Increase in axial spondyloarthritis diagnoses after the introduction of the ASAS criteria: a systematic review.

To explore the proportion of axial spondyloarthritis (axSpA) and psoriatic arthritis (PsA) diagnoses within all newly referred patients visiting rheumatology outpatient clinics. And more specifically, to analyze whether there is an effect of the introduction of the ASAS and CASPAR classification criteria for axSpA and PsA. We systematically searched Embase, Medline Ovid, Cochrane Central and Web of Science from database inception to November 2022. Articles that investigated new onsets of axSpA and PsA in adults from rheumatology clinics were included. In total, 170 out of 7139 studies were found eligible for full-text review, after which 33 unique studies were included. Seventeen studies reported new onsets of axSpA, and 20 studies of PsA. The pooled proportion of axSpA within all newly referred patients was 19% (95% CI 15-23%) and 18% (95% CI 14-22%) for PsA. The proportion of axSpA before 2009 was 3% (95% CI 0-6%) and increased up to 21% (95% CI 14-28%) after 2009. For PsA, limited data were available in order to analyze the proportions of PsA before 2006. Overall, heterogeneity was high (I 2  > 95%, p < 0.001) that was most likely caused by geographical area, study design, setting and use of different referral strategies. The pooled proportion of axSpA and PsA among patients referred to the rheumatology outpatient clinic was 19 and 18%, respectively. Although the proportion of diagnosed axSpA patients seemed to increase after the introduction of the ASAS criteria, due to the large heterogeneity our findings should be interpreted with caution.