Impetigo Workup

Updated: Jun 29, 2023
  • Author: Amanda T Moon, MD; Chief Editor: Russell W Steele, MD  more...
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Approach Considerations

Diagnosis of impetigo is usually based solely on history and clinical appearance. Bacterial culture and sensitivity are recommended (1) to identify possible methicillin-resistant Staphylococcus aureus (MRSA), (2) if an outbreak of impetigo has occurred, or (3) if poststreptococcal glomerulonephritis is present. Evidence of previous streptococcal skin infection may be sought in individuals in whom acute poststreptococcal glomerulonephritis (APSGN) is suspected.

In patients with nonbullous lesions, after cleansing the honey-colored crusted lesion and uplifting the scab, a bacterial culture of the fresh exudate underneath the scab may be obtained. In patients with bullous lesions, Gram stain and culture of the blister fluid is performed. On Gram stain, the presence of gram-positive cocci in chains indicates Streptococcus pyogenes; gram-positive cocci in clusters indicate S aureus. Culture and sensitivity results can help the physician choose appropriate antibiotic therapy.

Documentation of a recent streptococcal skin infection in the differential diagnosis of APSGN is accomplished best by obtaining antideoxyribonuclease B (anti-DNase B) and antihyaluronidase (AH) titers. More than 92% of patients with impetigo-associated APSGN have elevated anti-DNase B titers. Patients with impetigo have a poor antistreptolysin O (ASO) serologic response; only 51% of patients with impetigo-associated APSGN develop an increased ASO titer.

Urinalysis is necessary to evaluate for APSGN if the patient develops new-onset edema or hypertension. Hematuria, proteinuria, and cylindruria are indicators of renal involvement.

A potassium hydroxide wet mount may be performed to exclude bullous dermatophyte infection. A Tzanck preparation or viral culture may be performed to exclude herpes simplex infection.

A bacterial culture of the nares may be obtained to determine whether a patient is an S aureus carrier. If the nares culture is negative and the patient has persistent recurrent episodes of impetigo, bacterial cultures should be obtained from the axillae, pharynx, and perineum.

Obtain serum IgM levels in cases of recurrent impetigo in patients with negative S aureus carrier status and no predisposing factors such as a preexisting dermatosis. [29] Serum level determination of IgA, IgM, and IgG, including IgG subclasses, is necessary to rule out other immunodeficiencies.


Histologic Findings

Biopsy may be appropriate in doubtful or refractory cases of impetigo. [6] In bullous impetigo, few or no inflammatory cells are present within the bulla. A polymorphous infiltrate is present in the upper dermis. Acantholysis is noted in the granular layer.

In nonbullous impetigo, a serum crust is present above the epidermis. Neutrophils are common within the crust. In addition, gram-positive cocci are seen. Epidermal spongiosis and a severe dermal infiltrate of neutrophils and lymphoid cells are seen.