Socioeconomic inequalities in psoriatic arthritis treatment in Brazil: a nationwide analysis of coverage, use and expenditure

Abstract Background Psoriatic arthritis (PsA) is a chronic inflammatory immune-mediated disease associated with significant impairment and reduced quality of life. Despite the availability of a clinical protocol within the Brazilian Unified Health System (SUS), the coverage and socioeconomic distribution of pharmacological treatment for PsA across federative units remain uncharacterized. This study analysed territorial and socioeconomic inequalities in PsA treatment coverage, use, and expenditure in Brazil in 2025. Methods A nationwide cross-sectional study was conducted using administrative data from the SUS Outpatient Information System (SIA/SUS - APAC). The eligible population (PAT_ SUS ) was estimated from GBD 2023 psoriasis prevalence, adjusted by the PsA proportion among psoriasis patients in South America (21.5%) and SUS coverage (71%). For each federative unit, we estimated the number of treated patients (PT), the PT/PAT_ SUS ratio, drug consumption in defined daily doses (DDD), and mean per capita expenditure. Socioeconomic inequalities were measured using the Slope Index of Inequality (SII) and Relative Index of Inequality (RII), with the Human Development Index (HDI) as socioeconomic proxy. Population-weighted regressions were performed, with sensitivity analyses using log-transformed outcomes. Results Of an estimated 126,587 eligible individuals, 21,731 patient-equivalents received treatment based on DDD-derived APAC records, corresponding to a national coverage rate of 17%. PT/PAT_ SUS ranged from 0.02 to 0.37 (20-fold), per capita expenditure from USD 480 to USD 34,184 (71-fold), and DDD consumption from 2.4 to 78.8 per patient (33-fold). Statistically significant pro-rich gradients were identified for coverage (SII = 0.167; 95%CI: 0.053–0.280; RII = 5.304; 95%CI: 2.373–11.855) and consumption (SII = 48.065; 95%CI: 27.381–68.749; RII = 9.404; 95%CI: 4.550-19.439), with states at the upper socioeconomic extreme showing approximately four- and nine-fold higher values, respectively. Expenditure showed no significant gradient (RII = 0.986; 95%CI: 0.404–2.406). Sensitivity analyses confirmed the direction of all effects, with no gradient inversion. Conclusions PsA treatment coverage within SUS is markedly low and unevenly distributed across federative units, with statistically significant pro-rich gradients for coverage and consumption but no consistent pattern for expenditure, a finding consistent with territorial misallocation of resources and inequitable distribution of health benefits. Reducing these inequalities requires strengthening diagnostic and care capacity in lower-HDI states alongside systematic equity monitoring within the specialized pharmaceutical care component.