Your healthcare provider may order an RF blood test if they suspect you have RA or another autoimmune disease, such as if you have these signs and symptoms of RA:
- Joint pain
- Joint inflammation (tenderness, swelling, and warmth)
- Joint stiffness, typically occurring first thing in the morning, that lasts longer than 30 minutes
- Fatigue
The RF test involves a simple blood draw like any other blood test. This is typically conducted at a clinic, but at-home RF test kits — in which you prick your finger and mail the collected blood sample to a lab for analysis — are also available.
A positive (abnormal) test result means your body is producing higher-than-normal RF antibodies — more than 20 IU/mL (international units per milliliter). This result alone is not enough to diagnose you with RA or another autoimmune condition.
A negative (normal) result means your body produces RF at levels less than 20 IU/mL. You may still have RA if you receive a negative RF test result.
For RA, the RF test’s sensitivity (how well it correctly detects RA) varies widely between studies, ranging from 26 percent to 90 percent. One meta-analysis found an overall sensitivity of 69 percent, meaning that this percentage of people with RA will test positive for RF.
Similarly, the specificity of RF testing for RA (how well it avoids false positive results) varies, but it’s generally thought to be around 85 percent. This means the vast majority of the general population will test negative for RF, but 15 percent might test positive even though they don’t have RA.
Research also suggests that if your tests show really high levels of RF (more than three times the upper limit of normal), your likelihood of having RA increases. But if you have another inflammatory or rheumatic (affecting joints or other connective tissues) condition, your likelihood of receiving a false positive result increases. A test result showing a really low RF level does not automatically mean you do not have RA.
The RF test is just one part of an RA diagnosis, which will likely also involve tests to detect ACPA and inflammatory markers, as well as ultrasound or MRI (magnetic resonance imagery) scans to detect joint inflammation, erosion, and fluid buildup.