Skin Biopsies for Dermatopathology

The simplicity of dermatology is that the problem is easily visible, but the challenge is that everything looks similar. While we start with cytology slides under the microscope, this only provides information about the topmost layer of skin. For some dermatologic diseases, biopsy is the only way to obtain a definitive diagnosis.

There are some slowly-progressing, non-life-threatening diseases in which we may try treatment first, like zinc-responsive dermatosis (https://onlinelibrary.wiley.com/doi/abs/10.1111/vde.12369). However, indications for biopsy may include:

• severe/atypical skin lesions that may be life-threatening, especially with systemic signs (ex. vomiting, diarrhea, lethargy, inappetance, fever)
• acute-onset or rapidly progressing skin lesions
• skin lesions not responding to empirical therapy
• suspicion for a disease that can only be diagnosed via biopsy (ex. immune-mediated diseases)
  
• a need to rule out cancer

A veterinary dermatologist is specially trained to answer the following questions:

• When to biopsy? A biopsy always provides more information, never less. However, that information must be interpreted in a specific way. If the sample is taken too soon, too late, or while on interfering medications, then this may skew results.
  
• Where to biopsy? Location, location, location. Obtaining the correct part(s) of the lesion from the right area of the body is not only important for the dermatologist, but also for the pathologist. Depending on the patient's temperament, some locations (ex. trunk) can be sampled with just local anesthetic and lots of positive reinforcement (i.e. food). Some areas can be tricky to sample (ex. nose, pawpad, ears), and sedation or anesthesia may be required.
  
• What to biopsy? The lesion type and description for the pathologist is incredibly important. The lesion must never be scrubbed as this can wash away the answer. Newer/active lesions are ideal if present. Additional diagnostics may need to be obtained depending on the suspected disease; ex. tissue samples for cultures may be collected concurrently.
  
• How to biopsy? A 4-8 mm punch (circular) biopsy is typically used. However, in some cases, a small elliptical incision may be made with a scalpel. The samples are placed in formalin, and absorbable suture is used to close the incision. This is a relatively quick procedure.
  
• Who to send the biopsy to? A dermatopathologist should review most skin biopsies. Pathologists have one of the most difficult jobs in that they need to know every species, every organ, every disease. It is best to submit skin slides to someone who reads a lot of them.

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