Rheumatoid factor (RF) is a blood test performed in patients with suspected rheumatoid arthritis (RA). It is an antibody against IgG, which is itself an antibody; RF and IgG form immune complexes, which are part of the disease process of various rheumatological diseases.

Indications

---

RF is often determined in patients suspected in any form of arthritis. It has relatively little use there, as positive results can be due to other causes, and negative results do not rule out disease. Nevertheless, in combination with symptoms, it can be of help in distinguishing the causes. In addition, it is part of the disease criteria of RA and Sjögren's syndrome.

Interpretation

---

High levels RF (generally above 20 IU/mL, 1:40 or over the 95th percentile there is some variation among labs) are indicative of rheumatoid arthritis (present in 80%) and Sjögren's syndrome (present in 90%). There is a high rate of false positives due to other causes. These are:

• Chronic hepatitis
• Any chronic viral infection
• Leukemia
• Dermatomyositis
• Infectious mononucleosis
• Scleroderma
• Systemic lupus erythematosus (SLE)

External links

---

• MedlinePlus page on RF
• Labtestsonline page on RF
• PMID 1254316 ("A comparison of two tests for rheumatoid factor: latex test and l-agglutination")

Chemical pathology

Czynnik reumatoidalny