joanne, aloha
first off, if i may, i don't think consumption of starch is going to alter the results of any of your bloodtests. i know the 'dietistas' believe starch causes everything except the sunset; and i do think you've picked up a misconception here...a diet heavy in starch and/or sugars may alter you blood glucose readings- that's why they ask you to fast in the am prior to blood draw. you don't have to fast for days, weeks, months etc.
now, about the b27 test. it is NOT disponsive for diagnosis. the gene tests positive in about 10% of the general population, yet only like one tenth of one percent have as. thirty years ago many doctors believed b27+ was required for diagnosis; it's just not so.
we all get kind of conned in this CSI world into the 'genetics is destiny' conundrum. there are aprox fifteen different diagnostic scales used to determine AS, some of the newer models ignore the b27 status all together.
the problem with believing that the gene is the cause of AS is that so many people with the gene don't get AS.
good luck,
hope you have a younger rheumy with more modern approach.
