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Background MRI detected bone marrow edema (BME) plays a central role in the ASAS (Assessment of Spondyloarthritis International Society) classification criteria for axial spondyloarthritis (axSpA)[1, 2]. However, BME in the sacroiliac joints (SIJs) is also present in other conditions[3-7]. Objectives The aim was to investigate the diagnostic utility of MRI BME and its relation to different types of MRI SIJ lesions to separate patients with axSpA from persons with other conditions. Methods The MASH study is a prospective cross-sectional study of 204 participants, aged ≤45 yrs. The study included patients with axSpA (n=41), women with (n=46) and without (n=14) postpartum pain, patients with lumbar disc herniation (n=25), persons with hard physical labor (cleaning assistants) (n=25), long-distance runners (≥30 km/week) (n=23) and healthy men (n=29). Participants with pain should all have VAS pain >2 (on a scale 0-10) for ≥2 months. Participants in the non-axSpA groups were not allowed to have any clinical SpA features or rheumatological conditions. Participants underwent clinical, laboratory and MRI examination (semi-coronal STIR and T1W sequences) of the SIJs. MRIs were evaluated for BME, erosion, fat, ankylosis, and sclerosis according to the SPARCC MRI definitions of lesions[8, 9] by two independent readers. In nine slices of the cartilaginous compartment each SIJ was separately assessed for the presence of BME in relation to joint space and each of the above-mentioned structural lesions (range of total score per patient: 0-18). Results The table shows the clinical characteristics of each group, the mean MRI scores and MRI cut-off levels for the concordant reads. BME located adjacent to joint space, adjacent to erosions and adjacent to fat were more frequent in patients with axSpA, but these lesions were also seen in the other study groups, mainly women with postpartum pain. When increasing amounts of lesions were required (higher cut-offs), almost only AxSpA patients fulfilled the requirements. BME adjacent to sclerosis was most frequent in women with postpartum pain, whereas BME adjacent to ankylosis was only seen in patients with axSpA. Conclusion BME located adjacent to joint space, adjacent to erosion and adjacent to fat was most frequently, but not exclusively, occurring in patients with axSpA, whilst BME adjacent to sclerosis was most frequent in women with postpartum pain. Detailed analysis of lesions and their anatomical location may help differentiate axSpA from other conditions. References [1] Rudwaleit, et al. Ann Rheum 2009. [2] Lambert, et al. Ann Rheum 2016. [3] Weber, et al. Arthritis Rheum 2010. [4] Varkas, et al. Rheumatology (Oxford) 2018. [5] de Winter, et al. Arthritis Rheumatol 2018. [6] Arnbak, et al. Eur Radiol 2016. [7] Seven, et al. Ann Rheum 2018. [8] Maksymowych, et al. Arthritis Rheum 2005. [9] Maksymowych, et al. J Rheumatol 2015. Disclosure of Interests Sengul Seven: None declared, Pernille Hededal: None declared, Mikkel Ǿstergaard Grant/research support from: Abbvie, Celgene, Centocor, Merck, Novartis, Consultant for: Abbvie, BMS, Boehringer-Ingelheim, Celgene, Eli Lilly, Hospira, Janssen, Merck, Novartis, Novo, Orion, Pfizer, Regeneron, Roche, and UCB, Speakers bureau: Abbvie, BMS, Boehringer-Ingelheim, Celgene, Eli Lilly, Hospira, Janssen, Merck, Novartis, Novo, Orion, Pfizer, Regeneron, Roche, and UCB, Lone Morsel-Carlsen: None declared, Inge Juul Sorensen: None declared, Birthe Bonde: None declared, Gorm Thamsborg: None declared, Jens Jorgen Lykkegaard: None declared, Oliver Hendricks Grant/research support from: AbbVie, Novartis, Niklas Rye Jorgensen: None declared, Susanne Juhl Pedersen: None declared