050465), ARUP Laboratories
  2. Rheumatoid Factor (test 4418), Quest Diagnostics
  3. Rheumatoid Factor (RF) (test 006502), Labcorp
  4. Rheumatoid Factor, Serum (test RHUT), Mayo Clinic Laboratories
  5. Rheumatoid Factor Panel, Serum (test RFPN), Mayo Clinic Laboratories Extended Catalog, Mayo Clinic Laboratories
  6. 2010 Rheumatoid arthritis classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative, Annals of the Rheumatic Diseases (ACR/EULAR) (2010)
  7. Refining serological scores in rheumatoid arthritis classification: an international study (describes the 2010 low-positive and high-positive definitions), RMD Open (2026)
  8. Rheumatoid Factor (RF) Test, MedlinePlus (U.S. National Library of Medicine)
  9. Rheumatoid factor isotypes in rheumatoid arthritis diagnosis and prognosis: a systematic review and meta-analysis, RMD Open (2023)
  10. The rheumatoid factor: an analysis of clinical utility, The American Journal of Medicine (1991)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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