The HLA B27 blood test is a bit overrated in my opinion, and a negative result can really confuse the issue totally (especially if the test is done by your GP who doesn't understsnd it). While a fairly high proportion of folk with AS are positive, the percentage of positives drops dramatically in other forms of spondylitis. HLA B27 isn't part of the usual diagnostic criteria for AS, so really the only time it is of any use to check it is if a rheumatologist has a high level of suspicion of AS or spondylitis, but can't find any concrete radiographic evidence for it. In that case it can clinch a diagnosis that will allow you to get appropriate treatment.

Otherwise, I wouldn't push for it just yet. Wait and see if the rheumatologist thinks its necesary.