Baseline Serum Thymus and Activation‐Regulated Chemokine Levels and 1‐Year Relapse in Bullous Pemphigoid: A Retrospective Cohort Study

Baseline serological markers associated with relapse risk in bullous pemphigoid (BP) remain insufficiently defined. Serum thymus and activation-regulated chemokine (TARC) reflects type 2 inflammation and disease activity in BP, but whether its baseline level is associated with subsequent relapse has not been fully clarified. We retrospectively reviewed 109 Japanese patients with BP treated at Toyama University Hospital between January 2006 and September 2025. The primary endpoint was relapse within 1 year after treatment initiation. Associations between baseline serum TARC levels and time to relapse were evaluated using Cox proportional hazards models. During 1 year of follow-up, 31 patients relapsed and 9 died. Higher serum TARC levels were associated with relapse in univariable analysis (hazard ratio, 1.19 per doubling; 95% confidence interval, 1.02-1.39; p = 0.029). In the primary multivariable Cox model adjusted for age, dementia, total bullous pemphigoid disease area index (BPDAI) score, and anti-BP180NC16A antibody titer, the association was attenuated and was not statistically significant (hazard ratio, 1.16; 95% confidence interval, 0.95-1.42; p = 0.138). The corresponding Fine-Gray analysis yielded a similar result (subdistribution hazard ratio, 1.17; 95% confidence interval, 0.96-1.43; p = 0.120). Among patients without prior systemic glucocorticoid exposure, serum TARC remained associated with relapse in both Cox (hazard ratio, 1.34; 95% confidence interval, 1.03-1.73; p = 0.027) and Fine-Gray analyses (subdistribution hazard ratio, 1.35; 95% confidence interval, 1.03-1.75; p = 0.029). In this subgroup, adding serum TARC to the multivariable model increased the concordance index from 0.582 to 0.653 and improved model fit (likelihood ratio test, p = 0.025). These exploratory findings suggest that baseline serum TARC may provide modest complementary prognostic information when measured before systemic glucocorticoid treatment, although external validation is required.