Fig. 1.

Study design and patient selection. An overview of the study design is shown in panel a. The follow-up period (i.e., the period of time from back pain diagnosis to AS diagnosis) is outlined in red. Patient flow is depicted in panel b. Populations included and excluded from the main analysis are depicted by the blue and gray boxes, respectively. The orange boxes depict a patient population that was excluded from the main analysis owing to primary diagnosis by a non-rheumatologist but who had their diagnosis subsequently confirmed by a rheumatologist. AS = ankylosing spondylitis; CD = Crohn’s disease; HMO = health maintenance organization; PsA = psoriatic arthritis; PsO = psoriasis; RA = rheumatoid arthritis; UC = ulcerative colitis. ^aPatients with no interruption in insurance status