It is common to encounter some patients who have been diagnosed with rheumatoid arthritis (RA), turned out to be misdiagnosed. This is scary if you think the patients will be on a lot drugs for nothing but harmful for a long time. Most of these misdiagnosed patients have untypical symptoms or signs to RA. A positive blood test for rheumatoid factor (RF) usually influences doctors’ judgement.
If your blood test for RF turns to be positive, don’t panic, the possibility that you are suffering RA only for 30%!!!. This means that 70% of people who have high RF have no RA!!
A study published in 1992 observed that the positive predictive value for RA and any rheumatoid disease were 24% and 34% respectively and negative predictive value for the two were 89% and 86%. This means that positive RF test is not useful since the majority represented false positive results. So even the test said you have problem, but it is likely to be false in 76% possibility!!.
the author pointed out that given the test’s limitations, clinicians should reconsider their expectations when ordering an RF. The utility of the RF may improve if it is ordered more selectively for patients with typical symptoms and signs of RA.
Shmerling RH, Delbanco TL, Arch Intern Med 152(12):2417-2420 (1992) How useful is the rheumatoid factor? an analysis of sensitivity, specificity, and predictive value.