Background/Purpose: Psoriatic arthritis (PsA) incidence estimates vary considerably, and nationwide estimates are sparse. In Sweden, PsA is typically diagnosed in specialized care, although a limited subset with very mild disease may never be referred from primary care. The objectives of the current study were to estimate the mean annual incidence of PsA, diagnosed in specialized care, among adults in Sweden 2014-2016, overall and stratified by sex/age/education, and to describe the use of disease modifying anti-rheumatic drugs (DMARDs) among such cases.

Methods: Incident PsA cases ≥18 years (y) 2014-2016 were identified from the Swedish National Patient Register (NPR) and/or the Swedish Rheumatology Quality register (SRQ), based on five different definitions; A) Base case (BC): ≥1 ICD-10 code for PsA (L40.5/M07.0-M07.3) as main diagnosis from rheumatology or internal medicine (IM) in NPR or PsA diagnosis registered in SRQ during the relevant year, and no prior such diagnoses; B) Liberal: ≥1 ICD code for PsA as main or secondary diagnosis from any department in NPR or PsA diagnosis in SRQ during the relevant year, and no prior such diagnoses; C) Strict: ≥1 ICD code for PsA as main diagnosis from rheumatology/IM during the relevant year and no prior such ICD registration or PsA diagnosis in SRQ, plus ≥1 additional PsA ICD code (main or secondary from any department) within 2 years of the first in NPR; D) As BC, but reducing the number of PsA cases in line with a prior validation study in which 80% of 200 incident cases fulfilled PsA classification criteria; E) As BC, but excluding cases with ≥1 main diagnosis of rheumatoid arthritis from rheumatology/IM in NPR before or during 2 years after the incidence date. Irrespective of definition, cases < 18y at the incidence date or with a diagnosis of juvenile idiopathic arthritis (main/secondary) from any department in NPR or in SRQ ever, were excluded. Furthermore, DMARD use among cases within 2 years from the incidence date was assessed. The Swedish population the relevant years, demographics, education and treatments were retrieved from other national registers.

Results: The national, mean annual incidence of PsA, diagnosed in specialized care 2014-2016, in the population ≥18y was estimated at 21.8/100 000. Corresponding sensitivity analysis estimates ranged from 15.8 to 28.8/100 000 (Figure 1). The highest incidence was observed in the age-group 50-59y, followed by the age-groups 40-49 and 60-69y, regardless of sex (Figure 2). Irrespective of definition, the PsA incidence was numerically higher among females (Figure 2). Higher education was associated with lower age-/sex-standardized PsA incidence ( >12y, 10-12y, ≤9y education: 21.1 vs 28.9 vs 25.1/100 000, respectively, in adults ≥30y by the BC definition). Among incident PsA cases (BC definition), 22% received biologic or targeted synthetic DMARDs within two years of the incidence date, while an additional 49% received conventional synthetic DMARDs only (Figure 3).

Conclusion: The incidence of PsA, diagnosed in specialized care, in the adult Swedish population 2014-2016 was estimated at around 20/100 000. Almost 3/4 of cases received DMARD therapy within 2 years of the incidence date.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-national-incidence-of-clinically-diagnosed-psoriatic-arthritis-in-sweden-2014-2016/