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Background: Unlike axial spondyloarthritis (axSpA), no classification criteria are currently available for axial psoriatic arthritis (axPsA). This study aimed to evaluate the performance of the proposed cut-offs from the Assessment of Spondyloarthritis International Society (ASAS) data-driven definitions for active and structural magnetic resonance imaging (MRI) lesions typical of axSpA in distinguishing axial involvement in patients with psoriatic arthritis (PsA). Methods: Seventy-two consecutive PsA patients (67% male, aged 45± 14 years) meeting the CASPAR classification criteria, regardless of the presence of back pain, were included. All patients underwent radiography of the pelvis and spine, as well as sacroiliac joint (SIJ) MRI, while 52 (72%) of 72 patients also underwent whole-spine MRI. The final diagnosis of axPsA was ascertained by two experienced rheumatologists. One rheumatologist with expertise in imaging and one trained reader evaluated the radiography and MRI images. Results: AxPsA was diagnosed in 27/72 (38%) patients. The proposed cut-offs for active sacroiliitis demonstrated high specificity (95.6%) but relatively low sensitivity (51.9%) in distinguishing patients with and without axPsA. When structural lesions of the SIJ were included in addition to active lesions, the sensitivity significantly improved (96.3% vs. 51.9%) with a modest decrease in specificity (86.7% vs. 95.6%). Incorporating MRI spine lesions (using the proposed cut-offs for positive spine MRI from the SPACE cohort) alongside SIJ lesions did not further change sensitivity or specificity compared with assessing SIJ alone. Conclusion: The ASAS-proposed cut-offs for identifying active and structural lesions of SIJ demonstrated satisfactory performance in discriminating axial involvement in PsA patients.

Background: Sacroiliac joints (SIJ) radiography has lower sensitivity and specificity compared to magnetic resonance imaging (MRI) and low-dose CT (ldCT) of the SIJ for detecting axial spondyloarthritis. Whether this is also true for axial psoriatic arthritis (axPsA) remains uncertain. This study aimed to evaluate the performance of ldCT, MRI, and conventional radiography of SIJ and spine in distinguishing axial involvement in PsA patients. Methods: Fifty-one consecutive PsA patients (70.6% male, age 41 ± 13 years) meeting the CASPAR classification criteria, regardless of back pain, were included. All patients underwent SIJ and whole-spine radiography, SIJ MRI as well as SIJ ldCT, while 25 (49%) of 51 patients also underwent whole-spine MRI and ldCT. One rheumatologist with expertise in imaging and one trained reader evaluated images. The final diagnosis of axPsA was ascertained by two experienced rheumatologists. The sensitivity, specificity, and diagnostic accuracy of the three modalities for discriminating axPsA were compared. Results: AxPsA was diagnosed in 33.3% (17) of 51 patients. In discerning axial involvement in PsA patients, MRI exhibited superior sensitivity (92.9%) compared with ldCT (83.3%) and radiography (67.9%), while ldCT demonstrated higher specificity (97.1%) than MRI (86.4%) and radiography (72.7%). LdCT achieved the highest diagnostic accuracy (92%) among the three modalities, surpassing radiography (71%) and being comparable to MRI (89%). In the subgroup of patients with whole spine imaging, the combination of MRI and ldCT SIJ had the best diagnostic accuracy, reaching 93%. Including SIJ ldCT in addition to SIJ MRI further improved specificity (86.7% vs. 73.3%) with unchanged sensitivity (100%), while incorporating SIJ MRI alongside SIJ ldCT further improved sensitivity from 83% to 100%, with a modest decrease in specificity (86.7% vs. 93.3%). However, incorporating spine MRI/ldCT alongside SIJ MRI/ldCT did not further improve sensitivity or specificity in discriminating axial involvement in PsA patients. Conclusion: LdCT demonstrated superior diagnostic performance in distinguishing axial involvement in PsA patients compared to MRI or radiography, although MRI was more sensitive for detecting active sacroiliitis. The highest diagnostic performance was achieved with both MRI and ldCT of the SIJ. Additional spine imaging did not further increase diagnostic performance.

Background Unlike axial spondyloarthritis, no classification criteria exist for axial psoriatic arthritis (axPsA). In 2021, the Assessment of SpondyloArthritis International Society (ASAS) revised the magnetic resonance imaging (MRI) criteria for active sacroiliitis (2021 criteria: bone marrow edema [BME] present in [Formula: see text]4 sacroiliac joint [SIJ] quadrants or [Formula: see text]3 consecutive SIJ slices[1]). This study aimed to compare the utility of this new cut-off in discriminating PsA patients with/without axPsA versus the 2009 ASAS criteria[2] for active-MRI-SIJ (BME [Formula: see text] 2 consecutive slices or [Formula: see text]1 location in a single slice). Methods Consecutive patients who fulfilled the classification criteria for PsA were recruited into this cross-sectional study, regardless of back pain. Sixty-seven patients underwent radiography (including pelvis, cervical/thoracic/lumbar-spine) and MRI-SIJ. Additionally, 47 underwent whole-spine MRI. AxPsA diagnosis was based on clinical information and imaging findings, as determined by an expert rheumatologist and a radiologist, and used as the reference standard. Two independent readers evaluated the MRI images based on two criteria for active sacroiliitis (BME cut-off: [Formula: see text] 4[1] vs [Formula: see text] 2[2]) and spondylitis (BME cut-off: [Formula: see text] 5[3] vs [Formula: see text] 3[4]). The agreement between the two MRI BME cut-offs for active sacroiliitis/spondylitis and the reference standard was evaluated. Results Sixty-seven patients (mean age: 47±12 years, 44 (65.7%) male, psoriasis and PsA disease duration: 13.5±10.3 and 3.8±6.1 years respectively) were recruited (Table 1). Twenty-three (34.3%) were diagnosed with axPsA, including 13 (56.5%) with radiographic sacroiliitis and 10 (43.5%) with non-radiographic axPsA. 12/67 (17.9%) had active MRI-sacroiliitis based on the 2021 ASAS criteria, while 4/47 (8.5%) had spondylitis based on the 2016 proposed definition[3]. Compared with the reference standard, the agreement increased after applying a more stringent threshold to define active sacroiliitis (BME cut-off: [Formula: see text] 4 vs [Formula: see text] 2; Kappa: 0.514 vs 0.392, respectively; Fig. 1A-B). The agreement with the reference standard further increased by applying a more stringent criteria for active spondylitis in addition to active sacroiliitis (BME cut-off for MRI-SIJ and spine: [Formula: see text] 4 and [Formula: see text] 5 vs [Formula: see text] 2 and [Formula: see text] 3; Kappa: 0.717 vs 0.342, respectively; Fig. 1C-D), resulting in higher specificity (active-sacroiliitis: 97.7% vs 81.8%; active-sacroiliitis and/or spondylitis: 100% vs 72.7%) and higher positive predictive value (91.7% vs 61.9%; 100% vs 50.0%, respectively). Conclusion At least four BME lesions on MRI-SIJ and five inflammatory lesions on MRI-spine allow acceptable discrimination of axPsA and no axPsA while assuring [Formula: see text]95% specificity.