Alternate Names: RF
How Performed: Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (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 tourniquet 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.
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: Adults:
No special preparation is usually necessary.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:
How It Feels: 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.
Risks:
- 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
Why Performed: The RF test is used mainly in the diagnosis of rheumatoid arthritis, although it is not very specific for this purpose. Other rheumatologic diseases (such as systemic lupus erythematosus) can cause elevated RF tests. In rheumatoid arthritis, abnormal immunoglobulin G (IgG) antibodies produced by lymphocytes in the synovial fluid act as antigens for the production of other IgG and IgM antibodies. Antibodies reacting against antigens form immune complexes. These immune complexes can activate the complement system and other inflammatory processes. Tests for RF measure the reactive IgM molecules.
About 80% of patients with rheumatoid arthritis have positive RF tests, that is, titers > 1:80 (this means that the sample can be diluted 80 times while still being able to measure the presence of the IgM antibody).
Normal Values: negative:
- < 60 u/ml (nephelometric method) or
- < 1:80 titer (agglutination method)
Note: U/ml = units per milliliter
Abnormal Results: A positive test may indicate:
Additional conditions under which the test may be performed:
Cost:
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.
