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    Poddubnyy, D., Brandt, H., Vahldiek, J., Spiller, I., Song, I., Rudwaleit, M., & Sieper, J. (2012). the frequency of non-radiographic axial spondyloarthritis in relation to symptom duration in patients referred because of chronic back pain: results from the Berlin early spondyloarthritis clinic. Annals of the rheumatic diseases, 71(12), 1998–2001. http://access.bl.uk/ark:/81055/vdc_100177554955.0x000046