Incidence of Clinically Diagnosed Psoriatic Arthritis in Sweden

Objective Prior incidence estimates of psoriatic arthritis (PsA) vary considerably. We aimed to assess the annual incidence of clinically diagnosed PsA among adults in Sweden in 2014-2016, overall and stratified by age/sex/education/geography, and to investigate potential time trends in incidence in 2006-2018. Use of disease-modifying antirheumatic drugs (DMARDs) during the 2 years after diagnosis was also examined.

Methods Patients (aged ≥ 18 years) with incident clinically diagnosed PsA in Sweden were identified from the National Patient Register (NPR) and/or the Swedish Rheumatology Quality Register (SRQ). Population statistics, stratification variables, and DMARD information were retrieved from other nationwide registers. Incidence was estimated according to a base case (BC) definition (ie, ≥ 1 main International Classification of Diseases, 10th revision, diagnosis of PsA [L40.5/M07.0-M07.3] from rheumatology/internal medicine in NPR, or a PsA diagnosis in SRQ during the relevant year, and no prior such diagnoses) and 4 different sensitivity analysis case definitions.

Results The mean annual incidence of clinically diagnosed PsA among adults in Sweden in 2014-2016 was estimated at 21.77 per 100,000 person-years (PYs) at risk, according to the BC definition; 17.41 per 100,000 PYs at risk after accounting for diagnostic misclassification; and 15.78 to 28.83 per 100,000 PYs at risk across all sensitivity analyses. Incidence was slightly higher in female individuals, was lower in those with higher education (aged > 12 years), and peaked during the ages of 50 to 59 years. No apparent increasing or decreasing time trend was observed in 2006-2018. Within 2 years of diagnosis, 71.03% of patients had received DMARD therapy (22.37% biologic or targeted synthetic DMARDs).

Conclusion From 2014 to 2016, the annual incidence of clinically diagnosed PsA in the adult Swedish population was approximately 20 per 100,000 PYs at risk. Two years after diagnosis, almost three-quarters of patients had received DMARD therapy.