Clinical and imaging characteristics of 135 cases of infectious sacroiliitis: a retrospective cohort study in China.

To study the clinical, imaging, and computed tomography (CT)-guided biopsy pathology of patients with infectious sacroiliitis (ISI). We retrospectively analysed 135 patients diagnosed with ISI between 2008 and 2020, comprehensively evaluating clinical characteristics, laboratory test outcomes, pathological examination results, and magnetic resonance images (MRI). Among the 135 patients with ISI, 90 (66.7%) were diagnosed with pyogenic sacroiliitis (PSI), 28 (20.7%) with tuberculous sacroiliitis (TSI), and 17 (12.6%) with brucella infectious sacroiliitis (BSI); 69 (51.1%) met the 2009 ASAS criteria for axial spondyloarthritis. The rate of back pain is lower in the PSI group (18.9%) than in the TSI (39.2%) and BSI (64.7%) groups. The Berlin MRI scoring in the sacroiliac joint showed erosion and bone marrow oedema in the PSI (3.11 ± 2.33, 8.55 ± 2.66) and TSI (3.14 ± 2.08, 7.88 ± 3.90) groups, with higher values than in the BSI group (1.62 ± 1.54, 5.23 ± 3.05). The erythrocyte sedimentation rate (ESR) was higher in the PSI (52.71 ± 29.63 mm/h) and TSI (56.22 ± 19.39 mm/h) groups than in the BSI group (33.29 ± 25.12 mm/h). Our study is crucial because all patients underwent CT-guided sacroiliac joint biopsy; 130 patients (86.7%) had positive results through tissue culture and pathological examination, and one (0.8%) had a confirmed Malassezia fungal infection by tissue next generation sequencing. Two patients (1.5%) were diagnosed through blood culture, and another (0.8%) was diagnosed through the brucellosis agglutination test. ISI is diagnostically challenging as it can mimic spondyloarthritis, particularly when relying exclusively on imaging and clinical parameters. CT-guided sacroiliac joint biopsy is indispensable as a diagnostic intervention for precisely differentiating infectious sacroiliitis and identifying the specific pathogens involved, especially in female patients who exhibit negative HLA-B27 status and manifest systemic symptoms such as fever, elevated ESR, and unilateral lesions on imaging. Key Points • To the best of our knowledge, this is the largest single-centre cohort study on infectious sacroiliitis in China. • CT-guided biopsy of the sacroiliac joint is necessary for identifying infectious sacroiliitis and pathogens. • This study provides insights into the clinical and imaging features of infectious sacroiliitis based on a large number of cases.