How is epidemiology of SpA?

Most common one is ankylosing spondylitis (AS) 60% have + HLAB27 (except undifferentiated only 25%) however chance of pt w/ + HLAB27 to get SpA is 1-5% could get to 20% if a relative has it. More common in males (except psoriatic where equal proportions) Mean age 26 years from late adolescence to 40 years, we usually have delay in dx in HLAB27 - pt.