RHEUMATOID FACTOR (R.A. TEST)
1) Abnormal in more than 2/3rd of adults with R.A. It becomes abnormal usually after several months of appearance of clinical features.
2) Only 1/3rd of patients with a positive test actually have R.A. which means that the specificity is also poor.
3) 10-20% of normal individuals >65 years of age have normal R.A. test.
4) R.A. test can be positive in several other rheumatological disorders as well as in tuberculosis, hepatitis B, interstitial lung disease, cirrhosis.
5) Prognostic significance lies in the fact that high titers correlate with more severe & progressive disease.
6) Always ask for R.A. test with titer.
7) The titer of Rheumatoid factor (RF) can be converted to International Units (IU) by multiplying the titer with the conversion factor provided in the kit. For eg. if the highest dilution factor is 1/20, the titer is 20 and if the conversion factor is 8, then RF level is calculated as 20 x 8= 160 IU.
ANTI – CCP (Cyclic Citrullinated Polypeptides) ANTIBODIES
1) More specific than R.A. Specificity is 95%.
2) May become abnormal earlier than rheumatoid factor. In 50% of RF negative RA patients, anti CCP is positive.