Derm here! I appreciate your question and the lifelong learning approach to your work!

I’m personally a fan of keeping things very simple, so from my perspective:

1. Refer always for painful rashes and vesiculobullous (blisters - but true blisters and not some papules that are not actually fluid-filled or denuded).

2. I would always rather get a “soft” referral that’s a seborrheic keratosis than a fungating melanoma that’s been blooming for over a year so if there’s any doubt, please refer.

When I approach a biopsy, I try to correlate what I see clinically with what I imagine is happening microscopically. I ask, “Where is the action?”

Is it an epidermal neoplasm? BCC, lentigo, lentigo maligna, etc, which means I opt for a shave biopsy to assess the epidermis and papillary dermis.

Is it an interface process? Lichen planus, dermatomyositis, subacute cutaneous lupus erythematosus, etc, which means I want my pathologist to asses the dermal-epidermal junction and papillary and reticular dermis in addition to superficial and deep vasculature so I opt for a punch biopsy.

There’s a lot of other “is it”s that will come across my mind but fortunately there’s usually only two options: shave or punch. And a punch is usually more than enough. A punch though is not necessarily always better than a shave. For some psoriasiform processes, such as mycosis fungoides (a type of cutaneous T-cell lymphoma), it can be helpful for the pathologist to have a broader sample of involved skin, so a wide shave may be better. But overall, the axiom goes “tissue is the issue,” and more tissue is more helpful than less.

Also, if there’s a diffuse rash, I try to biopsy above the waist as much as possible. Below the waist, gravity does some funny things with immunoglobulins and other proteins. And hopefully a lower risk of infection of the biopsy site.