Skin biopsies are among the most commonly performed biopsies due to the frequency of skin conditions and easy access to lesions. However, the accuracy of skin biopsy interpretation and the resulting pathology report rely heavily on the proper execution of the biopsy procedure. Optimal biopsy procedure involves selecting the best locations to sample, using the most appropriate biopsy technique, ensuring correct storage and shipment of samples, and including comprehensive clinical information that helps the pathologist interpret histologic results.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n Skin biopsies are commonly performed because skin issues are common, the skin is readily accessible, and changes are often quickly noticed by the client or the veterinarian. Taking skin biopsies is a prevalent way to obtain detailed information about a skin lesion and guide the subsequent management of the case.<\/span><\/p>\n Skin biopsies are performed for various reasons, including achieving or confirming a diagnosis; guiding further testing; or, as a last resort, failure of lesion response to treatment. Some conditions, like demodicosis or other mite infestations, can be diagnosed without biopsies, and deep skin scrapes may suffice. However, a negative result from a skin scrape does not exclude the possibility of infection, which makes biopsies a potential subsequent step. <\/span><\/p>\n Biopsies may not always reveal pathogens, especially if pathogens are present in low numbers. Even when special stains are used, rare pathologic agents may remain undetected due to the low sensitivity of special stains on formalin-fixed, paraffin-embedded tissues. Cultures or polymerase chain reaction tests may be more sensitive for detecting a low number of pathogens. Nevertheless, a biopsy may identify specific agents even when scrapes and cultures return negative results, thus enabling a definitive diagnosis.<\/p>\n Before performing biopsies, first check for secondary pyoderma, which can obscure the primary lesions. For cases with secondary pyoderma, treating the infection before performing the biopsy should be considered.<\/span><\/p>\n If the patient was treated with a corticosteroid before biopsy, the inflammatory cell profile may be altered, thereby complicating interpretation of histologic lesions. For instance, eosinophil counts may be significantly decreased after steroid therapy, making the diagnosis of allergic dermatitis challenging. Before performing the biopsy, it is advisable to discontinue oral corticosteroids for at least 2 weeks and injectable steroids for 4 to 8 weeks, depending on the half-life of the drug. However, when lesions are severe and stopping the corticosteroid medication would result in patient suffering, it is acceptable to proceed with biopsies without discontinuing the drugs.<\/span><\/p>\n Hair should be trimmed using scissors; use of electric clippers should be avoided as they can cause artifacts and damage delicate features.<\/span><\/p>\n For superficial lesions, avoid scrubbing the lesion before performing the biopsy. Scrubbing may eliminate key diagnostic features (e.g., crusts containing acantholytic cells or infectious agents). The lesions should therefore either remain uncleansed or be gently cleansed with water and patted dry, ensuring that any crusts or scales remain undisturbed. <\/span><\/p>\n For deep dermal or nodular lesions, cleansing the skin by scrubbing the surface is acceptable because the diagnostic features in the deeper layers of the skin tissue remain visible.<\/span><\/p>\n Injecting local anesthetics (e.g., lidocaine) into the subcutaneous tissue instead of the dermis helps avoid artifacts that can mimic the appearance of dermal edema.<\/span><\/p>\n For areas affected by alopecia, marking the direction of hair growth by drawing a line with a Sharpie can greatly assist the pathologist during the trimming process (cutting the sample in smaller sections to fit the tissue cassettes). Marking allows for a cut that follows the hair follicles, which enables assessment of the entire length of the hair follicle (<\/span>FIGURE 1<\/b><\/span>). Note that a dot does not effectively indicate the direction of the cut.<\/span><\/p>\n Figure 1. Recommended step before performing a biopsy of an alopecic area. A line is drawn in the direction of hair follicles to facilitate trimming along the entire length of the hair follicles. Illustration: Kip Carter<\/p><\/div>\n It is crucial to biopsy fully developed primary lesions, if clinically available, because later stages of lesions may reflect only chronicity of the changes (e.g., fibrosis) and the original cause for the lesions may no longer be visible. For instance, a pustule containing acantholytic cells is typically indicative of pemphigus foliaceus, but older crusts might reveal only secondary bacterial contamination and degenerated cells, making a definitive diagnosis of pemphigus foliaceus impossible. Early lesions may or may not be diagnostic. Before a diagnosis can be reached, repeated biopsy samples may need to be submitted. In some cases, collecting lesions from numerous stages of maturation may be helpful.<\/span><\/p>\n Skin lesions can be primary or secondary (<\/span>TABLE 1<\/b><\/span>). However, the same type of lesion may be primary in one disease and secondary in another. For instance, alopecia is a primary lesion in endocrinopathies resulting from hair cycle arrest; however, it can also occur as a secondary lesion (e.g., in allergic dermatitis) as a result of mast cell degranulation or trauma to the hair follicles (e.g., scratching or overgrooming due to pruritus).<\/span><\/p>\n When lesions are multifocal, performing multiple skin biopsies enhances the likelihood of identifying a diagnostic feature in the samples. Typically, 3 to 4\u00a0biopsies are suggested, except when the lesion is focal and small enough to be entirely submitted through a single biopsy. If biopsy samples are collected from different locations on the body and lesions have a different appearance, submitting the samples in separate containers with proper labels will aid in the accurate interpretation of the lesions. <\/span><\/p>\n Biopsy samples should be collected from the center of the lesion. Additional samples may be obtained from the lesion\u2019s edge and unaltered skin (<\/span>FIGURE 2A<\/b><\/span>). Collecting biopsies solely from the margins of lesions should be avoided because margins may only contain fibrosis and granulation tissue and lack diagnostic features (<\/span>FIGURE 2B<\/b><\/span>). However, for ulcers, vesicles, and bullae, biopsies should be taken at the lesion\u2019s edge, including the healthy epidermis (<\/span>FIGURE 2C<\/b><\/span>), because for these lesions, it is crucial to examine the transition from the vesicle or ulcer to healthy skin. Doing so is key to identifying the precise location of the separation between the epidermis and the dermis (e.g., a suprabasilar split in pemphigus vulgaris, a subepithelial split in subepithelial bullous diseases) or detecting any signs of interface dermatitis that may predispose to a vesicular lesion or ulcer. If a biopsy sample is obtained from the center of a lesion, the overlying epidermis may not be included in the sample, making a definitive diagnosis unlikely.<\/span><\/p>\n\n
What to Consider Before Performing Biopsies<\/h2>\n
Secondary Conditions<\/h3>\n
Corticosteroid Use<\/h3>\n
Hair Removal<\/h3>\n
Cleansing<\/h3>\n
Local Anesthesia<\/h3>\n
Marking<\/h3>\n
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<\/a>Which Lesions to Biopsy<\/h2>\n
<\/a><\/p>\nHow Many Biopsy Samples to Submit<\/h2>\n
Identifying The Best Location to Biopsy<\/h2>\n
Figure 2. Recommended biopsy procedures. (A) Biopsy locations of healthy and lesional skin. The recommended sites ranked from most to least important are indicated. The first 3\u00a0primary biopsies (green) are from the center of multiple lesions, and submitting them increases the likelihood that samples will be diagnostic. It is advantageous to submit the other 2 biopsies (orange) in addition to the primary biopsies if the situation permits. Illustration: Kip Carter<\/span><\/a><\/div><\/div>