Rheumatoid Factor (RF)
How It Is Done
The health professional drawing blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Apply a gauze pad or cotton ball over the needle site as the needle is removed.
- Apply pressure to the site and then a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
A rheumatoid factor (RF) blood test measures the amount of the RF antibody present in the blood.
The results of the rheumatoid factor (RF) test may be reported in titers or units:
- A titer is a measure of how much the blood sample can be diluted before RF can no longer be detected. A titer of 1 to 20 (1:20) means that RF can be detected when 1 part of the blood sample is diluted by up to 20 parts of a salt solution (saline). A larger second number means there is more RF in the blood. So a titer of 1 to 80 shows more RF in the blood than a titer of 1 to 20.
- Nephelometry units show how much light is blocked by the blood sample in the tube. A high level of RF causes the sample to be cloudy, so less light passes through the tube than when the RF level is low. So an RF level of 100 units is higher than one of 40 units.