Clinical utility of the flow cytometric technique for shared rheumatoid epitope

The shared rheumatoid epitope (SRE) on the MHC class II antigen-presenting molecule constitutes a probable genetic risk factor for the occurrence of rheumatoid arthritis (RA) and may also determine disease severity. We have used a novel flow cytometric technique to determine the SRE in over 500 predominantly Caucasian patients attending a general rheumatology clinic. This technique has been validated against a polymerase chain reaction (PCR)/SSO molecular method. The SRE was observed in 90% of patients with Felty's syndrome (n = 10) and 75% of patients with RA (n = 178) as compared with 39% of patients with osteoarthritis or non-inflammatory rheumatic disorders (n = 73). Thus, the SRE determined by this method has a sensitivity for RA of 0.75, a specificity of 0.62 and an estimated positive predictive value of 0.02. In our RA cohort, there was no correlation between the functional outcome (health assessment questionnaire score) and SRE status. In conclusion, the determination of the SRE status by a flow cytometric method was found to have only modest sensitivity and specificity for RA; furthermore, the SRE did not correlate with functional outcomes. The clinical utility of the SRE assay is yet to be defined.