Background: The diagnostic process in a patient with early inflammatory back pain suggestive of axial spondyloarthritis (ax-SpA) requires assessment and integration of multiple aspects, including clinical examination, biological measurements and radiologic assessments. Among the physical examination features, alteration of spinal mobility is often observed in ax-SpA. However, whether mobility impairment might really increase diagnostic likelihood, and which of the measurements made have relevant diagnostic value remains unknown. Objectives: To describe the frequency and severity of mobility impairment in multiple traditional measurements in patients suspect of early ax-SpA at initial assessment time, and to analyze their individual diagnostic performances in reference to usual classification criteria applied after 2 years of follow-up. Methods: Data from the DESIR cohort, which included patients aged 18-50 with inflammatory back pain lasting for 3 months to 3 years and a clinical suspicion of ax-SpA diagnosis were used. Baseline measurements of Schober’s test (Schober), chest expansion (CEx), lateral spinal flexion (LatSpiFlex), cervical rotation (CervRot) and intermalleolar distance (IntMalDist) collected at baseline were classified according to reference data from the general population adjusted for age and -when appropriate- for height. Cutoffs were defined as above 2.5 th , 5 th , 10 th and 25 th percentiles. With ASAS classification for ax-SpA applied at 2 years follow-up visit as external reference, diagnostic performances (Sensitivity [Se], Specificity [Sp], Positive [PPV] and Negative [NPV] Predictive Values) were calculated. Results: Complete data were available for 575 patients (of whom 377 (66%) fulfilled the ASAS criteria at 2 years). Schober, CEx, LatSpiFlex, CervRot and IntMalDist were above 5 th percentile in respectively 278 (48%), 82 (14%), 220 (38%) and 93 (16%) patients. None of the measurements showed a clinically relevant compromise between both Se and Sp, but Sp was highest for CEx-most impaired cutoffs (Figure 1). The highest PPV (73.6%) and NPV (39.4%) were observed for LatSpiFlex. Conclusion: Measures of mobility and their levels of impairment do not show sufficient individual diagnostic value for ax-SpA among patients with early inflammatory back pain. However, highest degrees of impairment when compared to general population are more specifically observed in patients finally classified with ax-SpA for CEx, which was –consistently- 1 of the 2 mobility measures that was retained in the modified New York criteria for ankylosing spondylitis. Disclosure of Interests: Cédric Lukas Speakers bureau: AbbVie; Lilly; Merck; Novartis; Pfizer; Roche-Chugai;, Consultant of: AbbVie; Bristol-Myers Squibb; Janssen; Lilly; Merck; Novartis; Pfizer; Roche-Chugai; UCB; Sanofi;, Grant/research support from: Pfizer: Novartis, Gisèle Khoury Grant/research support from: Pfizer, Maria-Antonietta d’Agostino: None declared., Bernard Combe Speakers bureau: AbbVie; Bristol-Myers Squibb; Gilead; Janssen; Lilly; Merck; Novartis; Pfizer; Roche-Chugai; and Sanofi, Consultant of: AbbVie; Bristol-Myers Squibb; Gilead; Janssen; Lilly; Merck; Novartis; Pfizer; Roche-Chugai; and Sanofi, Grant/research support from: Novartis, Pfizer, and Roche-Chugai, Jacques Morel Speakers bureau: AbbVie; Bristol-Myers Squibb; Gilead; Janssen; Lilly; Merck; Novartis; Pfizer; Roche-Chugai; and Sanofi, Consultant of: AbbVie; Bristol-Myers Squibb; Gilead; Janssen; Lilly; Merck; Novartis; Pfizer; Roche-Chugai; and Sanofi, Grant/research support from: Novartis, Pfizer, and Roche-Chugai.