Inequality and determinants of access to phototherapy among patients with psoriasis in Thailand.

BACKGROUND

Ultraviolet (UV) phototherapy is a cost-effective, guideline-recommended treatment for moderate-to-severe psoriasis, supported by extensive evidence demonstrating its long-term safety and clinical efficacy. As a foundational therapeutic option in routine dermatological practice, access to phototherapy contributes meaningfully to sustained disease control and overall patient outcomes. Nevertheless, comprehensive nationwide data describing real-world utilization, particularly within universal health coverage systems, remain limited. A clearer understanding of patterns of access may therefore offer valuable insight into the organization and delivery of dermatological care.

OBJECTIVES

To quantify socioeconomic and regional inequality in UV phototherapy utilization and identify determinants of access within a universal health coverage system.

METHODS

This nationwide cross-sectional study (January-December 2023) integrated individual-level administrative health insurance data with a facility-based survey conducted across Thailand's 13 health regions to assess the availability of dermatologists and UV phototherapy devices. Patients with psoriasis (N = 49 219) were identified using International Classification of Diseases (ICD)-10 codes L40.0-L40.9. Moderate-to-severe disease (n = 16 188) was defined using treatment-based proxies, including receipt of phototherapy, systemic agents or biologic therapies identified through ICD-9 procedure codes and Thai Medicine Terminology drug codes. The primary outcome was phototherapy utilization. Multivariable logistic regression was applied to estimate adjusted odds ratios. Socioeconomic inequality was quantified using the Concentration Index and concentration curves.

RESULTS

Among 49 219 identified patients with psoriasis (202 776 encounters), 16 188 were classified as having moderate-to--severe disease. Of these, only 3.3% received UV phototherapy. Utilization was concentrated in higher-income provinces, as reflected by a positive Concentration Index of 0.213. In multivariable analysis, access to phototherapy was independently associated with age, psoriasis subtype, insurance scheme, geographical region, provincial income level, and the availability of dermatologists and phototherapy devices.

CONCLUSIONS

Access to phototherapy in Thailand remains limited and unequally distributed despite universal insurance coverage. Reducing disparities in workforce distribution and service capacity may enhance equitable access to this core dermatological treatment modality and support more equitable long-term disease control in routine clinical practice.