Background: SpA has historically been a difficult clinical diagnosis, especially early diagnosis. Two imaging techniques that address this problem are magnetic resonance imaging (MRI) and Single-Photon Emission Computed Tomography-Computed Tomography (SPECT-CT). Their accuracies have not been adequately compared. Objectives: The purpose of this study is to compare the sensitivities and specificities of SPECT-CT and MRI in SpA. Methods: This retrospective study assessed all patients who underwent SPECT-CT of the sacroiliac joint to assess for SpA. The results of SPECT-CT were compared against MRI for all patients in the cohort who underwent an MRI within 4 weeks of the SPECT-CT. A diagnosis of SpA in the discharge summary was considered the reference standard, and was based on a combination of clinical scenario, response to therapy, imaging,, patient history or lab index. Results: 200 patients (173 men; average 22±4 years of age) were included SpA was diagnosed in 189 (AS patients=99 and excluded in 11. SPECT-CT and MRI had similar (P >0 .05;k ¼ 0.74) sensitivities (0.94 vs 0.94),specificities (1.00 vs 1.00),positive predictive values (1.00 vs 1.00),negative predictive values (0.94 vs 0.80),and accuracies (0.97 vs 0.95) when compared to the reference standard. Conclusion: Although MRI remains the initial modality of choice in early diagnosing SpA, SPECT-CT appears diagnostically equivalent and should be considered a viable supplementary or alternative imaging modality particularly if there is contra-indication or inaccessibility to MRI. References: [1]Taurog J D, Chhabra A, Colbert R A. Ankylosing Spondylitis and Axial Spondyloarthritis[J]. N Engl J Med, 2016,375(13):1303. [2]van der Linden S, Valkenburg H A, Cats A. Evaluation of diagnostic criteria for ankylosing spondylitis. A proposal for modification of the New York criteria[J]. Arthritis Rheum, 1984,27(4):361-368. [3]Ward M M, Deodhar A, Gensler L S, et al. 2019 Update of the American College of Rheumatology/Spondylitis Association of America/Spondyloarthritis Research and Treatment Network Recommendations for the Treatment of Ankylosing Spondylitis and Nonradiographic Axial Spondyloarthritis[J]. Arthritis Rheumatol, 2019,71(10):1599-1613. [4]Boonen A, Sieper J, van der Heijde D, et al. The burden of non-radiographic axial spondyloarthritis[J]. Semin Arthritis Rheum, 2015,44(5):556-562. [5]Sieper J, Rudwaleit M, Baraliakos X, et al. The Assessment of SpondyloArthritis international Society (ASAS) handbook: a guide to assess spondyloarthritis[J]. Ann Rheum Dis, 2009,68 Suppl 2:i1-i44. [6]Bermo M, Behnia S, Fair J, et al. Review of Extraskeletal Activity on Tc-99m Methylene Diphosphonate Bone Scintigraphy and Value of Cross-Sectional and SPECT-CT Imaging Correlation[J]. Curr Probl Diagn Radiol, 2018,47(5):324-332. [7]Ward M M, Deodhar A, Akl E A, et al. American College of Rheumatology/Spondylitis Association of America/Spondyloarthritis Research and Treatment Network 2015 Recommendations for the Treatment of Ankylosing Spondylitis and Nonradiographic Axial Spondyloarthritis[J]. Arthritis Rheumatol, 2016,68(2):282-298. [8]Abdelhafez Y G, Hagge R J, Badawi R D, et al. Early and Delayed 99mTc-MDP SPECT/CT Findings in Rheumatoid Arthritis and Osteoarthritis[J]. Clin Nucl Med, 2017,42(11):e480-e481. Disclosure of Interests: Qing Han: None declared, Zhaohui Zheng: None declared, Kui Zhang: None declared, Zheng Yu: None declared, Fengfan Yang: None declared, Qiang Liang: None declared, Ping Zhu: None declared, Xenofon Baraliakos Grant/research support from: Grant/research support from: AbbVie, BMS, Celgene, Chugai, Merck, Novartis, Pfizer, UCB and Werfen, Consultant of: AbbVie, BMS, Celgene, Chugai, Merck, Novartis, Pfizer, UCB and Werfen, Speakers bureau: AbbVie, BMS, Celgene, Chugai, Merck, Novartis, Pfizer, UCB and Werfen