Erythema multiforme (EM), an acute dermatologic condition frequently encountered in the Emergency Department, classically presents with a targetoid rash. We reviewed all recent EM cases seen at the LAC-USC County Hospital in order to ascertain the proportion of Herpes associated EM (HAEM) cases and to inform the diagnostic workup of these patients.

ICD-9 and ICD-10 codes were used to extract a list of EM cases at our institution from 2013 to 2019. Two non-blinded abstractors screened records to confirm an EM diagnosis and entered patient data utilizing a standardized data abstraction form. Cohen's kappa statistic was used to measure inter-rater reliability on various variables. Kappa (κ) values ranged from 0.803 to 1.0.

70 pediatric and 56 adult EM patients were included in the study. A likely etiology was ascribed to 63% of pediatric and adult EM cases. Pediatric EM was most commonly attributed to upper respiratory infection (URI) (n = 23; 33%), Mycoplasma pneumoniae infection (n = 5; 7%), and medications (n = 4; 6%). Adult EM was most commonly attributed to HSV infection (n = 11; 20%), medications (n = 5; 9%), URIs (n = 4; 7%), and other infections (n = 4; 7%).

HSV-1/2 serologic testing should be considered in most EM patients to potentially prevent repeated ED visits. In EM cases not clearly attributable to herpes or drug exposure, physicians can consider further workup: Mycoplasma serology, nasal PCR, and a respiratory viral panel in pediatric patients. Identification of an etiologic cause may suggest a different treatment approach and prevent mislabeling of medication allergies in patient charts.