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Rheumatoid factor is the autoantibody most often used as a diagnostic marker for RA. Agglutination tests for RF such as the Rose–Waaler and latex tests are widely used and are of proven diagnostic value (4, 5). An abnormal serum RF titer by any method that has yielded a positive result in fewer than 5% of normal controls was included in the ACR criteria in 1987 (6). Agglutination tests fail to discriminate among RF isotypes, which can be separated by radioimmunoassay, nephelometry, or ELISA. These methods have been found helpful when used routinely (19, 20), despite variations across laboratories (21).
Most studies of RF isotypes looked for associations with specific clinical manifestations or with outcomes of recent-onset RA; few focused on differences in diagnostic value across assays (22–29), and none evaluated the diagnostic value of specific RF isotypes in a prospective cohort of patients with recent-onset arthritis. Classically, IgM-RF is the isotype most strongly associated with a diagnosis of RA. The functional affinity of RF has been found to be higher in RA patients than in healthy controls (30) and higher in recent-onset RA than in advanced RA (9), suggesting that functional affinity may help to discriminate between RF associated with RA and RF not associated with RA. The diagnostic value of the other RF isotypes remains unclear but may be low, according to a previous study (29). However, it has been shown that increases in IgA-RF and IgG-RF can precede the onset of clinical RA.
Other autoantibody–antigen systems may be useful in the diagnosis of RA, including antibodies to citrulline-containing peptides (called anticitrulline antibodies, antifilaggrin antibodies, antiperinuclear factors, or antikeratin antibodies, in reference to the different methods used for their detection), RA-33, Sa, p68 (or Bip), and calpastatin. The diagnostic value of anticalpastatin (31, 32) and anti–RA-33 seems low (11, 33), and that of anti-p68 has not been evaluated. Many studies have investigated the diagnostic value of antibodies to citrulline-containing antibodies (34–38). Two recent reports suggest that anti-Sa may be a useful diagnostic marker (39, 40).
Apart from autoantibodies, several laboratory tests have been suggested (but not evaluated) as markers for early RA. In particular, certain immunoglobulin glycosylation alterations (41, 42) and IgG subclass imbalances (43, 44) may be specific for RA. Also, assays for immune complexes have been developed (45), and circulating immune complexes have been detected in most patients with RA (46).
HLA typing is not only costly but also lacks specificity for the diagnosis of early RA. However, 2 reports suggest that HLA alleles may contribute to shape the clinical pattern of early inflammatory arthritis (47, 48). Thus, combining HLA typing with other tests may assist in the diagnosis of early arthritis.
The present study was designed to determine which combination of laboratory tests best discriminates between early RA and other forms of arthritis at presentation. Laboratory tests may have less diagnostic value when performed routinely than when done as part of a battery of tests for a clinical study, because variations may be observed across test kits. Consequently, we based our study on tests performed routinely at our laboratory.
To produce valid results, a study of the diagnostic value of laboratory tests in RA must meet 3 prerequisites: Disease duration must be short at the time the laboratory tests are performed, followup must be long enough to ensure that few patients have unclassifiable disease at the end of followup, and the test cutoffs must be determined in patients with inflammatory joint disease. Accordingly, we included only patients with recent-onset inflammatory joint disease. These patients were recruited by office-based physicians to ensure that the population was representative of patients seen in everyday clinical practice. They were followed up prospectively. Median followup was 30 months; to compensate for this fairly short duration, we collected a vast array of clinical, radiographic, and laboratory data. Because the diagnosis of RA is difficult, it was made in our study by a panel of 5 rheumatologists, based on all available data. This diagnosis was used as the benchmark for evaluating the value of first-visit laboratory tests for predicting RA 2 years later. The best tests were IgG-AKA, IgM-RF by ELISA, and the latex test. IgG-AKA was the most specific test and IgM-RF ELISA the most sensitive test; the latex test offered the best tradeoff between sensitivity and specificity.
IgG-AKA had a better diagnostic value than other AKA isotypes or APF. The best cutoff for IgG-AKA in our study was lower (≥ 1:10) than the cutoff established in our laboratory as 2 standard deviations under the mean in a control group (> 1:10). This may explain the discrepancy between our results and those of previous studies reporting low sensitivity of AKA in early RA (49). We did not look for antifilaggrin antibodies (36, 37) or anticitrulline antibodies (38). Although these antibodies are not perfectly correlated with each other, their diagnostic value seems similar; furthermore, assays for both antibodies are expensive.
The second and third most useful tests, even when combined with AKA, were the IgM-RF ELISA and the latex test. Although highly significant correlations were found among RF assays, in keeping with earlier studies (23, 49–53), differences in diagnostic value were apparent, and combining these tests produced a small improvement in diagnostic value.
Previous studies found that the latex test and IgM-RF ELISA were useful for diagnosing RA. However, these studies did not use cutoffs determined in patients with inflammatory joint disease. In our study, the ROC curve of the IgM-RF ELISA suggested that the 0.150 OD cutoff provided both good specificity and high sensitivity. The best cutoff for the latex test on the ROC curve was 20 IU, and any deviation from this value was associated with a dramatic decrease in diagnostic value. These cutoffs differ slightly from those established in our laboratory as 2 standard deviations under the mean in a control group; i.e., 0.200 OD for the IgM-RF ELISA and >20 IU for the latex test. With these cutoffs, the diagnostic value of the 2 tests was quite similar but lower than that observed with the cutoffs determined in the present study. However, the ROC curves showed a range of optimal IgM-RF ELISA cutoffs, whereas a single cutoff value was optimal for the latex test.
Previous studies suggested that IgG-RF and IgA-RF may be more specific than other RF isotypes and may assist in the diagnosis of RA (23, 49, 54). In the present study, both isotypes were of limited diagnostic value. The findings from the only previous study (29) of the diagnostic value of RF in early arthritis agree in part with our results: ELISAs for IgG-RF and IgA-RF were not useful in diagnosing RA, whereas the IgM-RF ELISA emerged as a reasonable alternative to the latex test. Data from several previous studies suggest that age at disease onset (29, 55–57) or sex (29, 58) may influence the diagnostic value of RF. This was not the case in our study (data not shown). Differences in patient population, followup duration, or sensitivity of the RF test used may explain these discrepancies.
Presence of ANA was significantly associated with RA but was not useful as a diagnostic tool for RA. HLA–DR4 and shared epitopes were associated with RA. However, their diagnostic value was limited in our population of patients with early arthritis. Anti–RA-33, CICs, and serum IgG levels were of no assistance in making the diagnosis of RA in our study. The same was true of IgG glycosylation. However, this finding may be ascribable to the difficulty of standardizing the IgG glycosylation assay in routine laboratory practice, as well as to the heterogeneity of our population in terms of age and disease activity.
In conclusion, combined positivity of IgG-AKA, IgM-RF by ELISA, and the latex test indicated RA with nearly complete certainty in our population characterized by a high prevalence of RA (36%): Sensitivity was 33%, specificity 99%, PPV 97%, and NPV 73%. Moreover, positivity of at least 2 of these 3 tests indicated that RA was extremely likely (PPV 79%) and negativity of at least 2 of these 3 tests that RA was extremely unlikely (NPV 80%). Positivity of 1 or more of the 3 tests was the criterion with the best tradeoff between sensitivity and specificity (sensitivity 75%, specificity 82%) in a strategy emphasizing sensitivity. The diagnostic value of this 3-test combination was slightly better than that of the constituent tests used alone or in pairs, and this combination performed better than the 1987 ACR criteria at the first visit (8). However, it is important to note that our results may not apply to populations in which the prevalence of RA is different from that in our study cohort. Studies in various settings are warranted. We are currently investigating the diagnostic value of these tests used in combination with clinical and radiographic data.
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We are grateful to the following rheumatologists for referring their patients to us: E. Blat, P. Busson, A. Castagné, J. P. Caumon, P. Chicault, V. Desmas, J. P. Elie, X. Filliol, C. Gauthier, J. Glemarec, J. Y. Grolleau, M. N. Guillermit, R. Guyader, M. Hamidou, P. Herrou, G. Lavel, F. LeJean, R. Lemaître, M. C. Lheveder, A. Martin, Y. Maugars, I. Nouy Trolle, J. Olivry, C. Paturel, N. Paugam, A. Prost, D. Rault, B. Ribeyrol, A. Rossard, D. Rodet, I. Valls, P. Vilon, and P. Voisin. We thank O. Meyer (Paris, France) for his help with anti-RA-33 detection. We are also grateful to A. Wolfe for reviewing the English in this article.