Specific HSV antigens have been detected within keratinocytes by immunofluorescence study. The HSV DNA has been identified primarily within the keratinocytes by polymerase chain reaction amplification. Direct immunofluorescence staining and examination may also identify an alternative diagnosis (eg, pemphigoid, immunoglobulin A linear dermatosis).
No specific laboratory tests are indicated to make the diagnosis of erythema multiforme, which should be arrived at clinically. The clinical picture can guide laboratory testing in severe cases. Cultures are indicated in severe cases and should be obtained from blood, sputum, and mucosal lesions.
The complete blood cell count with differential usually reveals moderate leukocytosis with atypical lymphocytes and lymphopenia, possibly secondary to the depletion of CD4 lymphocytes (90% of patients). An eosinophil count greater than 1000/mm3 may also be seen. Neutropenia (30% of patients) may occur and indicates a poor prognosis. A severely elevated total white blood cell count indicates infection. Mild anemia may be present, and thrombocytopenia is found in 15% of patients.
Histopathologic examination of a cutaneous punch biopsy may be used to confirm the diagnosis of erythema multiforme and to rule out the differential diagnosis. A shave biopsy of the cutaneous lesions may also exclude the presence of other blistering disorders, such as in equivocal cases, particularly in absence of target lesions.
For more on the workup of erythema multiforme, read here.
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Cite this: William James. Fast Five Quiz: How Much Do You Know About Erythema Multiforme? - Medscape - Feb 15, 2018.