Unit Converter
Rheumatoid factor (RF)
(Autoantibody Against IgG Fc Portion - Major Marker for Rheumatoid Arthritis & Autoimmune Diseases)
Synonyms
- Rheumatoid factor
- RF
- IgM-RF (most common)
- IgA-RF
- IgG-RF
- Latex RF
- RA factor
Units of Measurement
- IU/mL (International Units per milliliter)
- U/mL
- kU/L
- kIU/L
Unit Conversions
All of the above units represent identical antibody-activity units, expressed differently by assay manufacturers.
1 IU/mL=1 U/mL1\ \text{IU/mL} = 1\ \text{U/mL}1 IU/mL=1 U/mL 1 IU/mL=1 kU/L1\ \text{IU/mL} = 1\ \text{kU/L}1 IU/mL=1 kU/L 1 kIU/L=1 kU/L1\ \text{kIU/L} = 1\ \text{kU/L}1 kIU/L=1 kU/L 1 kU/L=1 U/mL1\ \text{kU/L} = 1\ \text{U/mL}1 kU/L=1 U/mL
(System-specific, but universally interchangeable.)
Description
Rheumatoid Factor (RF) is an autoantibody, usually IgM, directed against the Fc portion of IgG.
RF can appear in:
- Rheumatoid arthritis (RA)
- Autoimmune diseases
- Infections
- Healthy elderly adults (~5%)
RF forms immune complexes that contribute to chronic inflammation and joint damage.
Note: Anti-CCP antibodies are more specific for RA, but RF remains a major screening test.
Physiological Role
RF has no known physiological function; its presence represents immune dysregulation.
Clinical Significance
HIGH RF
Seen in:
1. Rheumatoid Arthritis (RA)
- Present in ~70–80% of cases
- High RF titers associated with:
- Severe disease
- Nodules
- Extra-articular involvement
- Poor prognosis
- Severe disease
2. Other Autoimmune Diseases
- Sjögren’s syndrome (75–95%)
- Mixed connective tissue disease
- Systemic lupus erythematosus
- Systemic sclerosis
- Vasculitis (Cryoglobulinemia type II)
3. Chronic Infections
- Hepatitis C (very common RF positivity)
- Tuberculosis
- Syphilis
- Subacute bacterial endocarditis
4. Liver Diseases
- Cirrhosis
- Primary biliary cholangitis
5. Healthy Elderly
Up to 3–5% show mild RF elevation.
LOW / NEGATIVE RF
Does not exclude rheumatoid arthritis.
~20–30% of RA patients are seronegative (RF-negative AND anti-CCP-negative).
Reference Intervals
(Tietz 8E + ACR/EULAR + Mayo + ARUP)
Rheumatoid Factor (RF)
- <14 IU/mL → Negative
- 14–30 IU/mL → Borderline or low-positive
- >30 IU/mL → Positive
- >60 IU/mL → High-titer
- >100 IU/mL → Strongly positive (suggests RA or autoimmune disease)
Interpretation Chart
| RF Level | Meaning |
| <14 IU/mL | Normal / negative |
| 14–30 IU/mL | Weakly positive; may be non-specific |
| 31–100 IU/mL | Moderately positive; evaluate for RA or mixed connective tissue disease |
| >100 IU/mL | High suspicion for RA or Sjögren’s; evaluate aggressively |
Diagnostic Uses
1. Suspected Rheumatoid Arthritis
Part of RA diagnostic panel:
- RF
- Anti-CCP (ACPA)
- ESR
- CRP
- Imaging
2. Sjögren’s Syndrome
RF frequently elevated.
3. Chronic Hepatitis C
Often very high RF—important for differential diagnosis.
4. Cryoglobulinemia
Type II mixed cryoglobulinemia often shows extremely high RF.
5. Juvenile Idiopathic Arthritis
RF-positive polyarticular JIA subtype.
6. Monitoring RA
High RF correlates with:
- Increased erosive disease
- Extra-articular complications
(RF is not ideal for monitoring treatment response.)
Analytical Notes
- Immunoturbidimetric, nephelometric, or ELISA assays
- Hemolysis, lipemia can interfere
- High RF may interfere with immunoassays (“rheumatoid factor effect”)
- Always pair RF with anti-CCP antibody for highest accuracy
- False positives common in infection (especially HCV)
Clinical Pearls
- RF alone cannot diagnose RA - anti-CCP is more specific.
- In hepatitis C, RF may be positive even without arthritis.
- Very high RF (>100 IU/mL) strongly predicts extra-articular RA.
- RF positivity in elderly individuals is often clinically insignificant.
- RF may appear years before clinical RA develops.
Interesting Fact
RF was first described in 1940 as the “Waaler-Rose reaction,” long before the modern understanding of autoantibodies and immune complexes.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Autoimmune Markers
- ACR/EULAR 2010 Rheumatoid Arthritis Classification Criteria
- Mayo Clinic Laboratories - RF
- ARUP Consult - Rheumatoid Factor
- NIH / MedlinePlus - RF Test
- CDC Rheumatology Resources