This patient has a herpes simplex virus (HSV) infection, and direct-fluorescent antibody testing (DFA) should be performed. The patient is immunocompromised due to HIV infection, and establishing the diagnosis is important in these patients. Although the diagnosis of HSV infection can often be made clinically, ancillary tests are available to confirm the diagnosis. Of these, DFA and polymerase chain reaction (PCR) are tests that can be rapidly completed (in less than 24 hours). They are also able differentiate between the various types of herpesvirus infections: HSV1, HSV2, and varicella infection. DFA and PCR are done by unroofing a vesicle and swabbing the base of the vesicle to provide an adequate sample.
Bacterial culture is incorrect as the lips are often contaminated with bacteria, given the presence of multiple bacteria in oral flora. In addition, the patient's lesions do not have honey-colored crust, suggesting that these are not impetiginized. Performing serologic testing (HSV IgG) is not recommended to diagnose herpes virus infections, as the seroprevalence for both HSV1 and HSV2 is high, and a positive result is not indicative of active infection.
Viral culture had been considered the gold standard in the past, but it can take over 48 hours (sometimes up to 1 week) for results to return. Culture is useful if resistance testing is necessary for recalcitrant infections. However, the recommended initial tests are now DFA and PCR to help establish or confirm the diagnosis.
