Axial psoriatic arthritis in patients not fulfilling the back pain entry features of the ASAS Classification Criteria for Axial Spondyloarthritis: findings from the ATTRACT Study.

OBJECTIVE Application of the ASAS classification criteria for axSpA in classifying axPsA is a topic of debate. In this study, we aimed to determine the prevalence of axPsA in patients with psoriasis and back pain who do not meet the entry pain features of the ASAS classification criteria. METHODS Patients reporting late-onset back pain (LoBP, after the age of 45) or non-chronic back pain (NcBP, lasting less than months) in the DCS screening tool were included in a group coined "Non-ASAS Back Pain" (non-ASAS/BP). They underwent clinical/instrumental assessment aimed at axPsA diagnosis and were compared with those patients fulfilling both two ASAS entry pain features at the screening (ASAS/BP). RESULTS After rheumatologic evaluation, 50/265 (18.8%) patients, 34/50 (68%) LoBP and 16/50 (32%) NcBP, were categorised as non-ASAS/BP group. In comparison with ASAS/BP patients, the mean age was higher, and the prevalence of IBP was lower. Clinical disease activity was similar between the two groups. AxPsA was confirmed in 6/50 (12%) non-ASAS/BP patients, which is a lower incidence than in the ASAS/BP group (29.0%). Finally, non-ASAS/BP axPsA patients showed a similar proportion of inflammatory and post-inflammatory radiographic and/or MRI changes as shown in ASAS/BP axPsA patients. CONCLUSION This study demonstrates that among psoriatic patients who experience late-onset or non-chronic back pain, thereby not fulfilling ASAS entry pain features, a considerable proportion may be affected by active axPsA.