In Reply We agree with Dr Miller that a well-performed Tzanck smear can be diagnostic of human herpes virus, but the test is fastidious and cannot identify the specific type of herpesviridae. Because of this, the test must be performed by a practitioner experienced with the technique, and clinical information or further laboratory testing is necessary to differentiate between HSV or varicella zoster virus, as both have similar appearance on Tzanck smear. In the case of a Tzanck smear performed in the setting of suspected erythema multiforme, HSV would be highest on the differential as the causative agent.
Ladizinski B, Lee KC. Diagnosis of Herpes Simplex Virus in Patients With Erythema Multiforme—Reply. JAMA. 2014;312(10):1061. doi:10.1001/jama.2014.8742