Diagnostic Tests: Tips from the Dermatologist #6

This is the sixth in our series of short articles with the aim to inform colleagues in practice about diagnostic tests done on our patients with dermatological problems. This article will focus on skin biopsy (technique and usefulness of the test).

Before taking skin biopsies, ask yourself questions and plan the procedure:

1 - Do I need a skin biopsy?

This is not part of a “routine” work up so before starting you should have:

  • Done less invasive techniques - a thorough clinical examination, scrapes and cytology to rule out infections
  • Recorded the major primary and secondary lesions
  • Listed differential diagnoses
  • Treated secondary bacterial infection
  • Withdrawn corticosteroids which may alter or mask lesions (as for intradermal allergy testing) unless the patient has uncontrollable pruritus or a life-threatening condition.

2 - Why biopsy?

  • Confirm a diagnosis - some diseases can only be diagnosed by biopsy
  • Rule outs
  • Assessing progress
  • Prognosis
  • Special cases
      • Persistent ulceration
      • Systemic illness
      • Unusual presentation
      • Failure to respond to logical therapy

3 - Before you start

Sedation or anaesthesia?

  • Local anaesthesia: for truncal biopsies from calm dogs. Inject lidocaine in the subcutis not dermis
  • Sedation if dogs are difficult to keep still
  • General anesthesia for aggressive dogs, cats, and nose and pads

Punch or ellipse?

  • Punches should routinely be 8mm, with 6mm (rarely 4mm) for difficult sites such as footpads and noses.
    • Ellipses are better for a whole lesion (e.g. pustule), across the edges of ulcers and for alopecia.
    • Blunt punches and blades will tear tissue and cause artifacts
    • Never use cautery on small biopsies.

4 - What to biopsy

  • Primary lesions
  • Three representative, different stages of lesions
  • Typical, rather than "interesting" lesions
  • Avoid traumatized areas
  • Biopsy alopecia in the direction of hair growth. Draw an orientation line for the pathologist. Make a note on the submission form that you used this technique.
  • All biopsies should be deep enough to reach subcutaneous fat
  • Include crust

Sample Sample

5 - How to biopsy

  • Never scrub the skin surface to avoid removing diagnostic lesions
  • Gently clip the fur with scissors
  • Hold the skin flat, without excessive stretching
  • Apply the punch perpendicular to the skin.
  • Press gently and rotate ONE way until it penetrates the fatty tissue
  • Hold the biopsy at the edge with tissue not rat-tooth forceps

6 - What to do with the biopsy

  • Place thin biopsies, sticky (fat) side down, on paper or card for a few seconds only before placing in fixative
  • Use enough fixative, a 1:10 ratio of tissue sample to neutral buffered formalin
  • In cold weather, fix biopsies at room or body temperature for 24 hrs before shipping to avoid freeze-thaw artifact
  • Package securely in accordance with UN Regulations. Labels should state: “UN3373 Biological Substance Category B” and “Exempt animal sample”. Special containers or bags are usually provided by the laboratory.

Always provide the pathologist with:

  • Your differential diagnoses
  • Detailed history and description of the lesions or any other systemic problem affecting the patient. This will help the dermatopathologist to put together the histological findings with the clinical signs and reach an accurate diagnosis. Sometimes a morphological diagnosis is given in the report and the dermatologist may be the best person to put together the pieces of the puzzle to achieve a definitive diagnosis.
  • Photos would be ideal. The pathologist may visualize the animal but occasionally they are a revelation!

7- Where to send the biopsy?

Not all pathologists have expertise in dermatopathology. It is strongly recommended to send your skin specimens to an experienced dermatopathologist as subtle lesions may be missed by pathologists who lack an in-depth knowledge of veterinary dermatology.


The skin biopsy is a very useful test which may help in diagnosing uncommon conditions but needs to be used wisely and following the correct protocol and technique.

1 - DVM, DipECVD, CertVD, MRCVS - Dick White Referrals,Veterinary Specialist Centre

2 - BVSc, PhD, MRCPath, DipECVP, MRCVS, RCVS: Specialist in Veterinary Pathology - Focus Veterinary Histopathology International, www.focuspathology.eu
Email Laboratory: lab@focuspathology.eu; Email Direct: joanrest@focuspathology.eu