Clinical and Experimental Rheumatology 2017; 35: 270-276. Clinical and genetic associations of radiographic sacroiliitis and its different patterns in psoriatic arthritis M. Haroon 1,2 , R. Winchester 3 , J.T. Giles 3 , E. Heffernan 4 , O. FitzGerald 1 1 Department of Rheumatology, St Vincent’s University Hospital, Dublin, Ireland; 2 Division of Rheumatology, Department of Medicine, University Hospital Kerry, Ireland; 3 Division of Rheumatology, Columbia University, College of Physicians and Surgeons, New York, USA; 4 Department of Diagnostic Imaging, St Vincent’s University Hospital, Dublin, Ireland. Abstract Objective We aimed to 1) identify clinical and genetic associations of sacroiliitis (SI) in patients with psoriatic arthritis (PsA), and 2) describe the different radiographic patterns of SI in PsA and their clinical and genetic associations. Methods 283 PsA patients, fulflling CASPAR criteria, underwent detailed skin and rheumatologic assessments. In addition, HLA-B*27 and B*080101 status was recorded, which have been shown as the key genetic markers of radiographic SI in PsA. Grade 2 Unilateral or bilateral radiographic changes of SI were required for inclusion and involvement was further defned as asymmetrical or symmetrical. Results 70 patients (25%) had radiographic SI; all either with a present or past history of backache. Regression analysis demonstrated a signifcant association of SI with peripheral joint erosions (p=0.043), PASI maximum (p=0.041), younger age of PsA onset (p=<0.001), presence of HLA-B*0801 (p=0.002) and only marginal signifcance with HLA-B*2705 (p=0.059). Asymmetrical SI was noted in 51 patients (73%). In striking contrast to those patients with symmetrical SI, patients with asymmetrical SI were more likely to be female (p=0.04), have a trend towards more severe nail disease (p=0.08) and peripheral joint erosions (p=0.08), more osteolysis (p=0.01), more HLA-B*0801 positivity (p=0.001) and much less HLA-B*270502 positivity (p=<0.001). Conclusion PsA developing at a younger age, severe skin disease, peripheral joint erosions, and HLA-B*0801 are signifcantly associated with SI, and there was only a marginal trend towards signifcance for HLA-B*2705. HLA-B*27 positive Axial-PsA patients resemble AS, while HLA-B*0801 positive Axial-PsA patients have asymmetrical and/or unilateral SI, which are typical of PsA. Key words psoriatic arthritis, HLA alleles, susceptibility, phenotype