What should you know about an RF blood test?

Rheumatoid factor is an immune protein found in some people with rheumatoid arthritis and other inflammatory or infectious conditions. A positive result can occur for reasons unrelated to rheumatoid arthritis, while a negative result does not exclude it. The result is interpreted with joint symptoms, examination findings and other tests, such as anti-CCP antibodies or inflammation markers when clinically appropriate.

The test may be part of an evaluation for persistent swollen joints, morning stiffness or unexplained symptoms. It is not the same as a general full blood count, and the number on the report needs the laboratory's reference range and your clinical context.

Usually, a blood sample is taken from a vein and no special preparation is needed unless other tests are being done at the same visit. Tell the clinician about infections, medicines and autoimmune conditions. Bring the full report to follow-up rather than interpreting an isolated positive or negative result online.

Your clinician may repeat or add tests, examine affected joints, or refer you to a rheumatology service. Conditions such as anaemia, coeliac disease, Crohn's disease or chronic kidney disease can also affect the wider assessment of fatigue or inflammation.

Seek prompt medical advice for a hot, very swollen joint with fever, rapidly worsening weakness, chest pain, breathing difficulty, or a new severe rash. These symptoms need assessment regardless of the RF result.

Ask why the test was ordered, what your result means with your symptoms, whether other tests are needed, and what signs should bring your appointment forward.