Understanding Your Dog's Skin Biopsy Results

By Emiel Maddens  ·  Reviewed in consultation with licensed veterinary professionals  ·  Updated June 2026  ·  10 min read

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After your veterinarian removes or samples a skin growth, lump, or abnormal skin area from your dog, the tissue is sent to a veterinary pathologist for examination under a microscope. The resulting pathology report provides the definitive diagnosis, which guides treatment decisions and informs you about your dog's prognosis. For many pet owners, receiving a pathology report can feel overwhelming, as the document is written in medical terminology that may seem impenetrable without a background in medicine or biology.

Understanding the basic structure and key terms of a pathology report can help you participate more meaningfully in conversations with your veterinarian about your dog's diagnosis and treatment plan. While your vet will always interpret the results and explain them to you, having a foundational understanding allows you to ask better questions, understand the reasoning behind treatment recommendations, and feel more confident in the decisions you make for your dog's care.

This guide walks you through the main components of a veterinary skin pathology report, explains the most commonly used terminology, and discusses what different findings mean for your dog's treatment and outlook.

The Biopsy Process: From Tissue to Diagnosis

Types of Skin Biopsies

There are several types of skin biopsies, and the type performed depends on the clinical situation. An excisional biopsy removes the entire lesion, including a margin of surrounding normal tissue, and is both diagnostic and potentially therapeutic. This is the most common approach for discrete skin masses where complete removal is the goal. An incisional biopsy removes only a representative portion of a larger lesion and is used when complete excision is not immediately feasible, when the lesion is too large for excision without first knowing the diagnosis, or when the clinical presentation suggests a condition best diagnosed by sampling the edge where abnormal meets normal tissue. A punch biopsy uses a circular cutting instrument (typically 6 to 8 millimeters in diameter) to obtain a cylindrical core of tissue through the full thickness of the skin. Punch biopsies are ideal for diffuse skin diseases, rashes, and areas of abnormal pigmentation or texture where a representative sample is needed rather than complete removal.

How Tissue Is Processed

After collection, the tissue is placed in formalin, a preservative that fixes the cells in place and prevents decomposition. The fixed tissue is then embedded in paraffin wax, which allows it to be sliced into extremely thin sections (typically 4 to 5 micrometers thick) using a precision instrument called a microtome. These tissue sections are mounted on glass slides and stained, most commonly with hematoxylin and eosin (H&E), which colors cell nuclei blue-purple and cytoplasm pink. The stained slides are then examined by a board-certified veterinary pathologist who evaluates the cellular architecture, identifies abnormal cells, assesses tissue organization, and formulates a diagnosis. Special stains and immunohistochemistry may be applied to additional sections if the initial evaluation requires further characterization of the tissue or cells.

Turnaround Time and Communication

Standard pathology results typically take five to ten business days from the time the tissue arrives at the laboratory, though rush processing may be available for urgent cases at additional cost. Some general practice veterinary clinics have in-house cytology capabilities for quick preliminary assessments, but histopathology is always performed at a reference laboratory by a board-certified veterinary pathologist. Your veterinarian receives the official report and will contact you to discuss the findings, typically by phone for significant diagnoses. If you request a copy of the full pathology report (which you are entitled to), you will receive the same document your veterinarian received, which is what this article will help you understand.

Reading the Pathology Report: Section by Section

Patient and Sample Information

The top of the report contains identifying information: your dog's name, breed, age, and sex, along with your veterinarian's name and clinic, the date the sample was received, and the laboratory accession number. The clinical history section summarizes the information your vet provided to the pathologist, including the location of the biopsy site, clinical appearance of the lesion, duration, and any relevant medical history. This context is crucial because the pathologist's interpretation is guided not only by what they see under the microscope but also by the clinical scenario. A round cell tumor in the skin of a two-year-old dog has a different differential diagnosis than the same cells in a ten-year-old dog, and providing accurate clinical history helps the pathologist reach the most precise diagnosis.

Gross Description

The gross description details the physical appearance of the tissue sample as it arrived at the laboratory, before any microscopic processing. It includes the number of tissue pieces, their dimensions (measured in centimeters or millimeters), shape, color, consistency (firm, soft, friable), and surface characteristics. For excisional biopsies, the gross description notes whether surgical margins are inked (the pathologist applies colored ink to the outer surfaces of the tissue so that margins can be assessed under the microscope). The gross description may also note whether the specimen was sectioned (cut into slices) for processing and how many tissue blocks and slides were prepared. While this section may seem mundane, it provides important documentation of the sample's physical characteristics and processing.

Histopathological Description

This is the most detailed section of the report and describes what the pathologist sees under the microscope. It typically begins with the overall tissue architecture, describing the arrangement of cells within the skin layers (epidermis, dermis, subcutis). For tumors, it describes the cell type, growth pattern, cellular organization, nuclear characteristics, and relationship to surrounding normal tissue. For inflammatory conditions, it describes the type and distribution of inflammatory cells, any changes to the skin layers, and associated features such as fibrosis (scar tissue formation) or necrosis (tissue death). This section uses specialized medical terminology that can be difficult for non-medical readers to parse, but key terms are explained later in this article.

Diagnosis and Comments

The diagnosis line provides the pathologist's conclusion, which is the most important piece of information in the report. It may be a single definitive diagnosis (such as "cutaneous mast cell tumor, low-grade") or a list of differential diagnoses ranked by likelihood if the findings are not entirely specific. The comments section, present in many reports, provides the pathologist's interpretation of the findings in a clinical context. This is where the pathologist may discuss prognosis, recommend additional testing (such as immunohistochemistry, c-kit mutation analysis, or Ki-67 proliferation index), comment on surgical margins, or suggest follow-up. The comments section is often the most readable part of the report for non-medical professionals and provides valuable guidance for clinical decision-making.

Working with your vet to understand biopsy results ensures the best possible treatment decisions for your dog.

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Key Pathology Terms Explained

Terms Describing Cell Behavior

Benign means the growth is non-cancerous and not expected to spread to other parts of the body. Malignant means the growth is cancerous and has the potential to invade surrounding tissues or metastasize (spread) to distant sites. Well-differentiated describes tumor cells that closely resemble normal cells of their tissue of origin, suggesting less aggressive behavior. Poorly differentiated describes tumor cells that look significantly different from normal cells, suggesting more aggressive behavior. Mitotic index or mitotic count refers to the number of cells actively dividing (undergoing mitosis) in a specified number of microscopic fields, with higher counts generally indicating faster growth and more aggressive potential. Anaplastic describes severely abnormal-looking cells with loss of normal structure, typically associated with high-grade malignancy.

Terms Describing Tissue Changes

Hyperplasia is an increase in the number of cells in a tissue, resulting in enlargement but maintaining normal cell appearance and organization. Dysplasia refers to abnormal cell growth with disordered arrangement, which can be a precancerous change. Neoplasia means new growth and is the medical term for a tumor, whether benign or malignant. Inflammation is described by the type of inflammatory cells present: neutrophilic (bacterial infection), lymphocytic or lymphoplasmacytic (immune-mediated or chronic), eosinophilic (allergic or parasitic), and granulomatous (foreign body or specific infections). Fibrosis refers to scar tissue formation. Necrosis is tissue death. Ulceration means loss of the surface epithelial layer. Acanthosis is thickening of the epidermis. Hyperkeratosis is excessive buildup of keratin on the skin surface.

Margin Terminology

Margin evaluation is one of the most critical aspects of a tumor biopsy report. Complete (or clean) margins mean that normal tissue surrounds the tumor on all sides, with no tumor cells extending to the cut edge of the specimen. This indicates the tumor was fully removed. Incomplete (or dirty) margins mean tumor cells extend to or very near the cut edge, suggesting that tumor cells may remain in the patient. Close margins describe a narrow band of normal tissue between the tumor edge and the cut edge, which may or may not be sufficient depending on tumor type. The pathologist typically measures the nearest margin in millimeters. For mast cell tumors, a minimum margin of 2 millimeters of clean tissue is generally considered adequate, while other tumor types may have different requirements based on their biology.

Special Stains and Advanced Testing

Beyond standard H&E staining, the pathologist may employ additional techniques to refine the diagnosis. Immunohistochemistry (IHC) uses antibodies that bind to specific cell markers, helping identify the cell type of origin when it is not clear from morphology alone. For example, CD117 (KIT) staining helps confirm mast cell tumors and assess staining pattern, while melan-A and PNL2 help identify melanocytic tumors. The Ki-67 proliferation index uses IHC to quantify the percentage of tumor cells that are actively proliferating, providing prognostic information beyond what the mitotic count alone can offer. AgNOR (argyrophilic nucleolar organizer region) counting is another proliferation marker. PCNA (proliferating cell nuclear antigen) staining serves a similar purpose. These advanced tests add to the cost and turnaround time but can provide invaluable prognostic and treatment-planning information for borderline or ambiguous cases.

Common Diagnoses and What They Mean

Benign Tumors

If the biopsy report returns a diagnosis of a benign tumor, the news is generally very good. Common benign skin tumor diagnoses include lipoma (fatty tumor), histiocytoma (Langerhans cell tumor in young dogs), sebaceous adenoma or epithelioma, benign melanocytoma, fibroma, and various cyst types (infundibular cyst, dermoid cyst). Benign tumors that are completely excised (clean margins) are considered cured, and no further treatment is needed for that specific lesion. However, dogs predisposed to certain tumor types may develop additional tumors at other sites over time. Your veterinarian may recommend regular monitoring for new growths, particularly in breeds or individuals with a history of multiple skin tumors.

Malignant Tumors

A malignant tumor diagnosis initiates a more involved process of staging, prognosis assessment, and treatment planning. The report's tumor grade, mitotic index, margin status, and any special staining results all feed into the overall prognostic picture. Common malignant skin tumor diagnoses include mast cell tumor (with Kiupel low or high-grade designation), soft tissue sarcoma (with grading), melanoma (with location-specific behavior assessment), squamous cell carcinoma, and hemangiosarcoma. Your veterinarian, possibly in consultation with a veterinary oncologist, will review the pathology findings alongside staging results (lymph node assessment, imaging) to recommend a treatment plan tailored to your dog's specific situation. It is important to remember that "malignant" does not automatically mean a poor prognosis. Many malignant tumors are highly treatable, especially when caught early.

Inflammatory and Immune-Mediated Conditions

Not all skin biopsies are performed for tumors. Biopsies of inflamed or abnormal-appearing skin can diagnose a wide range of conditions. Common inflammatory diagnoses include pyoderma (bacterial skin infection), dermatophytosis (fungal infection), demodicosis (Demodex mite infestation), allergic dermatitis, and various patterns of non-specific dermatitis. Immune-mediated diagnoses include pemphigus foliaceus (with characteristic acantholysis), discoid lupus erythematosus (with interface dermatitis), sebaceous adenitis (with granulomatous inflammation targeting sebaceous glands), and alopecia areata. Endocrine-related skin changes, such as the follicular atrophy and excessive telogenization seen in hypothyroidism, can also be identified on biopsy. Each diagnosis carries specific treatment implications and prognosis that your veterinarian will discuss with you.

When the Diagnosis Is Uncertain

Not every biopsy produces a clear-cut, definitive diagnosis. Sometimes the pathologist provides a list of differential diagnoses rather than a single conclusion, indicating that the microscopic findings are consistent with multiple conditions and additional information is needed to narrow the diagnosis. This may happen when the biopsy sample is small, when the lesion was at an atypical stage of development, or when the condition is uncommon. In these cases, the pathologist may recommend additional biopsies, special stains, immunohistochemistry, or correlation with specific clinical tests (such as serology or fungal culture). An uncertain initial result is not a failure of the biopsy process but rather an honest assessment that guides the next diagnostic steps toward a conclusive answer.

What Comes After the Biopsy Results

Discussing Results with Your Veterinarian

When your veterinarian calls with biopsy results, prepare a few key questions to make the most of the conversation. Ask about the specific diagnosis and what it means for your dog's health. If it is a tumor, ask about the grade, margin status, and whether additional treatment or staging is recommended. Ask about the expected course of the condition and what signs to watch for going forward. If the terminology in the report is confusing, ask your vet to explain it in plain language, and request a printed or emailed copy of the full report for your records. If the diagnosis involves a malignancy or complex condition, ask whether a referral to a specialist (veterinary oncologist or dermatologist) would be beneficial for your dog's care.

Understanding Next Steps for Tumors

For tumors, the biopsy results dictate the next steps. A benign tumor with complete margins typically needs no further treatment, just regular monitoring for new growths. A benign tumor with incomplete margins may warrant monitoring (for types with very low recurrence risk) or revision surgery (for types that tend to regrow). A malignant tumor with complete margins may need staging to determine whether adjuvant therapy is warranted. A malignant tumor with incomplete margins usually requires additional treatment, either revision surgery to widen the margins, radiation therapy to the surgical site, or both. High-grade malignancies typically undergo full staging and are discussed with a veterinary oncologist to develop a comprehensive treatment plan.

Seeking a Second Opinion

If you are uncertain about the diagnosis or the recommended treatment plan, seeking a second opinion from another veterinary pathologist is entirely reasonable and within your rights as a pet owner. Your veterinarian can arrange for the original tissue blocks and slides to be sent to another reference laboratory for independent review. This is particularly worthwhile for borderline diagnoses (such as a Grade II mast cell tumor where additional prognostic testing might change the treatment approach), rare conditions where specialist expertise may be beneficial, or any situation where the stakes of the diagnosis are high. Second opinions do not reflect negatively on your primary veterinarian, as pathology interpretation involves professional judgment, and peer consultation is a standard and valued practice in medicine.

Keeping Records for the Future

Maintain a complete file of your dog's pathology reports, including the laboratory name, accession number, date, and diagnosis. If your dog ever changes veterinary clinics, these records provide crucial medical history. The laboratory accession number allows the original tissue blocks and slides to be retrieved from the lab's archives, often for years after the initial biopsy, which can be valuable if a new pathologist needs to review the material or if additional testing is needed at a later date. For dogs with a history of skin tumors, the cumulative pathology record helps future veterinarians understand the pattern of tumor development and tailor screening and treatment accordingly.

Understanding Your Dog's Skin Biopsy Results FAQ

Q: How long does it take to get skin biopsy results?

Standard histopathology results typically take five to ten business days from the time the tissue arrives at the reference laboratory. If special stains or immunohistochemistry are needed, the turnaround time may extend by an additional three to five days. Rush processing is available at most laboratories for urgent cases, usually with results in one to three business days at additional cost. Your veterinarian will let you know the expected timeline and will contact you as soon as results are available.

Q: What does 'complete margins' mean on a biopsy report?

Complete (or clean) margins mean that when the pathologist examined the edges of the tissue that was removed, no tumor cells were found at or near the cut boundaries. This indicates that the tumor was fully contained within the excised tissue and, by extension, is likely to have been completely removed from your dog. Complete margins are one of the most important positive findings in a tumor biopsy report, as they significantly reduce the risk of local recurrence.

Q: What happens if margins are incomplete?

Incomplete margins mean that tumor cells extend to the edge of the excised tissue, indicating that some tumor cells likely remain in your dog. The next steps depend on the tumor type and grade. For low-grade, slow-growing benign tumors, close monitoring may be sufficient. For malignant tumors or tumors with higher recurrence risk, options include revision surgery to remove additional tissue, radiation therapy to the surgical site, or a combination of both. Your veterinarian will discuss the specific recommendations based on the tumor type and your dog's overall situation.

Q: Should I ask for a copy of the pathology report?

Yes, it is a good idea to request a copy of every pathology report generated for your dog. The report becomes an important part of your dog's medical record and provides crucial baseline information for future comparisons. If you ever change veterinarians or seek a specialist referral, having your own copy ensures continuity of care. You are entitled to copies of your pet's medical records, including pathology reports, and most veterinary clinics will provide them at no charge or a minimal copying fee.

Q: Can biopsy results be wrong?

While veterinary pathologists are highly trained specialists and their diagnoses are generally reliable, pathology interpretation involves professional judgment, and borderline cases can be challenging. Factors that can affect diagnostic accuracy include small or poorly representative biopsy samples, unusual presentations of common conditions, and rare diseases that may not be initially considered. If you or your veterinarian have questions about a diagnosis, requesting a second opinion from another board-certified pathologist is a standard and appropriate practice. The original slides and tissue blocks can be sent to another laboratory for independent review.

Sources

Gross, T.L., Ihrke, P.J., Walder, E.J., & Affolter, V.K. (2005). Skin Diseases of the Dog and Cat: Clinical and Histopathologic Diagnosis, 2nd Edition. Blackwell.

Kiupel, M. et al. (2011). Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors. Veterinary Pathology, 48(1), 147-155.

Meuten, D.J. (2016). Tumors in Domestic Animals, 5th Edition. Wiley-Blackwell.

Miller, W.H., Griffin, C.E., & Campbell, K.L. (2013). Muller and Kirk's Small Animal Dermatology, 7th Edition. Elsevier.

Sharkey, L.C., Dial, S.M., & Matz, M.E. (2007). Maximizing the diagnostic value of cytology in small animal practice. Veterinary Clinics of North America: Small Animal Practice, 37(2), 351-372.

Related Reading

• Mast Cell Tumors in Dogs, Understanding grading and prognosis after a mast cell tumor diagnosis
• Melanoma in Dogs, How biopsy results guide treatment for different melanoma types
• Monthly Skin Check Guide, Early detection starts with regular at-home skin examinations
• Sebaceous Cysts in Dogs, What biopsy reveals about different types of skin cysts

Emiel Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.