Background: Pemphigoid gestationis (PG) is a rare pregnancyassociated subepidermal immunobullous disease that targets hemidesmosomal proteins, particularly BP180. Clinically, PG can resemble the eruption known as polymorphic urticarial papules and plaques of pregnancy (PUPPP), and accurate differentiation between these 2 pruritic pregnancy dermatoses has important implications for fetal and maternal prognoses. Results of epitope mapping studies show that IgG autoantibodies in up to 90%of PG serum samples target the well-defined membrane-proximal NC16a domain of BP180. Objective: To examine the usefulness of a commercially available NC16a domain enzyme-linked immunosorbent assay in the serodiagnosis of PG and in the differentiation of PG from PUPPP. Participants: A total of 412 women consisting of pretreatment patients with PG (n=82), patients with PUPPP (n=164), and age-and sex-matched controls (n=166). Methods: All serum samples were assayed in duplicate. Receiver operating characteristic analyses were performed to determine a cutoff value for the diagnosis of PG and for differentiation from PUPPP and controls. Results: A cutoff value of 10 enzyme-linked immunosorbent assay units was associated with specificity and sensitivity of 96%. Conclusions: The NC16a enzyme-linked immunosorbent assay is highly sensitive and highly specific in differentiating PG from PUPPP, and it is potentially a valuable tool in the serodiagnosis of PG. Background: Pemphigoid gestationis (PG) is a rare pregnancyassociated subepidermal immunobullous disease that targets hemidesmosomal proteins, particularly BP180. Clinically, PG can resemble the eruption known as polymorphic urticarial papules and plaques of pregnancy (PUPPP), and accurate differentiation between these 2 pruritic pregnancies dermatoses has important implications for fetal and maternal prognoses. Results of epitope mapping studies show that IgG autoantibodies in up to 90% of PG serum samples target the well-defined membrane-proximal NC16a domain of BP180. available NC16a domain enzyme-linked immunosorbent assay in the serodiagnosis of PG and in the differentiation of PG from PUPPP. Participants: A total of 412 women consisting of pretreatment patients with PG (n = 82), patients with PUPPP (n = 164), and age-and sex-matched controls (n = 166). Methods: All serum samples were assayed in duplicate. Receiver operating characteri Results: A cutoff value of 10 enzyme-linked immunosorbent assay units were associated with specificity and sensitivity of 96%. Conclusions: The NC16a enzyme was used to determine the cutoff value for the diagnosis of PG and for differentiation from PUPPP and controls. -linked immunosorbent assay is highly sensitive and highly specific in differentiating PG from PUPPP, and it is potentially a valuable tool in the serodiagnosis of PG.