Investigations
1st investigations to order
clinical diagnosis
Test
Most cases of EM can be diagnosed by history and clinical examination alone, and no further investigations are needed.
Result
typical clinical features
Investigations to consider
haematoxylin and eosin biopsy
Test
Biopsy is not required in clinically typical cases. When Rowell's 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 oedema; lymphocytic infiltration and non-specific immune deposits
immunofluorescence biopsy
FBC
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
Possible elevated blood urea nitrogen/creatinine if decreased fluid intake.
Result
may show evidence of volume depletion
herpes simplex virus (HSV) serology
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
Detects DNA in fluids and tissues. Samples from lesions are useful in differentiating from herpes simplex. Herpes zoster-associated EM can be differentiated from generalised extension of herpes by rapid PCR.[12]
Result
positive for varicella DNA
cold-haemagglutination serology
Test
Can be used to confirm diagnosis of Mycoplasma pneumonia.
Result
positive if M pneumoniae infection
M pneumoniae titres
Test
Can be used to confirm diagnosis of Mycoplasma pneumonia.
Result
positive if M pneumoniae infection
CXR
Test
Mycoplasma pneumonia can cause many different radiological changes. The most common are bronchial thickening with areas of interstitial infiltration and subsegmental atelectasis involving a lower lobe.
Result
highly variable appearance
auto-antibody titres
Test
Subacute cutaneous lupus erythematosus is associated with the presence of auto-antibodies and photosensitivity in sun-exposed areas. Rowell's syndrome might be a morphological variant.[23]
Result
increased levels of anti-Ro (SSA), anti-La (SSB) and antinuclear antibodies, and rheumatoid factor
Emerging tests
anti-desmoplakin antibodies
Test
Have been noted in patients with recurrent EM.[36]
Result
may be present
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