What to Expect with Your Dog's Skin Biopsy

A skin biopsy is a small surgery that removes a piece of skin, or a whole lump, so a pathologist can examine it under a microscope. Vets use it in two main situations: to find out exactly what a lump is, and to diagnose skin disease that hasn't responded to treatment.

For most skin biopsies, your dog gets local anesthetic, often with sedation, and goes home the same day with a stitch or two that comes out in 10 to 14 days (VCA Animal Hospitals, n.d.). Results usually take about 3 to 7 days (American Animal Hospital Association [AAHA], 2026). A biopsy tells you far more than a needle sample can, including whether a tumor is benign or malignant, how aggressive it is, and, after removal, whether the edges are clean (AAHA, 2026). If your dog has a lump that hasn't been diagnosed yet, ask whether a biopsy should come before surgery to remove it. That order can change the whole plan.

Key Points

• A skin biopsy removes tissue for a pathologist to examine. It's the most reliable way to diagnose many skin lumps and skin diseases.
• Histopathology, the lab exam of biopsy tissue, can grade tumors and measure surgical margins. Cytology from a needle sample generally can't (AAHA, 2026).
• Needle samples are still useful. In one study of 243 skin and under-the-skin masses, needle cytology agreed with biopsy results 90.9% of the time (Ghisleni et al., 2006).
• For skin disease, vets usually take several samples, ideally three to five 6 mm punches, and the skin isn't scrubbed first (Cornell University College of Veterinary Medicine, n.d.).
• Steroids can change biopsy results. Pathologists recommend stopping them for weeks beforehand when that's safe for your dog (Wiener, 2025).
• For an undiagnosed lump, a biopsy before full removal is usually recommended, because malignant tumors often need much wider margins than benign ones (Ettinger, 2017).

Detailed Explanation

What a skin biopsy is

A biopsy is any procedure that removes tissue to examine. In a skin biopsy, the vet removes a small piece of skin, or sometimes the whole lump, and puts it straight into a preservative called formalin. A veterinary pathologist then slices it very thinly, stains it, and studies it under a microscope. Our guide to reading your dog's biopsy report explains what comes back.

It's different from a fine needle aspirate, where a needle collects loose cells. Cells alone can tell you a lot. A biopsy shows how those cells are arranged and how deep they go, which is often what the diagnosis depends on.

Why vets recommend skin biopsies

There are two common reasons, and they're handled a bit differently.

The first is a lump. Lumps on and under the skin are extremely common in dogs, and many are harmless. Some are cancer, though, including mast cell tumors, melanoma, and squamous cell carcinoma, and you can't tell which by looking or feeling. Oncologist Dr. Sue Ettinger's rule of thumb is "See Something, Do Something": any skin lump about the size of a pea, roughly 1 cm, that's been there for a month should be sampled (Ettinger, 2017). Our guide to lumps that aren't cancer and our 10-minute dog cancer check cover what to look for at home.

The second is skin disease that won't settle. Hair loss, crusting, sores, or changes in skin color that haven't responded to treatment can have many causes, from allergies and infections to immune disorders and, occasionally, cancers of the skin such as cutaneous lymphoma. A biopsy can sort these out when other tests haven't.

Biopsy or needle aspirate?

These two tests work together. A needle aspirate is usually the first step for a lump because it's quick, cheap, and often done without sedation. If cost is part of your decision, our article on the cost of treating cancer in dogs can help you plan. Results can come back in as little as a day or two (AAHA, 2026). If you've wondered whether it hurts, our article on whether fine needle aspirates hurt dogs covers that.

Aspirates are more accurate than many owners expect. In a study of 243 skin and under-the-skin masses from dogs and cats, needle cytology matched the biopsy diagnosis in 90.9% of cases. When cytology identified a tumor, it was right 99.4% of the time (Ghisleni et al., 2006). But it missed some: when cytology found no tumor, it was correct only 68.7% of the time (Ghisleni et al., 2006). And 49 of the original 292 samples didn't contain enough cells to read at all (Ghisleni et al., 2006).

That's where a biopsy comes in. According to the AAHA oncology guidelines, histopathology can determine benign versus malignant with more confidence, identify nearly all tumor types, and report tumor grade and margins, while cytology generally can't grade a tumor (AAHA, 2026). Grade matters enormously for cancers such as mast cell tumors and soft tissue sarcomas, because it shapes the treatment plan and outlook. When cytology is nondiagnostic or unclear, a biopsy before treatment is recommended (Ettinger, 2017).

Why a lump should usually be biopsied before it's removed

This is the part of the process owners most often push back on. "Can't we just take it off?"

Sometimes, yes. For a lump with no diagnosis, though, specialists generally advise against it. The reason is margins. A benign lump may need only a narrow margin of skin around it. Malignant tumors often need margins of 2 to 3 cm (Ettinger, 2017). If a cancer is removed as though it were benign, cancer cells are often left behind. For soft tissue sarcomas, a review notes that removing a tumor without wide margins commonly leaves microscopic disease behind, which makes later treatment more aggressive, and that a biopsy before treatment is preferred when feasible (Ehrhart, 2005).

So the usual approach depends on size. Dr. Ettinger's guidance is that a lump small enough to fit inside an 8 mm biopsy punch can simply be punched out, while anything larger should have an incisional biopsy, meaning a sample taken from part of the lump, before it's removed (Ettinger, 2017). Knowing the diagnosis first lets the surgeon plan the right margins the first time. Our article on what clear margins mean explains why that first surgery counts.

The main types of skin biopsy

Punch biopsy is the most common. A small round blade, a bit like a tiny cookie cutter, removes a circle of skin. Pathologists generally prefer a 6 or 8 mm punch, and recommend a 4 mm punch only for delicate spots like the ear flap, paw pads, or near the lips and eyes, because smaller samples are often too small to diagnose (Cornell University College of Veterinary Medicine, n.d.; Wiener, 2025).

An incisional or wedge biopsy removes a slice from part of a larger lump or lesion. Wedges are often used on the ear margin, paw pads, or nodules (Wiener, 2025).

An excisional biopsy removes the entire lump or lesion. It's useful for small lesions or larger pustules, nodules, and ulcers (Wiener, 2025), and for lumps already known to be benign. For undiagnosed lumps, see the section above.

Needle core biopsies, which use a special needle to pull out a thin core of tissue, are another option for some lumps (VCA Animal Hospitals, n.d.).

Preparing for a skin biopsy

Preparation depends on why the biopsy is being done.

For skin disease, steroids are the big one. Corticosteroids change the inflammation pathologists look for. One veterinary pathologist recommends stopping oral steroids for at least 2 weeks and injectable steroids for 4 to 8 weeks before a biopsy, depending on the drug, unless stopping would cause your dog to suffer (Wiener, 2025). Cornell's lab gives similar guidance (Cornell University College of Veterinary Medicine, n.d.). Never stop a medication on your own, though, even if you've read that it can affect results. Some steroids have to be tapered, and your vet will decide what's safe. Our article on steroids for dog cancer explains why they're used.

Infection is the other. Secondary bacterial skin infections can hide the underlying problem, so vets often treat the infection first, sometimes for 2 to 3 weeks, before taking biopsies (Cornell University College of Veterinary Medicine, n.d.; Wiener, 2025).

For lumps, preparation is usually simpler. Your vet will tell you whether your dog needs to fast for sedation.

What happens during the procedure

Many skin biopsies are done with local anesthetic, often combined with sedation. Larger biopsies may need general anesthesia (VCA Animal Hospitals, n.d.).

If you're used to surgery prep, one detail may surprise you. For skin disease biopsies, the vet usually doesn't scrub the skin, because the crusts and surface changes can hold the clues the pathologist needs (Cornell University College of Veterinary Medicine, n.d.). Hair is trimmed gently with scissors rather than clippers, which can damage the sample (Wiener, 2025). Deeper lumps are a different case, and scrubbing is fine there (Wiener, 2025).

For skin disease, expect several samples. Pathologists recommend three to five biopsies for widespread disease (Cornell University College of Veterinary Medicine, n.d.). Vets aim for fresh, fully developed lesions, because older ones may show only scarring (Wiener, 2025). Some biopsies are repeated if the first samples didn't capture the disease well (Wiener, 2025).

Each sample goes into formalin within seconds (Cornell University College of Veterinary Medicine, n.d.). Your vet sends it with your dog's history, photos, and a list of suspected diagnoses, which helps the pathologist enormously (Wiener, 2025). If your dog has seen a specialist, a veterinary oncologist or dermatologist may choose a specialty pathology service for complex cases (Wiener, 2025).

Recovery at home

Most dogs go home the same day after a small skin biopsy (VCA Animal Hospitals, n.d.). Mild soreness is common, and your vet may send home pain relief. Many dogs wear a cone or recovery suit so they can't lick or scratch the site (VCA Animal Hospitals, n.d.).

Check the site once or twice a day. Call your vet if you see redness, discharge, swelling, or signs of pain (VCA Animal Hospitals, n.d.). Small biopsies usually have a stitch or two removed in 10 to 14 days (VCA Animal Hospitals, n.d.). Our article on recovery after cancer surgery covers incision care in more detail.

How long results take

Histopathology results usually take about 3 to 7 days (AAHA, 2026). Cornell's dermatopathology service reports a typical turnaround of about 4 days from when the lab receives the sample (Cornell University College of Veterinary Medicine, n.d.). Special stains, extra tests, or a second opinion from another pathologist can add time.

The wait is hard. It helps to ask your vet ahead of time how and when they'll share results, and to write your questions down while you wait.

What the results can tell you

A good biopsy report can tell you what the lesion is, whether it's benign or malignant, and for many cancers its grade. If the whole lump was removed, it also reports how much healthy tissue surrounds it, which shows whether more surgery may be needed (AAHA, 2026). For skin disease, it can identify the pattern of inflammation, infections, immune disease, or cancer.

Sometimes the answer is less clear. A report may describe changes without naming a single disease, or say the sample didn't capture enough. That's frustrating, but it isn't a failure. Pathologists encourage vets to call them when results don't fit the clinical picture (Wiener, 2025), and sometimes a repeat biopsy is the right next step.

Plenty of results are good news. Many skin lumps, such as histiocytomas in young dogs, turn out to be benign. If the report does confirm cancer, our page for when your dog was just diagnosed and our article on cancer staging explain what usually comes next. A second opinion on the pathology is always reasonable for a diagnosis that will drive big decisions.

When to Contact a Veterinarian

Book an appointment for any new lump that's about the size of a pea and has been there for a month, or any lump that grows, changes, bleeds, or ulcerates (Ettinger, 2017). Our article on what to do when a tumor bleeds can help in the meantime. Skin problems that haven't improved with treatment are worth a conversation about biopsy too.

After a biopsy, call your vet if the site becomes red, swollen, hot, or painful, if there's discharge or the wound opens, or if your dog won't leave it alone despite a cone. Call right away for bleeding that doesn't stop with gentle pressure, or if your dog seems lethargic, stops eating, or vomits after sedation.

Questions to Ask Your Veterinarian

• Should we start with a needle aspirate, or go straight to a biopsy?
• If this lump might be cancer, should we biopsy it before removing it?
• What type of biopsy do you recommend, and how many samples will you take?
• Does my dog need sedation or general anesthesia, and should I withhold food beforehand?
• Do any of my dog's medications, especially steroids, need to be stopped first, and how?
• Will the sample go to a specialty dermatopathology or oncology pathology service?
• When should I expect results, and how will you share them?
• If the results are unclear, what's the next step?

Our full list of questions to ask your vet about dog cancer can help you prepare, and the Dog Cancer Tracker is an easy place to record lump sizes and dates.

Sources

American Animal Hospital Association. (2026). Table 3.3: Overview of cytologic versus histopathologic examination. In 2026 AAHA oncology guidelines for dogs and cats. https://pages.aaha.org/hubfs/Guidelines%20PDFs/2025oncology/OncologyGL_Table3.3.pdf

Cornell University College of Veterinary Medicine. (n.d.). Dermatopathology. Animal Health Diagnostic Center. https://www.vet.cornell.edu/animal-health-diagnostic-center/testing/testing-protocols-interpretations/dermatopathology

Ehrhart, N. [AUTHOR TO CONFIRM]. (2005). Soft-tissue sarcomas in dogs: A review. Journal of the American Animal Hospital Association, 41(4), 241-246. 

Ettinger, S. (2017, June 9). Veterinary oncology: What to do with lumps and bumps on dogs and cats. Today's Veterinary Practice. https://todaysveterinarypractice.com/oncology/tumor-detection-dogs-cats/

Ghisleni, G., Roccabianca, P., Ceruti, R., Stefanello, D., Bertazzolo, W., Bonfanti, U., & Caniatti, M. (2006). Correlation between fine needle aspiration cytology and histopathology in the evaluation of cutaneous and subcutaneous masses from dogs and cats. Veterinary Clinical Pathology, 35(1), 24-30. 

VCA Animal Hospitals. (n.d.). Tissue biopsies. https://vcahospitals.com/veterinary-care/specialty/departments/surgery/tissue-biopsies

Wiener, D. J. (2025). How to take and submit skin biopsies: Getting the most out of your pathology report. Today's Veterinary Practice. https://todaysveterinarypractice.com/dermatology/how-to-take-and-submit-skin-biopsies-getting-the-most-out-of-your-pathology-report/