Anti-CCP Antibody Unit Converter
Anti-CCP Antibody Unit Converter
U/mL and kU/L are the same number — convert between them, and then treat the number as meaningless without its method, because anti-CCP units differ between second- and third-generation assays and between manufacturers.
Anti-CCP Antibody converter
U/mL = kU/LAnti-CCP 40 U/mL, on an assay whose upper limit of normal is 17 U/mL
The units are interchangeable; the numbers are not
and no conversion exists between a CCP2 unit, a CCP3 unit and another manufacturer’s unit
- U/mL = kU/L
- the same number, so an anti-CCP of 40 U/mL is 40 kU/L. Some manufacturers write EU/mL or RU/mL for their own arbitrary units; those are not interchangeable with each other either
- an arbitrary unit
- the assay measures binding to a synthetic cyclic citrullinated peptide, calibrated against the manufacturer's own standard. There is no international reference preparation that makes the units comparable, and no mass or molar equivalent
- CCP2 against CCP3
- second- and third-generation assays use different synthetic peptide antigens, so they detect overlapping but different populations of anti-citrullinated protein antibodies and report on different numerical scales. A result of 40 on one is not a result of 40 on the other
- why the criteria use multiples
- the 2010 ACR/EULAR criteria define low-positive as above the assay's own upper limit of normal and high-positive as more than three times it, precisely so that the scoring survives the incomparability of the units
- the family it belongs to
- anti-CCP is one member of the anti-citrullinated protein antibody family, which is what is really being detected. Other members — anti-mutated citrullinated vimentin, anti-citrullinated fibrinogen — are measured in yet other units and add little in routine practice
Worked example
Anti-CCP 40 U/mL, on an assay whose upper limit of normal is 17 U/mL
40 U/mL = 40.0 kU/L — identical, and the only arithmetic on this page
Against this assay's upper limit of 17 U/mL, 40 is low-positive in ACR/EULAR terms — above the limit but below three times it (51 U/mL) — and scores 2 points in the serology domain rather than 3
A result of 60 U/mL on the same assay would be high-positive and score 3, while 60 on an assay with an upper limit of 25 would not be. The multiple of the local limit is what the criteria score, never the bare number
The clinical reading is stronger than it would be for a rheumatoid factor of the same standing. Anti-CCP's pooled specificity is 95% against RF's 85%, so a positive result in a patient with an inflammatory polyarthritis is much harder to explain away
And it is not only diagnostic. A positive anti-CCP predicts more erosive disease, and in stored blood-donor samples the antibody was detectable years before any symptoms began
Anti-CCP against rheumatoid factor — why both are measured
| Anti-CCP | IgM rheumatoid factor | |
|---|---|---|
| Pooled sensitivity for RA | 67% (95% CI 62–72) | 69% (95% CI 65–73) |
| Pooled specificity | 95% (94–97) | 85% (82–88) |
| What it binds | A synthetic cyclic citrullinated peptide — a post-translationally modified self-antigen | The Fc portion of IgG, so it is an antibody against antibodies and interferes with other immunoassays |
| Positive in healthy people and in chronic infection | Rarely. Hepatitis C, Sjögren’s and chronic infection do not produce it in the way they produce RF | Frequently — up to about 4% of healthy people, and as many as 76% in hepatitis C |
| Timing relative to symptoms | Can precede symptoms by years — detectable in stored blood-donor samples well before the first joint complaint | Also detectable pre-symptomatically, but less specifically |
| Prognostic value | Predicts erosive, more severe disease and supports earlier, more intensive treatment | High titres also predict erosive disease and extra-articular features |
| ACR/EULAR 2010 serology domain | Scored identically to RF: low-positive 2 points, high-positive 3 | Scored identically to anti-CCP; only the highest-scoring result in the domain counts |
Why no cut-off is printed here
| Source of incomparability | Consequence |
|---|---|
| Different synthetic peptide antigens between assay generations — CCP2 against CCP3 | The two detect overlapping but different antibody populations and report on different scales. A CCP3 result cannot be read against a CCP2 threshold |
| Different calibrators between manufacturers, with no international reference preparation | Nominally identical U/mL figures from two platforms are not the same measurement, and published upper limits of normal differ several-fold |
| Different units in the first place — U/mL, EU/mL, RU/mL | A unit that looks convertible often is not. EU/mL and RU/mL are each a manufacturer’s own arbitrary unit |
| Manufacturer-set thresholds, and locally validated ones | Some laboratories raise the cut-off above the manufacturer’s to improve specificity, which changes the meaning of “positive” within a single assay |
| The criteria’s own solution | The 2010 ACR/EULAR criteria score against a multiple of the reporting assay’s upper limit of normal, not against a number — which is the practice this page follows by printing no interval |
More specific than rheumatoid factor, and earlier
The conversion is trivial: units per millilitre and kilo-units per litre are the same number, so an anti-CCP of 40 U/mL is 40 kU/L. There is no mass or molar unit, because the assay measures binding to a synthetic cyclic citrullinated peptide and reports it in arbitrary units against the manufacturer’s own calibrator. Some manufacturers write EU/mL or RU/mL, each of which is their own arbitrary unit and none of which is interchangeable with the others.
That is the first thing to understand about the number, and it is more severe here than for most immunoassays. Second- and third-generation assays — CCP2 and CCP3 — use different synthetic peptide antigens, detect overlapping but different populations of anti-citrullinated protein antibodies and report on different numerical scales. There is no international reference preparation to tie them together. A result of 40 U/mL on one platform is not a result of 40 U/mL on another, published upper limits of normal differ several-fold, and some laboratories deliberately raise the manufacturer’s threshold to improve specificity. An anti-CCP figure is therefore meaningless without the method that produced it, which is why this page prints no reference interval under the answer and why the 2010 ACR/EULAR classification criteria score each result against a multiple of its own assay’s upper limit of normal rather than against a number: low-positive above the limit, high-positive above three times it, worth 2 and 3 points respectively out of the 6 needed to classify definite rheumatoid arthritis.
What makes the test valuable is its specificity. In Nishimura and colleagues’ meta-analysis anti-CCP and rheumatoid factor had almost identical sensitivity — 67% against 69% — but anti-CCP’s specificity was 95% against rheumatoid factor’s 85%. The conditions that generate false-positive rheumatoid factors in quantity, principally hepatitis C, Sjögren’s syndrome and chronic infection, do not generate anti-CCP in the same way. A positive anti-CCP in a patient with an inflammatory polyarthritis is therefore much harder to explain away than a positive rheumatoid factor.
Two further properties matter clinically. The antibody can appear long before the disease does: in stored serum from blood donors who later developed rheumatoid arthritis, anti-citrullinated protein antibodies were detectable years before the first symptom, which is the basis of current interest in pre-clinical rheumatoid arthritis. And it carries prognostic weight — anti-CCP positivity, particularly at high titre, is associated with more erosive joint damage and supports earlier and more intensive treatment. None of which makes it a stand-alone test. Around a third of patients with rheumatoid arthritis are anti-CCP-negative, so a negative result does not exclude the disease, and the antibody is read with rheumatoid factor, the pattern and number of involved joints, the symptom duration and the acute-phase response. Rheumatoid factor and anti-CCP together are more informative than either alone: double positivity carries both a higher probability of the diagnosis and a worse prognosis.
Frequently asked questions
Is 40 U/mL of anti-CCP the same as 40 kU/L?
Yes — U/mL and kU/L are numerically identical, so nothing needs converting between them. There is no molar or mass unit, because the result is binding activity in arbitrary units against the manufacturer’s calibration. EU/mL and RU/mL, used by some manufacturers, are each their own arbitrary unit and are not interchangeable with U/mL.
Why does this page show no reference interval?
Because anti-CCP units are not comparable across methods. Second- and third-generation assays (CCP2 and CCP3) use different synthetic peptide antigens and report on different scales, there is no international reference preparation, manufacturers’ upper limits of normal differ several-fold, and some laboratories raise the threshold locally. The 2010 ACR/EULAR criteria handle this by scoring each result against a multiple of its own assay’s upper limit of normal rather than against a number, and this page follows the same logic.
Is anti-CCP better than rheumatoid factor?
It is more specific, not more sensitive. In Nishimura and colleagues’ meta-analysis the two had similar sensitivity — 67% for anti-CCP and 69% for rheumatoid factor — but specificity was 95% for anti-CCP against 85% for RF, because hepatitis C, Sjögren’s syndrome and chronic infection do not produce anti-CCP in the way they produce RF. They are not alternatives, though: measuring both is more informative than either alone, and double positivity implies both a higher probability of rheumatoid arthritis and a worse prognosis.
Can anti-CCP be positive before any joint symptoms?
Yes. In serum stored from blood donors who later developed rheumatoid arthritis, anti-citrullinated protein antibodies were detectable years before the first joint complaint. That is the basis of current research interest in pre-clinical rheumatoid arthritis. A positive anti-CCP in someone with no arthritis is not a diagnosis and is not by itself a reason to treat, but it does mark a substantially raised risk and warrants rheumatological assessment if symptoms appear.
Does a high anti-CCP titre mean worse disease?
It points that way. Anti-CCP positivity, and high titres particularly, is associated with more erosive joint damage, and the ACR/EULAR classification criteria weight a high-positive result — more than three times the assay’s upper limit of normal — at 3 points against 2 for a low-positive. Because units are assay-specific, “high” has to be read as a multiple of the local upper limit rather than as an absolute number.
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 — anti-CCP pooled sensitivity 67% (95% CI 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 — serology domain: negative 0, low-positive (>ULN) 2, high-positive (>3× ULN) 3 points; ≥6 of 10 classifies definite RA.
- Nielen MMJ, van Schaardenburg D, Reesink HW, et al. Specific autoantibodies precede the symptoms of rheumatoid arthritis: a study of serial measurements in blood donors. Arthritis Rheum. 2004;50(2):380–386. doi:10.1002/art.20018
- van der Helm-van Mil AHM, Verpoort KN, Breedveld FC, Toes REM, Huizinga TWJ. Antibodies to citrullinated proteins and differences in clinical progression of rheumatoid arthritis. Arthritis Res Ther. 2005;7(5):R949–R958.
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.