Autoantibodies in Rheumatoid Arthritis - Laboratory and Clinical Perspectives

Abstract

Measurement of two groups of autoantibodies, rheumatoid factor (RF) and anti-citrullinated protein/peptide antibodies (ACPA) have gained increasing significance in the diagnosis and classification of rheumatoid arthritis (RA) over the last 65 years. Despite this rising importance of autoimmune serology in RA, there is a palpable lack of harmonization between different commercial RF and ACPA tests. While a minimal diagnostic specificity has been defined for RF tests, which almost always are related to an international reference preparation, neither of this applies to ACPA. Especially assays with low diagnostic specificity are associated with very low positive predictive values or post-test probabilities in real world settings. In this review we focus on issues of practical bearing for the clinical physician diagnosing patients who potentially have RA, or treating patients diagnosed with RA. We advocate that all clinically used assays for RF and ACPA should be aligned to a common diagnostic specificity of 98-99% compared to healthy controls. This high and rather narrow interval corresponds to the diagnostic specificity seen for many commercial ACPA tests, and represents a specificity that is higher than what is customary for most RF assays. Data on antibody occurrence harmonized in this way should be accompanied by test result-specific likelihood ratios for the target diagnosis RA on an ordinal or interval scale, which will provide the clinical physician with more granular and richer information than merely relating numerical values to a single cut-off point. As many physicians today are used to evaluate autoantibodies as positive or negative on a nominal scale, the introduction of test result-specific likelihood ratios will require a change in clinical mindset. We also discuss the use of autoantibodies to prognosticate future arthritis development in at-risk patients as well as predict severe disease course and outcome of pharmacological treatment.

Keywords: ACPA; anti-CCP; diagnosis; prognosis; rheumatoid arthritis; rheumatoid factor.

Figure 1

Excerpt from the response from the British External Quality Assessment provider UK NEQAS to one individual laboratory on the October 2020 distribution of rheumatoid factor. Responses had been submitted from 312 laboratories, out of which 308 were correctly positive and four incorrectly reported as negative. The histogram bars show the quantitative distribution for all participating labs, with the distribution of labs using the same commercial assay as this individual laboratory in grey. The figure is published with the permission of Dina Patel, UK NEQAS.

Figure 2

Distribution of (A, C, E) anti-CCP2 and (B, D, F) IgM RF among 268 RA patients and 100 healthy blood donors from Sweden. In (A, C). dot blots are shown with the medians depicted as horizontal solid lines. The dotted horizontal lines depict the cut-off points for clearly positive responses, as suggested by the manufacturer. In (B, D), the corresponding Receiver Operator Characteristics (ROC) curves are shown; including information about Area Under the Curve (AUC). In (E, F) the distribution of the 100 controls is depicted for anti-CCP2 and IgM RF, with vertical arrows depicting the 95th percentile among the 100 controls (95th), the company-suggested cutoffs (CC), and in (E) the value three times higher than the 95th percentile (3x 95th, in red). Figures within parentheses show the corresponding measurement values.

Figure 3

When diagnostic sensitivities are compared between different tests, they should be aligned to the same diagnostic specificity, preferably in the high specificity range. In this schematic figure, the Area Under the Curve (AUC) is highest for the red and lowest for the blue Receiver Operator Characteristics (ROC) curve. However, at the predefined diagnostic specificity (vertical dotted line) the blue ROC curve represents the test with the highest sensitivity, which should be preferred when laboratory results are reported in relation to one single cutoff. The original picture was obtained from Allan Wiik, Copenhagen, and published in modified form with his permission.