Funny thing happened on the way to making HLA-B27 a "requirement" they started doing more studies and found some huge varients among different populations. They discovered that about 90% of primary Caucasians with AS were positive but by the time they got to the Moroccans fewer than 40% of them were positive. It got into all this linear relationship stuff.
I know a number of folks who have an AS Dx and have never had a HLA-B27 test. you get points for a positive, but you don't get deductions if you don't. FWIW there are other things MUCH more critical. Quoting from the text below: "genome scanning has shown that the major histocompatibility complex (MHC),
including HLA-B27, contributes less than 40% to the recurrence risk ratio in AS"
I'd be awfully suspicious of any Doc who thinks HLA-B27 is a requirement. Incidentally the AS Dx is likley to go away anyway really making the water muddy.
(heres a pretty good text: http://www.vumc.nl/afdelingen-themas/41463/27797/2089686/2090082/1608664/5.pdf