OBJECTIVES: Our objective was to evaluate the relationship between bullous pemphigoid (BP) disease activity (DA), as measured by body surface area (BSA) and physician global assessment (PGA), and systemic therapy initiation (STI).

METHODS: BP patients from 5 specialty dermatology networks within the OMNY Health Dermatology Database were indexed at their first DA assessment with both BSA and PGA measurements. Patients with no recent history of traditional systemic or biologic therapy were included and followed from index DA assessment until STI or censoring. Cox regression with time-varying effects was employed to quantify the independent contributions of BSA, PGA, and their product on time to STI while controlling for patient characteristics.

RESULTS: A total of 779 patients were included (48% female, 48% ages ≥ 80 years, 12% non-white among known values). Index BSA (mean: 6.9, median: 0) and PGA (mean: 1.5, median: 0) exhibited skewed distributions. After controlling for patient characteristics, BSA exhibited a strong dose-response style relationship with earlier STI with hazard ratio (HR) point estimates of 1.5 (BSA: 2-15%) and 2.9 (BSA ≥ 15%). Similar results were obtained for PGA with HR point estimates of 0.98, 1.7, and 2.8 for mild, moderate, and severe categories, respectively. When both BSA and PGA were included together in the same model, relationships remained but were attenuated. Using the BSA-PGA product, HRs (95% confidence intervals) were 0.98 (0.67-1.4), 1.7 (1.1-2.6), and 2.5 (1.7-3.6) for 1-10, 11-30, and > 30 categories, respectively.

CONCLUSIONS: While the BSA-PGA product has been used in psoriasis and atopic dermatitis as a proxy for DA, its application in BP has not been well explored. Our results suggest meaningful independent contributions of BSA and PGA on STI in BP patients. The BSA-PGA product shows promise as a single metric for DA in BP.