0843: Predictive Validity of Data-driven Definitions for Active and Structural Lesions in the SI Joints Typical for Axial SpA: A 2-year Follow-up in the SPondyloArthritis Caught Early Cohort

Liese de Bruin¹, Mary Lucy marques², Miranda Van Lunteren³, Manouk de Hooge⁴, Sofia Exarchou⁵, Floris Van Gaalen³, Désirée van der Heijde⁶ and Sofia Ramiro⁶, ¹Leiden University Medical Center, Leiden, Netherlands, ²Leiden University Medical Center; Centro Hospitalar e Universitário de Coimbra (Department of Rheumatology), Coimbra, Portugal, ³Leiden University Medical Center, Department of Rheumatology, Leiden, Netherlands, ⁴Ghent University Hospital, Hesperange, Luxembourg, ⁵Lund University, Åkarp, Sweden, ⁶Department of Rheumatology, Leiden University Medical Center, Leiden, Netherlands

Background/Purpose: The Assessment of Spondyloarthritis international Society MRI (ASAS MRI) group has proposed definitions for active and structural lesions typical for axial SpA (axSpA) on SI joint MRI (MRI-SIJ), based on high specificity and positive predictive values (PPV; ≥95%) for the rheumatologist’s diagnosis of axSpA (Maksymowych et al., 2021). These cut-offs have not been validated in other cohorts nor assessed in early disease. Here, we analyze the predictive validity of the proposed cut-offs for active and structural MRI-SIJ lesions in early axSpA after 2 years follow-up.

Methods: Patients with chronic back pain (unknown origin; ≥3 months; ≤2 years; onset < 45 years) from the SpA Caught Early (SPACE) inception cohort were followed-up and diagnosed (axSpA/no axSpA) by a rheumatologist after 2 years (Marques et al., 2023). Three central readers scored baseline MRI-SIJ for bone marrow edema (BME), erosion, fat lesions, and sclerosis. Patients with a certain (level of confidence about the diagnosis ≥7) or very probable (level of confidence < 7 and a consistent diagnosis in the last 2 available visits) diagnosis were included for analysis. When data were available for ≥2 readers, the MRI SIJ were analyzed for consensus between ≥2 readers on scored lesions and each individual reader. The lesions were assessed for quadrants, consecutive slices, or combinations between different lesions (Table 1 & 2). We calculated sensitivity, specificity, PPV and negative predictive values (NPV) for each cut-off. Based on consensus, a combination of specificity ≥95% and PPV ≥95% was required for a cut-off to be considered validated. For the individual readers, a PPV of ≥94% in ≥2 readers was accepted.

Results: We analyzed 447 patients (age 30 (SD 8) years; 45% males; 66% axSpA). BME in ≥1 quadrant, or ≥2 consecutive slices met the threshold based on consensus (Table 1). BME in ≥2 quadrants or ≥2 consecutive slices met the threshold in ≥2 individual readers (Table 1 & 2). Erosions in ≥2 quadrants or ≥4 consecutive slices, and fat in ≥1 quadrant or ≥2 consecutive slices met the threshold based on consensus. Erosion in ≥3 consecutive slices met the threshold in ≥2 individual readers, and fat in ≥2 quadrants or ≥3 consecutive slices met the threshold in all individual readers. All combinations of BME-fat met the threshold based on consensus (Table 1). Combinations of BME-erosions and fat-erosion in ≥1 quadrants, each and/or one of both ≥2 quadrants, or ≥3 quadrants with fat lesions and/or ≥2 quadrants with erosion met the threshold based on consensus. Combined lesions did not outperform single item lesions.

Conclusion: In early axSpA, the proposed ASAS MRI cut-offs for active SIJ lesions have been validated in consensus and individual reading. For structural SIJ lesions, the proposed cut-offs for fat in quadrants and consecutive slices as well for erosions in quadrants have been confirmed. However, we propose a higher cut-off of ≥4 consecutive slices for erosions. All these cut-offs have now shown a high specificity and PPV in both early and established axSpA.






L. de Bruin: None; M. marques: None; M. Van Lunteren: None; M. de Hooge: UCB, 6; S. Exarchou: Amgen, 2, Janssen, 2, Novartis, 2, UCB Pharma, 2; F. Van Gaalen: AbbVie, 12, Personal fees, BMS, 12, Personal fees, Eli Lilly, 12, Personal fees, Jacobus Stichting, 5, MSD, 12, Personal fees, Novartis, 5, 12, Fees, Stichting ASAS, 5, Stichting Vrienden van Sole Mio, 5, UCB Pharma, 5; D. van der Heijde: AbbVie, 2, Bayer, 2, BMS, 2, Eli Lilly, 2, Galapagos, 2, Gilead, 2, GSK, 2, Imaging Rheumatology BV, 12, Director, Janssen, 2, Novartis, 2, Pfizer, 2, Takeda, 2, UCB Pharma, 2; S. Ramiro: AbbVie, 2, 5, Eli Lilly, 2, Galapagos, 5, MSD, 2, 5, Novartis, 2, 5, Pfizer, 2, 5, Sanofi, 2, UCB Pharma, 2, 5.