Normal Rheumatoid Factor Levels
A normal rheumatoid factor (RF) level is less than or equal to the upper limit of normal (ULN) for the laboratory and assay, which is typically considered to be ≤14-15 IU/mL. 1
Understanding Rheumatoid Factor Values
The American College of Rheumatology (ACR) and European League Against Rheumatism (EULAR) categorize RF results into three groups:
- Negative: Values less than or equal to the upper limit of normal (ULN) for the laboratory and assay 2, 1
- Low positive: Values higher than ULN but ≤3 times ULN 2, 1
- High positive: Values more than 3 times ULN 2, 1
Clinical Significance of RF Levels
- RF is one of the most characteristic laboratory parameters in rheumatoid arthritis (RA), with its specificity for this disease increasing when the titer is high 3
- When RF information is only available as positive or negative (without specific values), a positive result should be scored as low positive for RF in clinical assessment 2
- High RF titers (>300 IU/mL) are strongly associated with rheumatoid arthritis (80% of cases) and are significantly associated with the presence of rheumatoid nodules 3
Diagnostic Value of RF
- RF testing is included in the 2010 ACR/EULAR classification criteria for RA, contributing up to 3 points in the scoring system 1, 4
- The sensitivity of RF for rheumatoid arthritis is relatively low (28%), but it has better specificity (87%) 5
- RF should be interpreted alongside anti-citrullinated protein antibody (ACPA) status, as both are important serological markers for RA diagnosis and prognosis 1
RF in Other Conditions
RF positivity is not specific to RA and can be observed in:
The likelihood of finding a healthy patient or someone with osteoarthritis or soft tissue rheumatism with high RF titers is very low 3
Clinical Application
- When a patient has a high RF titer, RA should be ruled out first, followed by other inflammatory diseases, collagenosis, and liver diseases 3
- The positive predictive value of RF for RA is relatively low (24%), meaning most positive RF results represent false positives 5
- The negative predictive value of RF for RA is higher (89%), suggesting that a negative RF result may be more helpful in excluding RA than a positive result is in confirming it 5
Caution in Interpretation
- RF levels alone may not predict rheumatological disease, and RF can be found asymptomatically in the general population 6
- The rate of being diagnosed with any rheumatic disease is significantly lower in patients with RF levels between 20 and 50 IU/mL compared to those with higher levels 6
- Always interpret RF results in the context of clinical findings and other laboratory parameters 6