The histological changes in the skin in dermatomyositis can mimic the changes seen in lupus. Immunohistochemistry is not very helpful to differentiate between the two.

There are some features in histology (can be seen on routine haematoxylin and eosin) that would favour lupus over dermatomyositis. These include features like follicular plugging, perifollicular inflammation, vacuolar change of outer root sheath epithelium and  basement membrane thickening. However, these features will not be present in all cases of lupus and differentiation between the two on histology alone can be difficult.

Immunofluorescence can sometimes be helpful. The presence of a ‘lupus band’ on direct immunofluorescence favours lupus erythematosus, whereas deposition of C5b-9 (membrane attack complex) in the vessels along the dermo epidermal junction favours dermatomyositis. Immunofluorescence requires fresh tissue. Routine skin biopsies are usually sent to the lab in formalin.