Over 50% of AS+B27- presenting to the AS Middlesex Clinic, actually retested B27+; the serological test is highly flawed and many people who 'think' they are negative would become positive with proper (Fc or ELISA) testing.
However, a physician using the antigen as a criterion for diagnosis is not conforming with current standards (which fact does not help those of us who have been stuck with and by time and money-wasting doctors).
Agarwal (Pittsburgh) proposed guidelines for a diagnosis of "Pre-AS;" before radiological changes appear and this includes antigen status, plus several other aspects common in AS.
When I got AS, the connection was not yet known, but it was a great help in diagnosis: Not that anybody then could tell me what to do to avoid permanent damage. In fact, the ultimate diagnosis may have done more bad than good!