SAT0449 Progression from undifferentiated spondyloarthritis to ankylosing spondylitis: Is USPA a proxy for non-radiographic axial SPA?

Background Axial spondyloarthritis (axSpA) is a new umbrella term including patients with ankylosing spondylitis (AS), as well as those with non-radiographic axSpA (nr-axSpA). The new classification provides accepted evidence-based criteria to diagnose these patients. However, in practice, patients may be diagnosed using an earlier disease concept—undifferentiated SpA (uSpA)—for which an ICD-9 code exists. While clinical studies have found that a proportion of nr-axSpA patients experience radiographic progression (ie, to AS), little is known about progression to AS in a real-world setting. Objectives Explore the rate at which patients diagnosed with uSpA were subsequently diagnosed with AS using a large claims database. Methods Patients were selected from a de-identified US private insurer claims database (2000–2010). Potential nr-axSpA patients were defined as patients with ≥1 diagnosis for uSpA (ICD-9: 720.9) and a history of back disorders (724.0-724.9) prior to uSpA diagnosis. The date of first uSpA diagnosis was used as the study index date. Patients were required to have ≥6 months of continuous enrollment in a health plan prior to index (baseline period); be ≥18 years old on index; have no diagnoses for rheumatoid arthritis (714.0) or any type of SpA (AS: 720.0; Psoriatic Arthritis: 696.0; Reactive Arthritis: 099.3, 711.1; Inflammatory Bowel Disease: 713.1) on or before index; and have no diagnoses for any type of SpA other than AS post-index. Patients were followed until the earliest of: (a) diagnosis of AS (progression); (b) end of continuous eligibility; or (c) end of data availability. Baseline characteristics were measured separately for patients who were subsequently diagnosed with AS, and compared with those with no diagnosed progression. Results 6045 patients met the sample selection criteria. Median (IQR) available follow up was 14.2 (5.8-29) months. Two years post index, 8.9% of patients were diagnosed with AS, increasing to 13.7% by the 7 year mark. 5590 patients had no subsequent AS diagnosis. These patients averaged 47.6 years of age, and 61.4% were female. In comparison, patients progressing to AS were younger (43.8 years, P