Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
Skin biopsies can provide the skilled clinician with a detailed image of the pathologic processes occurring in diseased skin. Unfortunately, all too often the clinician receives a histologic description and interpretation that may not be clinically helpful. While bad biopsies can certainly happen to good clinicians, there are a few techniques that you can use to maximize the likelihood of obtaining a diagnostic sample.
Understand what you can and cannot reasonably expect from a biopsy. Unfortunately, many dermatologic conditions do not have a pathognomonic histologic appearance. For example, most hypersensitivity disorders can be impossible to distinguish histologically. In these cases, a biopsy will be useful to distinguish a possible hypersensitivity from a non-allergic cause of pruritus (e.g., infection, neoplasia), but cannot be expected to identify the specific hypersensitivity involved (e.g., food allergy.)
Know when to biopsy. Biopsies are most useful when secondary infections and self-trauma are resolved. Often, clients delay in presenting a pet for veterinary care until the problem becomes almost unbearable. As a result, the underlying problem is often overwhelmed or masked by damage/scarring due to self-trauma or by secondary infections with bacteria and/or yeast. It is often prudent to initiate antibiotic/antifungal therapy and place the patient in a restraining cone to prevent self-trauma for 1–2 weeks prior to taking the biopsy. Whenever possible, antiinflammatory medication (such as glucocorticoids) should be avoided for a couple of weeks prior to a biopsy to avoid artifactual alteration of the histologic pattern.
Know how to select sites for biopsy. Some clinical lesions are more likely to yield diagnostic samples than other ones. As a general rule, primary lesions (plaques, pustules, vesicles, nodules and the edges of eroded or ulcerated lesions) tend to yield better diagnostic results. In contrast, biopsies of lichenified skin, excoriated skin, scars and the center of ulcers are unlikely to provide useful information.
Don’t be afraid to take multiple biopsies. There are a number of dermatologic conditions in which the “diagnostic lesion” can be very difficult to find. The likelihood of finding this diagnostic lesion increases with every sample provided. It increases even more when the samples represent a spectrum of the disease process, including older lesions (or the center of lesions), developing lesions (or the edge of spreading lesions) and sometimes even “normal skin” immediately adjacent to the affected area. Similarly, the biopsy must include the affected area. If the lesion to be sampled appears to be subcutaneous in location (as might be the case for some tumors or panniculitis/cellulitis cases), samples that only include the overlying skin may not include the information that the pathologist needs. In this case, an excisional wedge biopsy may be appropriate. Alternately, deep tissue can sometimes be sampled by first taking an 8-mm punch biopsy, then a 6-mm biopsy punch taken through the primary biopsy site. Inform the pathologist if this technique is used.
Know how to prepare for a biopsy. As a general rule, less disruption is better. If the area to be biopsied is in haired skin, the hair should be carefully clipped no shorter than 4–8 mm in length to avoid disturbing surface scales and crusts. The skin to be biopsied should not be scrubbed or soaked in antiseptic unless the biopsy is to be used only for culture. The area should be blocked by subcutaneous (not intradermal) injection of lidocaine, with or without bicarbonate. In some cases, it may be preferable to perform a line or ring block a few centimeters away from the site itself. Examples where this would be appropriate include biopsies to be taken for culture (the preservative in the lidocaine may kill microbes) or lesions that appear to extend to the deep dermis or subcutis. Topical application of anesthetic creams may cause distortion of the epidermis and should be avoided.
Optimize the biopsy procedure itself. The use of cautery to excise tissue for biopsy should be avoided, as it creates coagulative necrosis of the sample. Punch or wedge biopsies are preferred. As a general rule, bigger biopsies provide more and better information than smaller ones. While small biopsies may sometimes be necessary (e.g., 4-mm punches are often used to sample the planum nasale), 6- or 8-mm punches are preferred. New, sharp biopsy punches produce less shearing artifact than dull ones. Punches should be pressed down into the skin and twisted in a single direction until the blade pops through to the subcutaneous tissue. The sample should be carefully grasped by the subcutaneous tissue from underneath the biopsy. The dermis itself should not be grabbed or pinched, as this induces substantial artifact. The biopsy can then be gently blotted to remove excess blood and then placed in at least a 10–fold volume excess of 10% buffered formalin.
Provide the pathologist with as much information as possible. It can be difficult for even the most skilled pathologist to provide a meaningful interpretation from a tiny 4- or 6-mm slice of skin. It can be virtually impossible when there is no context provided for the sample. More information is always better. Provide a complete signalment for the patient, and indicate where the biopsy sample(s) were taken. A thorough history and gross description of the problem should be provided. This should include the duration of the condition, the clinical appearance, any treatment provided (and the response to that treatment), and any current medications. Differential diagnoses should be included as well. If you think that a piece of information is important for you to know, it’s important for the pathologist to know. If possible, include photographs of the patient and the lesions.
The judicious acquisition of skin biopsies provides the clinician with a very powerful diagnostic tool. Knowledge of factors that can contribute to optimal or suboptimal samples will help the clinician maximize the likelihood of obtaining a diagnostic specimen. Most of all, it is important to remember that the pathologist and the clinician must work together as a team for the best results. Having a thorough history, clinical description and differential list allows the pathologist to more accurately narrow down possible differentials. This can minimize the requirement for adjunctive diagnostic tests and will help the pathologist to provide the most reliable information in a timely manner.