Obtaining diagnostic biopsy samples of the skin can be a difficult task. Yet, when a skin biopsy is performed correctly, these samples can yield a wealth of information and, perhaps, hold the key to diagnosis. Perform skin biopsies in cases of suspected neoplasia, vesicular or ulcerative diseases, or unusual or atypical cases as well as in cases that have not responded to conventional trial therapy. In general, perform skin biopsies within three to four weeks of disease onset, since more chronic lesions can be difficult to interpret because of changes secondary to infection, scarring, or corticosteroid therapy.
Since secondary infection can alter histologic findings (e.g. discoid lupus erythematosus and mucocutaneous pyoderma appear similar histologically), pretreatment with antibiotics for two to three weeks may be necessary. Corticosteroid therapy can also change biopsy results, and ideally patients should not receive oral or topical corticosteroids within two to three weeks and injectable long-acting corticosteroids within six to eight weeks before the biopsy procedure (severe, life-threatening cases would obviously be an exception to this rule).
If possible, choose primary lesions such as papules, pustules, vesicles, macules, or nodules. In suspected cases of discoid lupus erythematosus, select early depigmented lesions (before erosion or scarring occurs). Even if primary lesions are not present, diagnostic information can be obtained from crusts, which should be carefully preserved with the skin biopsy samples. Lesions less likely to be diagnostic include excoriations, ulcers, or chronically scarred areas. More information is gained by obtaining multiple (three to five) samples from a variety of lesions.
Administer mild sedation if needed. Hair overlying the lesion may be gently clipped, but the clipper blades should not touch the skin or crusts, if present. Then, nodular lesions may be lightly prepared, but do not scrub or prepare crusted, papular, or pustular lesions, as important surface pathology may be destroyed. Next, inject about 1 ml of 1% to 2% lidocaine subcutaneously under the lesion using a 25-ga needle. Allow enough time for the lidocaine to reach full effect before beginning the biopsy.
For most cases, 6- or 8-mm biopsy punches are preferred; 4-mm biopsy punches may be necessary for difficult areas such as near the eye, on the ear, and on the nasal planum or footpads of smaller patients.
Use new, sharp biopsy punches since older used ones tend to shear and distort tissue and create artifacts. Excisional biopsy with a scalpel may be indicated for larger or nodular lesions or for diseases of the subcutaneous fat.
Place the area of interest in the center of the biopsy punch. Do not include a large amount of normal skin with the biopsy sample or the lesion may be missed when the biopsy sample is processed and cut, resulting in only normal skin being placed on the slide. If normal skin is wanted for comparison, take a sample from an adjacent area. The only time a lesion should be biopsied on the margin is in the case of an ulcerative skin disease. If a large lesion appears to be different in the center from the leading edges, obtain biopsy samples from both areas.
Rotate the biopsy punch firmly in one direction until the blade penetrates all skin layers and rotates freely in the subcutaneous fat. Rotating it back and forth instead of in one direction can cause a shearing artifact.
Use thumb forceps to grasp the underlying fat, avoiding iatrogenic damage to the dermis.
Close the biopsy site with simple interrupted sutures or a cruciate suture pattern.
After obtaining biopsy samples but before fixation, place the samples fat side down on pieces of a wooden tongue depressor to prevent tissue folding and to aid in orientation when samples are processed. Tag biopsy samples from radically different lesions or nodules with a suture or place them in individually labeled containers for differentiation.
Within five minutes of obtaining the samples, fix them in 10% neutral phosphate buffered formalin (minimum 10 parts formalin to 1 part tissue for adequate fixation). When obtaining a sample for tissue cultures, place the sample in a sterile red top tube with a small amount of sterile (not bacteriostatic) saline solution.
Skin histology results are optimized by using experienced dermatopathologists. It is also essential to give a complete signalment and history, including description and distribution of lesions; other signs; the results of pertinent diagnostic tests; current or past therapy and the response to therapy; and your differential diagnoses. These elements are important to allow the pathologist to formulate an accurate diagnosis. If there are questions, most pathologists are happy to discuss their findings with clinicians.