Skin Biopsy for Diffuse Dermatologic Disease in Cats & Dogs

Elizabeth R. Drake, DVM, DACVD, University of Tennessee

ArticleLast Updated June 20234 min read
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Knowing which diseases are best diagnosed via skin biopsy may aid in selection of cases that should be submitted for pathology. Careful clinical description of dermatologic lesions (eg, erythematous and alopecic lesions, crusted lesions, pustular or papular eruptions) from multiple parts of the body, a clear and concise patient history, and a brief differential diagnosis list can help increase the likelihood of diagnostic success. In addition, submission of samples to a veterinary pathologist adept at dermatohistopathology can enhance results, as these specialists are accustomed to reviewing skin biopsies and have greater awareness of newly reported diseases.


Case Selection 

Skin biopsy samples submitted for histopathology are most reliable for diagnosing neoplasia, immune-mediated and autoimmune diseases, some alopecic disorders, and deep infections (eg, fungal and mycobacterial diseases). Histopathology is not sensitive or specific for diagnosing allergy or endocrine diseases, which are common; skin biopsy for these types of suspected lesions is therefore not regularly used in the clinic (Table). 

Table: Diseases Best Diagnosed via Skin Biopsy

Differential Diagnoses 

Primary/Secondary Lesions 

Samples  

Features  

Autoimmune disease

Crusts 

Crusts, pustules 

Subcorneal pustules with neutrophils and acantholytic cells 

Infectious, sterile inflammatory, and neoplastic disease 

Nodules, draining tracts 

Deep sample to include panniculus (requires collection of adipose tissue)  

Panniculitis (subcutaneous fat is target of disease process) 

Drug reactions, pemphigus vulgaris, blistering skin diseases, cutaneous lymphoma 

Ulcers, erosions 

Vesicle (if present), ulcerated lesion with and without diseased epidermis 

Cleft formation, apoptotic keratinocytes, necrosis 

Color dilution alopecia, follicular dysplasia, hair cycling disorders (eg, alopecia X, cyclic flank alopecia)  

Alopecia 

Alopecic lesion, comedones (if present), pigmented and nonpigmented areas (if present) 

Follicular atrophy, pigment clumping 


Step-by-Step: Skin Biopsy for Diffuse Dermatologic Disease

Step 1: Sedate the Patient (Optional)

In fractious or painful patients, administer a sedative to ensure comfort and help the patient remain still for collection of high-quality samples.

Step 2: Select the Lesions

Identify and use a permanent marker to mark lesions of interest with 4 framing dots before performing a local block to ensure anesthetized areas are selected.

Step 3: Administer Local Anesthesia

Dilute 2% lidocaine with sterile water in a 1:1 ratio. Locally infuse the subcutaneous tissue directly below the area to be sampled until a small bleb is formed (the maximum amount to safely administer is based on patient body weight).  

Author Insight 

A small amount of sodium bicarbonate may be added to buffer the lidocaine and minimize stinging during injection. Caution should be used with small dogs and cats to avoid a toxic lidocaine dose (dogs, 10 mg/kg; cats, 5 mg/kg). 

Step 4: Prepare the Skin 

If collecting samples for culture, gently scrub the surface with 2% chlorhexidine solution and rinse thoroughly with sterile saline.

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If collecting samples for histopathology, do not scrub or disrupt lesional skin. Trim the hair if needed, but do not shave with clippers. 

Step 5: Collect Tissue Samples

Use sterile instruments and materials to collect at least 3 samples (if possible) with a punch biopsy instrument (6 mm for routine skin biopsy samples; 4 mm for nasal planum and pinnal biopsies [smaller instruments here can help avoid disfiguring the patient]); only a single 4- to 6-mm tissue sample is needed for culture submission. Rotate the instrument in one direction to avoid sample disruption. Place the sample in a sterile container.

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Author Insight 

The sample for tissue culture should be collected first and placed in a sterile container for submission to a microbiology laboratory for macerated tissue cultures (usually aerobic, anaerobic, fungal/dermatophytes). Collecting samples for tissue cultures first ensures any growth reported by the laboratory is part of the disease process, not contamination. Samples for histopathology are collected after those for tissue culture. 

Step 6: Preserve Samples for Histopathology

Place each sample on an index card or tongue depressor, and allow time for sample adherence. Place each sample upside down in a leak-proof, formalin-filled container. Ensure the samples remain submerged in formalin.

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Author Insight 

Sterility is not needed at this point, but clean techniques should be used. The focus is sampling lesions at different stages of disease progression (Figure 1); therefore, lesions should be separated (eg, collection of a pustule, crust, and erythematous lesion) for better interpretation of the sample. Which sample came from which clinical lesion cannot be discerned once the sample is placed in formalin.

image source
FIGURE 1

Stages of lesion development involving the nasal planum and muzzle. The abnormal border between haired and nonhaired skin showing erosions, hypopigmentation, and abnormal loss of cobblestone architecture (arrowhead); nonhaired skin exhibiting erythema and loss of pigment also affecting the nasal planum and suggestive of epidermal–dermal junction inflammation (dashed arrow); and abnormal haired skin with erythema and raised plaques (black arrow) can be seen. Each of these areas, as well as the outside edge of the nostril (ie, alar fold; solid white arrow), should be sampled if possible. 

Step 7: Close the Skin 

Depending on the location biopsied and patient temperament, use nonabsorbable sutures to close each site with a simple interrupted or cruciate suture pattern.

Author Insight 

Nonabsorbable sutures can minimize the chance of suture reactions in cats and diseased tissue. Absorbable sutures can be substituted in patients in which sutures would be difficult to remove.