 Intervention if applicable: GPs are randomized in clusters either to use directly the referral rule or use the referral rule after 4 months. The referral rule consists out of four variables, a positive ASAS inflammatory back pain questionnaire, a positive family history for spondyloarthritis, a good reaction to NSAIDs non-steroidal anti-inflammatory drugs and back pain duration longer than 5 years. If at least two out of four variables are present a referral to the rheumatologist is advised. 