Pediatric emergency medicine trisk 1302 1302
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LABORATORY EVALUATION
If there is no clear etiology of what led to the blister, the laboratory tests
described next can be helpful.
Gram Stain
The Gram stain of fluid from an intact blister will be positive in impetigo and in a
secondarily infected lesion. It will be negative, however, in all other conditions.
Tzanck Smear
Multinucleated giant cells will be present on a Tzanck smear of material scraped
from the base of an intact, freshly opened vesicle caused by herpes simplex,
herpes zoster, and primary varicella. The Tzanck smear is not sensitive so
additional testing is required if negative.
Rapid Slide Test for Direct Immunofluorescence
Fluorescent-tagged monoclonal antibody is applied to cells scraped from the
blister base and can differentiate HSV-1, HSV-2, or varicella-zoster virus. Results
can be available in 1 to 2 hours.
Bacterial or Viral Cultures
Cultures help confirm an etiologic diagnosis when Gram stain, Tzanck smears,
and DIF are negative or indeterminate.
Polymerase Chain Reaction
An alternative or adjunct to traditional culture techniques, polymerase chain
reaction techniques allow for amplification of DNA or RNA present within a
specimen and rapid identification of the etiologic pathogen, including HSV, VZV,
and enterovirus. The technique is useful even when the pathogen present is no
longer viable.
Skin Biopsy
For perplexing cases undiagnosed by clinical and/or simple laboratory evaluation,
dermatologic consultation and skin biopsy are required.
If the picture on histology is compatible with erythema multiforme, DIF should
be considered. DIF will be negative in erythema multiforme but will be positive
in bullous pemphigoid (linear IgG and C3 on the basement membrane), DH
(granular IgA at tips of dermal papillae of uninvolved perilesional skin), and
CBDC (linear IgA on the basement membrane) though DIF can occasionally be
negative in CBDC.
