From the Guidelines
Rheumatoid factor (RF) has a specificity of approximately 70-80% for rheumatoid arthritis (RA), as indicated by the 2010 ACR/EULAR Classification Criteria for Rheumatoid Arthritis 1. This means that 20-30% of patients without RA may test positive for RF, resulting in false positives. RF can be detected in other autoimmune diseases like Sjögren's syndrome, systemic lupus erythematosus, and mixed connective tissue disease, as well as in chronic infections, certain malignancies, and even in some healthy individuals, particularly the elderly. The specificity of RF increases with higher titers, meaning that strongly positive RF results are more likely to indicate RA than weakly positive results. For this reason, RF testing is typically used in conjunction with anti-citrullinated protein antibody (ACPA) testing, which has a higher specificity of about 95-98% for RA, as mentioned in the context of RA management guidelines 1. When both RF and ACPA are positive, the specificity for RA diagnosis increases significantly. RF testing remains valuable in the diagnostic workup for RA, but its limitations in specificity highlight the importance of considering clinical presentation and additional laboratory markers when establishing a diagnosis, as emphasized in the management of early arthritis recommendations 1.
Some key points to consider when interpreting RF results include:
- The presence of RF is not exclusive to RA and can be found in other conditions
- The specificity of RF increases with higher titers
- RF testing is often used in conjunction with ACPA testing to increase diagnostic specificity
- Clinical presentation and additional laboratory markers should be considered when establishing a diagnosis of RA.
It is essential to note that the management of RA has evolved, and guidelines have been developed to address various aspects of RA treatment, including the use of traditional and biologic disease-modifying antirheumatic drugs (DMARDs) and the role of glucocorticoids 1. However, the specificity of RF remains an important consideration in the diagnostic workup for RA.
From the Research
Specificity of Rheumatoid Factor (RF) in Rheumatoid Arthritis (RA)
- The specificity of RF in RA has been reported to vary across different studies, with values ranging from 74.4% to 90.4% 2, 3.
- A study published in 2016 found that the specificity of RF was 79%, while the specificity of anti-cyclic citrullinated peptide antibody (anti-CCP) was 98% 3.
- Another study published in 2003 reported a specificity of 76.4% for RF, which is lower than that of anti-CCP (91.5%) 4.
- Raising the RF cutoff point can help increase specificity, with a study published in 2005 finding that a cutoff point of 25 mIU/ml resulted in a specificity of 85.2% 5.
- The use of a common diagnostic specificity of 98-99% for RF assays has been advocated, which is higher than what is customary for most RF assays 6.
- The specificity of RF can be improved by combining it with other tests, such as anti-CCP, with a study published in 2016 finding that the presence of either anti-CCP or RF increased the sensitivity to 85%, and when both were present, the specificity increased to 98% 3.