My GP was fine about doing the SI xray, but you do need to make sure they understand that this is a longstanding chronic problem - "the lower back pain" pathways in the UK really do focus very strongly on mechanical pain, and a very conservative approach (with recommendations NOT to do xrays or MRIs), which is something that NASS I think has been questioning, as there is too little suspicion of inflammatory back pain. Thats why that S-factor poster is quite useful - it very clearly indicates "inflammatory back pain" criteria, so if you meet all those, then you can very easily argue for your GP to refer you to rheumatology. Thats the first main hurdle really - getting that referral for "possible inflammatory arthritis". I don't know how to deal with GPs who only go by ESR and CRP - mine are always normal too, and its been incredibly difficult to get my GP to understand that doesn't rule out IBP.

If you did get the HLA B27 test done privately and it was positive, it might be a little help, but I still think that is best left for a rheumatologist to order if other testing is borderline and they need one more positive to swing a diagnosis. Also you wouldn't necessarily know how reliable the particular test the private lab does is, and there may be difficulties in getting the blood specimen to the lab in the right condition and time frame as it is a fairly sensitive test. If you are considering anything privately, it could possibly be useful to consider seeing a private rheumatologist for a one-off appointment, especially if your GP isn't keen on referring you to an NHS one. They are sometimes more likely to refer privately if you are going to pay. The only thing to watch there is to make sure you do your research and find someone who is a spondyloarthritis specialist. If you select a private rheumatologist who also works in your local NHS hospital, then it could make bridging from private back to NHS much easier. The benefits of a private appointment first off are that you get far more time than an NHS appointment usually is, so are more likely to get all the puzzle pieces fitted together - especially as you aren't in the usual age range for first presentation.

But, first step is to consult the GP with that S-factor back pain poster in your hand and ask for a rheumatology referral.