Prospective comparison of laser nephelometry with standard agglutination techniques for detection of rheumatoid factor

The assessment of rheumatoid factor levels and hemogram parameters as predictors for rheumatologic disease

AIM

Many markers are used for clinical diagnosis in rheumatic diseases; rheumatoid factor (RF) is the most frequently used marker. However, RF is not specific to rheumatoid arthritis (RA). RF positivity is widely observed in patients with advanced age, infectious, autoimmune, and lymphoproliferative diseases. In this context, the objective of this study is to investigate the demographic characteristics, frequency of antinuclear antibody (ANA) and anti-cyclic citrullinated peptide (anti-CCP) positivity, hemogram parameters and distribution of the diagnoses in RF-positive patients followed at the rheumatology clinic.

METHODS

The population of this retrospective study consisted of patients above 18 years of age who were referred to have RF positivity by nephelometric method at Kahramanmaraş Necip Fazıl City Hospital Rheumatology Clinic between January 2020 and June 2022.

RESULTS

The mean age of the 230 patients with a positive RF test result, 155 (76%) male and 55 (24%) female, was 52.7 ± 15.5 years. There were 81 (35.2%), 54 (23.5%), 73 (31.7%) and 22 (9.6%) patients with RF levels between 20 and 50 IU/mL, 50 and 100 IU/mL, 100 and 500 IU/mL, and above 500 IU/mL, respectively. There was no significant difference detected between the groups that were created based on the RF titers regarding demographic characteristics (P > 0.05). The rate of being diagnosed with any rheumatic disease was significantly lower in the group with RF levels between 20 and 50 IU/mL compared to other groups (P = 0.001). The distribution of rheumatic and non-rheumatic disease diagnoses according to RF levels did not reveal any significant difference between the groups (P = 0.369 and P = 0.147, respectively). RA was the most common (62.2%) rheumatic disease diagnosis among the patients included in the study. The leukocyte count was significantly higher in the group with RF levels above 500 IU/mL compared to the group with RF levels between 20 and 50 IU/mL (P = 0.024). There was no significant difference between the groups in other laboratory results, that is, hemogram, sedimentation, C-reactive protein, platelet, and lymphocyte/monocyte ratio (P > 0.05).

CONCLUSION

The study findings indicate that RF positivity can be seen in the context of different rheumatological diseases; hence RF levels alone may not predict rheumatological disease. There was also no significant relationship between RF levels and ANA and anti-CCP positivity. The most common diagnosis in patients presenting with elevated RF levels was RA. Nevertheless, it should be noted that RF can be found asymptomatically in the general population.

Autoantibodies in Rheumatoid Arthritis - Laboratory and Clinical Perspectives

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.

Anti-citrullinated peptide antibody assays and their role in the diagnosis of rheumatoid arthritis

Increasingly, assays for the detection of anti-citrullinated peptide antibodies (ACPA) are used in RA diagnosis. This review summarizes the biologic basis and development of ACPA assays, available ACPA assays and their performance characteristics, and diagnostic properties of ACPA alone and compared to rheumatoid factor (RF) in early RA. We also review correlations, precision, costs and cost-effectiveness, availability, stability and reproducibility of the available assays. Taken together, data indicate that ACPA has a higher specificity than RF for early RA, good predictive validity, high sensitivity, apparent cost-effectiveness and good stability and reproducibility. Given its superior performance characteristics and increasing availability, ACPA is emerging as the most useful single assay for the diagnosis of RA.

The rheumatoid factor: an analysis of clinical utility

The rheumatoid factor (RF) is a frequently ordered diagnostic test, yet it possesses significant limitations in sensitivity, specificity, and predictive value. Recognition of these limitations could improve the test's utility by encouraging more selective test ordering and more circumspect interpretation of test results. An analysis of 563 requests for RF from a teaching hospital revealed a positive predictive value of only 24% to 34%. The RF performs best under conditions of moderate pretest likelihood of rheumatoid arthritis, and otherwise has rather limited clinical utility.