Rheumatoid Factor Unit Converter
Rheumatoid Factor Unit Converter
IU/mL and kIU/L are the same number — convert between them, then read the result honestly: rheumatoid factor is neither necessary nor sufficient for a diagnosis of rheumatoid arthritis, and results are not comparable between assays.
Rheumatoid Factor converter
IU/mL = kIU/LRheumatoid factor 20 IU/mL, on an assay whose upper limit of normal is 14 IU/mL
The units are interchangeable; the results are not
and no conversion exists between one manufacturer’s IU/mL and another’s
- IU/mL = kIU/L
- the same number, so an RF of 20 IU/mL is 20 kIU/L. One international unit per millilitre is one kilo-international unit per litre by definition of the prefixes, and that is the whole of the arithmetic
- an arbitrary unit
- no mass or molar equivalent exists. The assay measures the binding of autoantibody — classically IgM — to the Fc portion of IgG, calibrated against the manufacturer's own standard, so a number without its method is not interpretable
- titres are a different scale again
- older latex agglutination and Rose–Waaler methods report a dilution titre such as 1:80. A titre cannot be converted to IU/mL: they are different measurements of the same phenomenon, and the correspondence between them varies by method
- what the assay actually detects
- an antibody against IgG. That is why RF interferes with other immunoassays and causes false-positive specific IgM results — it is an antibody that binds antibodies, and immunoassays are built out of antibodies
- high-positive has a definition, but a relative one
- the 2010 ACR/EULAR classification criteria define low-positive as above the assay's upper limit of normal and high-positive as more than three times it. The definition is deliberately expressed as a multiple of the local threshold rather than as a number
Worked example
Rheumatoid factor 20 IU/mL, on an assay whose upper limit of normal is 14 IU/mL
20 IU/mL = 20.0 kIU/L — identical, and that is the whole of the conversion
Against this assay's upper limit of 14 IU/mL, 20 is low-positive in ACR/EULAR terms: above the limit but below three times it (42 IU/mL), so it scores 2 points rather than 3 in the classification criteria
Change only the laboratory and the interpretation changes with it. On a platform whose upper limit is 20 IU/mL the same result is negative and scores nothing — which is why this page prints no interval under the answer
Now the clinical reading. A low-positive RF on its own is weak evidence: RF is found in a few per cent of healthy people, more often with age, and in hepatitis C in up to 76% of patients, in Sjögren's syndrome, in cryoglobulinaemia and in chronic infection
And a negative RF does not exclude rheumatoid arthritis. Pooled sensitivity for IgM RF is 69%, so roughly three patients in ten with the disease are RF-negative — which is why an anti-CCP is measured alongside it
Why no cut-off is printed here — published RF thresholds and what they are worth
| Item | Figure | Source |
|---|---|---|
| Pooled sensitivity of IgM RF for rheumatoid arthritis | 69% (95% CI 65–73) — so roughly three patients in ten with RA are RF-negative | Nishimura et al meta-analysis, Ann Intern Med 2007 |
| Pooled specificity of IgM RF | 85% (95% CI 82–88) — appreciably lower than anti-CCP’s 95% | Nishimura et al, 2007 |
| Reported sensitivity across individual studies | 26% to 90%, depending on the population studied and the cut-off chosen | StatPearls, Rheumatoid Factor |
| RF positivity in healthy people | Up to about 4% of young healthy individuals, and of the elderly; prevalence rises as the chosen cut-off falls | StatPearls, Rheumatoid Factor |
| RF positivity in hepatitis C | As high as 76% — the single most important non-rheumatological cause of a positive RF | StatPearls, Rheumatoid Factor |
| Manufacturers’ upper limits of normal | Commonly quoted between about 10 and 20 IU/mL, varying by platform and by the population used to set them | Assay-specific; this is why no interval is printed on this page |
| ACR/EULAR definition of high-positive | More than three times the assay’s own upper limit of normal, not a fixed number | 2010 ACR/EULAR classification criteria |
What a positive rheumatoid factor is found in besides rheumatoid arthritis
| Setting | Comment |
|---|---|
| Healthy people, increasingly with age | A few per cent of healthy individuals are positive, and the proportion rises as the cut-off is lowered. In an unselected population with a low prevalence of RA, most positive RF results are not RA |
| Hepatitis C | Positive in up to 76% of patients, often with mixed cryoglobulinaemia. A patient with arthralgia and a positive RF needs hepatitis C considered before rheumatoid arthritis is assumed |
| Primary Sjögren’s syndrome | Commonly positive, and a positive RF in a patient with dry eyes and dry mouth should prompt anti-Ro/anti-La rather than a diagnosis of RA |
| Mixed cryoglobulinaemia and other cryoglobulinaemic states | Rheumatoid factor activity is part of the pathophysiology rather than an incidental finding |
| Systemic lupus erythematosus, mixed connective tissue disease | Positive in a substantial proportion, which is one reason RF alone cannot distinguish between inflammatory arthritides |
| Chronic infection — tuberculosis, subacute bacterial endocarditis, chronic parasitic infection | Chronic immune stimulation produces rheumatoid factor; successful treatment of the infection often makes it disappear |
| Sarcoidosis, primary sclerosing cholangitis, chronic lung and liver disease | Recognised associations, again reflecting chronic immune stimulation |
Neither necessary nor sufficient
There is nothing to calculate. International units per millilitre and kilo-international units per litre are the same number, so a rheumatoid factor of 20 IU/mL is 20 kIU/L. There is no mass or molar unit, because the assay measures the binding of autoantibody — classically IgM — to the Fc portion of IgG, reported in arbitrary international units against the manufacturer’s own calibrator. Older agglutination methods report a dilution titre such as 1:80 instead, and a titre cannot be converted into IU/mL: they are different measurements of the same phenomenon and the correspondence between them varies by method.
Which leads to the first substantive point. Rheumatoid factor results are not comparable between assays. Manufacturers’ upper limits of normal are commonly quoted anywhere between about 10 and 20 IU/mL, set on different reference populations with different antigen preparations, so the same serum can be clearly positive on one platform and negative on another. The 2010 ACR/EULAR classification criteria handle this by refusing to name a number at all: low-positive means above the upper limit of normal of the assay used, and high-positive means more than three times that limit. This page prints no reference interval under the converted answer for the same reason, and puts the published figures in a labelled table with their sources instead.
The second point is clinical and needs stating bluntly, because the test is widely over-read. Rheumatoid factor is neither necessary nor sufficient for a diagnosis of rheumatoid arthritis. It is not sufficient because a few per cent of healthy people are positive — more often with increasing age, and more often the lower the cut-off — and because it is positive in hepatitis C in up to 76% of patients, commonly in primary Sjögren’s syndrome, in mixed cryoglobulinaemia, in lupus and mixed connective tissue disease, and in chronic infections including tuberculosis and subacute bacterial endocarditis. In a population where rheumatoid arthritis is uncommon, most positive results are something else. It is not necessary because its pooled sensitivity is 69%: roughly a quarter to a third of people with rheumatoid arthritis are rheumatoid factor-negative, and a negative result in a patient with a persistent symmetrical small-joint synovitis changes very little.
What the test is genuinely for is contributing weight to a clinical picture, and doing it alongside an anti-CCP antibody. Anti-CCP has a similar sensitivity — 67% in the same meta-analysis — with a much higher specificity of 95% against rheumatoid factor’s 85%, and the two together are more informative than either alone. Both appear in the ACR/EULAR serology domain, both score more heavily when the titre is high, and both are read with the number of involved joints, the symptom duration and the acute-phase response rather than in isolation. A high-titre RF also carries prognostic information: it is associated with more erosive joint disease and with extra-articular manifestations such as nodules, vasculitis and interstitial lung disease.
Frequently asked questions
Is 20 IU/mL of rheumatoid factor the same as 20 kIU/L?
Yes — IU/mL and kIU/L are numerically identical, so no conversion is needed between them. There is no molar or mass unit, because the result is binding activity in arbitrary international units against the manufacturer’s calibration. A dilution titre from an older agglutination method, such as 1:80, cannot be converted to IU/mL at all.
Why does this page show no reference interval?
Because the cut-off is a property of the assay rather than of the patient. Manufacturers’ upper limits of normal are commonly quoted between about 10 and 20 IU/mL, so a result of 15 IU/mL can be positive on one platform and negative on another. The 2010 ACR/EULAR classification criteria themselves avoid naming a number, defining low-positive as above the assay’s own upper limit of normal and high-positive as more than three times it. Published figures are tabulated above with their sources.
Can you have rheumatoid arthritis with a negative rheumatoid factor?
Yes, and it is common. The pooled sensitivity of IgM rheumatoid factor for rheumatoid arthritis is 69%, so roughly a quarter to a third of people with the disease are RF-negative — so-called seronegative rheumatoid arthritis. A negative RF in a patient with persistent symmetrical small-joint synovitis changes very little, and an anti-CCP antibody should be measured too, though that is also negative in around a third of cases.
What else causes a positive rheumatoid factor?
A great deal. Up to about 4% of healthy people are positive, more often with increasing age and more often the lower the cut-off used. Hepatitis C is positive in as many as 76% of patients and is the most important non-rheumatological cause; primary Sjögren’s syndrome, mixed cryoglobulinaemia, systemic lupus erythematosus and mixed connective tissue disease are frequently positive; and chronic infections such as tuberculosis and subacute bacterial endocarditis, along with sarcoidosis and chronic liver disease, all produce it through chronic immune stimulation.
Is rheumatoid factor or anti-CCP the better test?
Anti-CCP, on specificity: in the same meta-analysis the two had similar sensitivity (67% for anti-CCP, 69% for RF) but specificity was 95% for anti-CCP against 85% for RF. They are not alternatives, though. Both appear in the ACR/EULAR serology domain, both score more heavily at high titre, and measuring the two together is more informative than either on its own — a patient positive for both has a higher probability of rheumatoid arthritis and a worse prognosis than one positive for either alone.
Related calculators
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. Ann Intern Med. 2007;146(11):797–808 — pooled IgM-RF sensitivity 69% (95% CI 65–73), specificity 85% (82–88); anti-CCP sensitivity 67% (62–72), specificity 95% (94–97).
- Aletaha D, Neogi T, Silman AJ, et al. 2010 Rheumatoid arthritis classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative. Ann Rheum Dis. 2010;69(9):1580–1588 — negative 0 points, low-positive (>ULN) 2, high-positive (>3× ULN) 3; total ≥6 of 10 classifies definite RA.
- Ingegnoli F, Castelli R, Gualtierotti R. Rheumatoid factors: clinical applications. Dis Markers. 2013;35(6):727–734. doi:10.1155/2013/726598
- Sharif K, Sharif A, Jumah F, Oskouian R, Tubbs RS. Rheumatoid factor. In: StatPearls. Treasure Island (FL): StatPearls Publishing — RF positive in up to 4% of young healthy individuals and of the elderly, and in as many as 76% of patients with hepatitis C; reported RA sensitivity 26–90% across studies.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.