ACPA-positive and ACPA-negative RA take the same total time from first symptom to arthritis — but split it in mirror image. Seropositive patients reach the rheumatologist later and convert faster; seronegative patients arrive sooner and linger, leaving roughly twice as long an actionable window once they are in front of you. Each entity carries its own signature before arthritis exists.
A population-based MRI study of 11,163 German adults found sacroiliac joint bone marrow oedema in about one in three people — roughly 50 times the prevalence of axial spondyloarthritis. The signal tracked with BMI, occupational loading, age and, in women, pregnancy, while chronic back pain barely moved the rate, providing the missing denominator for interpreting a positive SIJ MRI.
A 2026 pictorial review of the conditions that mimic sacroiliitis on SIJ MRI — bone marrow oedema is sensitive but the least specific sign, and overcalling it risks committing patients to lifelong biologic therapy. Erosion, lesion combinations, topographic location, semi-axial imaging and clinical context are the defences against false-positive reads.