Both clinical and laboratory criteria are useful for establishing the diagnosis of HSV infections. Clinical diagnosis can be made accurately where characteristic multiple vesicular lesions on an erythematous base are present. Scrapings of the base of the lesions and subsequent staining with Wright, Giemsa (Tzanck preparation), or Papanicolaou’s stain will demonstrate characteristic giant cells or intranuclear inclusions of a herpesvirus infection. These cytologic techniques are often useful as a quick office procedure to confirm the diagnosis. Limitations of this method are that it does not differentiate between HSV and varicella-zoster infections and is only about 60 percent as sensitive as viral isolation. The laboratory confirmation of HSV infection is best performed by isolation of virus in tissue culture. HSV causes a discernible cytopathic effect in a variety of cell culture systems, and most specimens can be identified within 48 to 96 h after inoculation. The sensitivity of viral isolation varies with the stage of lesions (higher in vesicular than in ulcerative lesions), whether the patient has a first or recurrent episode of the disease (higher in first episodes), and whether the sample is from an immunosuppressed or immunocompetent patient (more antigen in immunosuppressed). Immunofluorescent assays using monoclonal antibodies and some DNA hybridization procedures have approached the sensitivity of viral isolation for detecting HSV from genital or oral-labial lesions but appear only about 50 percent as sensitive as viral isolation for the detection of asymptomatic HSV in cervical or salivary secretions. Laboratory confirmation allows for subtyping the virus, which may be useful epidemiological^ as well as in helping to predict the frequency of reactivation after first-episode oral-labial or genital HSV infection. Restriction endonuclease analysis of viral DNA can also be used to differentiate between HSV-1 and HSV-2 as well as to differentiate between strains within the same subtypes, information which may be very useful in identifying common source outbreaks of HSV.

Acute and convalescent serum can be useful in documenting seroconversion during primary HSV-1 or HSV-2 infection. However, only 5 percent of patients with recurrent mucocutaneous HSV infections show a fourfold or greater rise in anti-HSV antibodies between acute and convalescent serums. As such, serologic assays have little utility in diagnosing acute mucocutaneous HSV infection, and are best used to identify persons with past infection.