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4 When, Where, and How to Biopsy Skin
Dawn Logas
KEY POINTS
1 Skin biopsies should be taken when a lesion appears atypical or is not responding as expected to therapy.
2 Most nodular lesions should be biopsied for both histopathology and culture.
3 Be sure to use a veterinary dermatopathologist.
4 Histopathology is just a diagnostic test and may need to be repeated.
When
Although skin biopsies are not a common procedure, they are essential for the diagnosis of various dermatologic conditions. A biopsy should be performed when a probable diagnosis is not clear based on signalment, history, physical exam, or less invasive diagnostic tests. An easy rule of thumb is that a skin biopsy should be taken when the skin lesions appear atypical or the skin is not responding the way you think it should to current therapy. A biopsy should be taken for histopathology if you suspect neoplasia, pre‐ or para‐neoplastic disease, systemic disease with dermatologic markers/manifestations, autoimmune disease, or noninflammatory alopecia (Figure 4.1). Nodular lesions should usually be biopsied for both culture and histopathology, since clinically it can be impossible to tell whether the lesions are infectious or sterile (Figure 4.2). Culturing for aerobic bacteria, anaerobic bacteria, mycobacteria, fungi, and oomycetes may be indicated.
Where
Take multiple samples, preferably at least three to four that represent the variations in the lesions, including early to mature lesions. Try to biopsy primary lesions such as macules, papules, plaques, pustules, vesicles, wheals, nodules, and tumors (Figure 4.3). Secondary lesions such as alopecia, crusting, comedones, follicular casts, and erosions/ulcers should be biopsied if they are representative of the disease (Figure 4.4). Only abnormal tissue should be collected when using punch biopsies (Figure 4.5), otherwise when the biopsy is cut in the lesion may be missed and only normal tissue viewed. For ulcerative or erosive lesions, be sure a little intact epithelium on the edge of the lesion is included in the biopsy if an elliptical biopsy is taken. If punch biopsies are used, one sample should be just of the erosion/ulcer and another sample should be of the lesional area immediately adjacent to and including a small rim of intact epithelium. If a thick crust becomes separated from the underlying tissue of the biopsy sample, place it in the jar of 10% formalin and notify the lab that the specimen is in two pieces. For very crusty lesions, a piece of crust can be included along with the biopsy in the formalin jar, but be sure to inform the pathologist so they will cut it in.
How
The most common biopsy technique used in veterinary dermatology is a punch biopsy (Figure 4.6). Punch biopsies are simple procedures that require minimal instrumentation (Figure 4.6). The biopsy punches range from 2 to 10 mm in diameter, but for most lesions a 6–8 mm punch will suffice. The area chosen for biopsy should consist entirely of affected skin, with little to no normal tissue included. Once you have selected a lesion, gently remove excess hair with scissors or clippers, but do not shave close and do not scrub or prep the area. The only time a lesion should be disinfected is when the biopsy is for a culture. Most punch biopsies can be performed with local anesthesia (lidocaine) only. If the patient is fractious