THe usual blood tests for inflammation, ESR(sed rate) OR CRP (C-reactive protein) often are not reliable indicators for inflammation in AS, and normal values for these tests does not rule out AS or other spondyloarthropathies.

Here's a link to a a previous post on ESR and CRP:
https://www.kickas.org/cgi-bin/w3t/showthreaded.pl?Cat=&Board=support1&Number=87063&page=&view=&sb=&o=&vc=1#Post87063