33399 C3 immunoperoxidase staining in the diagnosis of bullous pemphigoid: A retrospective diagnostic test accuracy study

Abstract

Bullous pemphigoid (BP) diagnosis is typically based on histopathologic findings (H&E) plus ≥1 supportive test eg, direct immunofluorescence (DIF) and/or BP180/230 ELISA. C3 immunoperoxidase (IPOX) staining is a newer supportive test with practical advantages. The aim of this retrospective diagnostic test accuracy study is to clarify the role of C3 IPOX in BP. Five hundred thirty-one paired H&E and DIF cases submitted by UCSF adult dermatologists between 2018-2020 were searched for reports in which H&E findings were suggestive of BP and/or clinical query included BP and H&E did not rule it out. All available 95 cases meeting criteria were sent for C3 IPOX. Chart review established final clinical diagnosis for all cases (reference standard). ELISA was available for 58 cases (61%). Sensitivity and specificity were 63.3% and 90.8% for C3 IPOX, 69.6% and 91.4% for ELISA, 83.3% and 100% for DIF, and 87.0% and 88.6% for a combined approach requiring either positive C3 IPOX or ELISA. C3 IPOX is selectively applied after initial biopsy is complete, potentially minimizing cost and scar length, though may need to be used in conjunction with other supportive tests (ie, ELISA) to reach optimal test performance in BP. Study limitations include potential for bias due to DIF and ELISA results influencing the final clinical diagnosis and small total N at a single institution. Future directions include extending our investigation to five years and analyzing the cost of various testing strategies, to further refine the role of C3 IPOX in the diagnosis of BP.