Tests

1st tests to order

clinical diagnosis

Test
Result
Test

Most cases of EM can be diagnosed by history and clinical exam alone, and no further investigations are needed.

Result

typical clinical features

Tests to consider

hematoxylin and eosin biopsy

Test
Result
Test

Biopsy is not required in clinically typical cases. When Rowell syndrome (overlap with systemic lupus erythematosus) is suspected, biopsy can aid in diagnosis.[32]

Result

satellite cell necrosis, vacuolar degeneration of the basement membrane, severe papillary edema; lymphocytic infiltration and nonspecific immune deposits

immunofluorescence biopsy

Test
Result
Test

Performed if hematoxylin and eosin biopsy is not clearly demonstrative of EM.[2][3]

Result

IgM and C3 at the basement membrane and perivascularly

CBC

Test
Result
Test

Can aid diagnosis when infection has not been well documented, especially when fever or symptoms of upper respiratory infection are found.

Result

abnormal white cell count

serum electrolytes

Test
Result
Test

Possible elevated blood urea nitrogen/creatinine if decreased fluid intake.

Result

may show evidence of volume depletion

herpes simplex virus (HSV) serology

Test
Result
Test

HSV serology can be useful if there have been recurrent episodes of EM but there are no specific HSV lesions found.

If serology is negative in recurrent disease, other infections should be considered.

Result

positive for HSV-1 or 2 IgM if HSV infection

rapid polymerase chain reaction (PCR)

Test
Result
Test

Detects DNA in fluids and tissues. Samples from lesions are useful in differentiating from herpes simplex. Herpes zoster-associated EM can be differentiated from generalized extension of herpes by rapid PCR.[12]

Result

positive for varicella DNA

cold-hemagglutination serology

Test
Result
Test

Can be used to confirm diagnosis of Mycoplasma pneumonia.

Result

positive if M pneumoniae infection

M pneumoniae titers

Test
Result
Test

Can be used to confirm diagnosis of Mycoplasma pneumonia.

Result

positive if M pneumoniae infection

CXR

Test
Result
Test

Mycoplasma pneumonia can cause many different radiologic changes. The most common are bronchial thickening with areas of interstitial infiltration and subsegmental atelectasis involving a lower lobe.

Result

highly variable appearance

autoantibody titers

Test
Result
Test

Subacute cutaneous lupus erythematosus is associated with the presence of autoantibodies and photosensitivity in sun-exposed areas. Rowell syndrome might be a morphologic variant.[23]

Result

increased levels of anti-Ro (SSA), anti-La (SSB) and antinuclear antibodies, and rheumatoid factor

Emerging tests

antidesmoplakin antibodies

Test
Result
Test

Have been noted in patients with recurrent EM.[36]

Result

may be present

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