Is a biopsy of either skin or muscle always indicated in the diagnosis of dermatomyositis?

A biopsy of either skin or muscle is not always necessary and the need for each depends on each patient's presentation. A typical history and physical exam, along with MRI or EMG findings, or more importantly, a positive myositis specific antibody can be adequate to make the diagnosis of dermatomyositis. Therefore, we now tend to order a biopsy to rule out other conditions or in the case of an atypical presentation.

In regards to the skin biopsy specifically, it is a relatively low risk procedure and easy to be performed during an outpatient visit. However, one needs to take into account that only a positive result can be evaluated, as the disease can be patchy; interface dermatitis is not specific for dermatomyositis (can be seen also in ...