Herpes simplex virus (HSV) keratitis remains primarily a clinical diagnosis based on characteristic features of the corneal lesion.  Laboratory studies may help to confirm the clinical suspicion in cases lacking typical findings, but they are not readily available in most clinical settings.
If the diagnosis is in doubt, however, laboratory diagnosis can be made using the following  :
Giemsa stain - Scrapings of the corneal or skin lesions show multinucleated giant cells
Papanicolaou stain - This shows intranuclear eosinophilic inclusion bodies
Immunohistochemistry looking for viral antigens
Polymerase chain reaction (PCR) assay 
Epithelial scrapings with Giemsa stain may show multinucleated giant cells, resulting from coalescence of infected corneal epithelial cells and intranuclear viral inclusions. However, negative cytology results do not exclude HSV infection.
Viral cultures obtained within several days of onset of disease and prior to antiviral therapy have a sensitivity of up to 70% and also allow for identification of the HSV subtypes. Various techniques (eg, conventional tube culture, shell vial assay, suspension infection method) are available.
HSV-antigen detection tests, such as the enzyme-linked virus-inducible system (ELVIS), are very specific for detecting herpes infection, but they are limited by their lower sensitivity. Cell culture for confirmation of HSV is recommended when the ELVIS test result is negative.
PCR assay using tear samples, corneal epithelium, anterior chamber tap, or corneal buttons may detect viral DNA in cases of herpetic keratitis or keratouveitis. However, it does not distinguish between latent or active HSV infections.
A study by Inoue et al indicated that an immunochromatographic assay (ICGA) kit developed for the identification of HSV is clinically useful in the diagnosis of HSV epithelial keratitis. The investigators found that the ICGA had moderate sensitivity and high specificity, showing positive results for 35 out of 75 patients (46.7%) diagnosed with epithelial keratitis and negative results for 39 out of 39 patients (100%) who did not have the disease. 
Slit-lamp photography may be used to document disease progression and response to treatment.
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