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.
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
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
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)
Alopecic lesion, comedones (if present), pigmented and nonpigmented areas (if present)
Follicular atrophy, pigment clumping
How to Improve Skin Biopsy Results
Skin biopsy is rarely a standalone diagnostic test, and results should be considered along with patient history, clinical presentation, and preliminary test results.
If possible, any immunosuppressive medications should be temporarily discontinued and a washout period observed prior to skin biopsy. Although the washout period varies depending on medication used and patient comfort, 30 days can be used as a starting point, with adjustments made based on variables and clinical judgement.
Secondary bacterial infections that may mask the underlying disease should be addressed prior to biopsy. In particular, mucocutaneous pyoderma and discoid lupus erythematosus of the nasal planum can appear identical on histopathology; cytology should be performed to rule out mucocutaneous pyoderma prior to biopsy.
At least 3 samples, relevant clinical history, and digital photographs of the skin lesions (if available) should be submitted.
Distribution of lesions should be described in addition to a short list of differential diagnoses.
A variety of clinical lesions (eg, erythematous and alopecic lesions, crusted lesions, pustular or papular eruptions) should be biopsied to provide samples in various stages of progression (Figure 2).
Stages of lesion progression on the lateral aspect of the trunk, left side. Although lesions appear similar, crust with haired skin (arrowhead), a central lesion with intact diseased epithelium (solid black arrow), and skin that appears healthy (dashed arrow) can be seen. A crusted lesion in a raised area (white arrow) is also present.
To preserve all aspects of the lesion that help aid in diagnosis, lesions collected for histopathology alone should not be surgically prepared or disturbed. Crusts that become separated from the tissue sample should be included.
Samples should be separated into individual containers to correlate lesion description with sample provided for evaluation.
When using a punch biopsy instrument, the center of the active lesion should be biopsied, unlike other biopsy types (eg, incisional biopsy for margin evaluation) in which the healthy/diseased interface is usually sampled. Results are most useful when the entire active disease is sampled. Samples centered on the edge of the lesion should not be submitted to avoid inadvertent evaluation of normal-appearing skin. A separate sample of skin that appears normal may also be submitted.
Samples from ulcerated lesions should include epidermis to determine why the epidermis is diseased and the ulcer developed.
If nodular lesions are present, a sample for deep tissue culture should be collected before samples for histopathology, as sterile materials are required for collection of tissue culture samples.
Well-preserved samples are key. Samples can be preserved by being placed on a tongue depressor or index card and suspended upside down in formalin to preserve architecture and orientation.1 Nontraumatized samples are also essential, as a crushed artifact may render the sample unreadable
Step-by-Step: Skin Biopsy for Diffuse Dermatologic Disease
What You Will Need
Sterile biopsy pack
Punch biopsy instrument (standard, 6 mm; paw pads and nasal planum, 4 mm; large patients, 8 mm)
Sterile Petri dish or tube (submission in culture medium is typically not acceptable)
2% chlorhexidine solution
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).
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.
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.
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.
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.
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.
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.