Rheumatoid factor

Alternative names
RF

Definition
Rheumatoid factor is a test that measures the presence and level of rheumatoid factor (RF) in the blood.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
No special preparation is usually necessary.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

The RF test is used mainly in the diagnosis of Rheumatoid Arthritis, although the test result can be positive in many other diseases as well as in healthy people.

RF is an Antibody that attaches to a substance in the body called immunoglobulin G (IgG), forming a molecule known as an immune complex. This immune complex can activate various inflammatory processes in the body.

About 80% of patients with Rheumatoid Arthritis have positive RF tests.

Normal Values

     
  • less than 60 u/ml (nephelometric method) or  
  • less than 1:80 titer (agglutination method)

Note: U/ml = units per milliliter

Different laboratories might have other reference values.

What abnormal results mean

A positive test may indicate:

Additional conditions under which the test may be performed:

What the risks are

     
  • Excessive bleeding  
  • Fainting or feeling lightheaded  
  • Hematoma (blood accumulating under the skin)  
  • Infection (a slight risk any time the skin is broken)  
  • Multiple punctures to locate veins

Special considerations

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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