The art of skin biopsy
THE ART OF THE SKIN BIOPSY
Dr Charlie Pye BSc, DVM, DVSc, DACVD
Atlantic Veterinary College
550 University Avenue
Prince Edward Island
Overview of the Issue
A skin biopsy is a powerful tool in veterinary dermatology. Many differential diagnosis lists can only be shortened via a skin biopsy. Even without a definitive diagnosis, a biopsy report can still guide you in your treatment/diagnostic decisions. In certain cases biopsies can be unrewarding but this is often due to timing of the biopsy, poor lesion selection or poor technique/processing. Obtaining a diagnosis via skin biopsy takes teamwork between the clinician and the pathologist; the clinician must carefully select, obtain and preserve the skin specimen and the pathologist will carefully process and interpret the specimen.
Objectives of the Presentation
To review biopsy site selection
To review biopsy technique
How to avoid errors in obtaining skin biopsies
When to biopsy?
There are no “hard and fast” rules as to when to take a skin biopsy. Some guidelines to follow include:1-2
1. Biopsy any obviously neoplastic or suspected neoplastic lesions
2. Biopsy any persistent ulcerations or nodular lesions
3. Biopsy any case where your differential diagnosis list can only be ruled in/out via biopsy
4. Biopsy any dermatosis not responding to rational therapy
5. Biopsy any unusual dermatosis or serious dermatosis
6. Biopsy any vesicular lesions
7. Biopsy any condition where the therapy is expensive, dangerous or time consuming
Appropriate antibiotics should be given prior to biopsy as secondary bacterial infections can obscure histopathological features of disease. If you are able, it is best to discontinue any anti-inflammatory therapy 2-3 weeks prior to biopsy (6-8 weeks for repositol corticosteroids) or these medications may affect the histologic appearance of many dermatoses. There will be cases where you may not be able to discontinue therapy.
Where to biopsy?
Pick the lesions or regions of the skin you think will show diagnostic change. For example, if you are suspicious of discoid lupus erythematosus, think of the pathology of the disease. DLE leads to depigmentation and pigmentary incontinence. Therefore it is best to select an area that is depigmenting (not completely depigmented; select the blue/grey areas). Skin histopathology varies depending on the site selected. For example, non-haired skin normally has fewer hair follicles and therefore is not the best place to biopsy if you suspect an endocrinopathy. It is better to select an area with more follicles such as the shoulder.2 If the dermatosis appears to have an unusual distribution, search for primary lesions such as pustules, vesicles, nodules, papules etc. If an ulcer is apparent, do not biopsy the ulcer itself. Try to take your sample from the side of the ulcer. There is still a debate in the literature as to whether obtaining the margin between normal and abnormal tissue is informative. Take multiple samples including different lesions if the clinical presentation is such with different lesions. Never biopsy just one lesion or site unless you have no other choice (e.g. feline nose, only one lesion apparent). Most laboratories will charge one fee for a number of samples so it is recommended to call the lab and check prior to obtaining the biopsy. If the disease has a waxing and waning nature and no primary lesions can be seen, consider asking the owner to bring their pet back when lesions are apparent. If the lesion has a crust, ALWAYS include this with your sample.1-2 If the crust falls off, you can include the crust in the formalin and then write on the submission sheet that the crust is separate but you would like it examined.
How to biopsy?
Selection of punch biopsies (size will depend on lesion), Adson thumb forceps, iris or small curved scissors, formalin vial(s), needles and suture material, needle holders, gauze, tongue depressors, scalpel blade.
Ideally chose a punch size that just fits over the lesion. If lesions are generalized select a site within the generalized area. If you are suspicious of neoplasia, the whole nodule/mass can be removed and sent for histopathology.
Smaller punch sizes should be used for smaller lesions or areas of the body more challenging to biopsy e.g. 4mm, for footpads, ear pinnae and nasal planum.
Circle sites to biopsy with a Sharpie.
If whole footpad affected, I recommend obtaining your sample from the edge of the pad (there will be less tension during healing).
Trim hair if needed, use caution using clippers as you can remove crusting.
Local anesthesia versus general anesthesia.
General anesthesia is often recommended for the nasal planum and footpads as these regions are generally more painful.3
Subcutaneous block with 0.5-1ml 1-2% lidocaine per site.1
Lidocaine toxic dose – try not to exceed 5 mg/kg for dogs or 2.5 mg/kg cat.1
Lidocaine solely has been found to be superior to 1% lidocaine with epinephrine or topical prilocaine.4
Lidocaine can sting upon injection so can use 1:10 ratio of lidocaine: 8.4% sodium bicarbonate.5
Taking the biopsy
Allow 5 minutes for the local anesthesia to take effect.
Make sure you have adequate light.
Turn biopsy punch in one direction to minimize shearing of tissue along with firm downward pressure as you turn.
May feel “give” as you go through to subcutaneous tissue.
You want some subcutaneous tissue on the bottom of the biopsy.
Use forceps to grasp the subcutaneous tissue – NOT the epidermis or you will crush it!!1,3
Cut underneath and then place immediately into 10% neutral buffered formalin (100ml 40% formaldehyde, 900ml tap water, 4 g acid sodium monohydrate and 6.5g anhydrous disodium phosphate).
Ratio of 10 parts formalin to 1 part specimen.
Close with cruciate or 1-2 single interrupted sutures and remove in 10-14 days.
When to use a scalpel:
Large lesions, vesicles or bullae where a punch could damage the lesions.
Centre lesion and cut elliptical shape around lesion.
Fixation causes tissue shrinkage – if larger than 4mm punch or elliptical, press sample down onto tongue depressor for 30-60 seconds.
Submission of biopsy
Avoid freezing – can add 95% ethyl alcohol as 10% fixative volume or allow 12 hours fixation before cold exposure.6
Section samples lager than 1cm in diameter (formalin can only penetrate to that depth).
Specimens should ideally be sent to a veterinary dermatopathologist or pathologist with interest in skin.1,2
Always include differential list, thorough history including response to medications, photos if possible.3
Can ask for special stains if infectious disease suspected.
Complications of biopsy
Bleeding usually minimal - always look for vessels prior to biopsy.
Delayed healing if patient has hyperadrenocorticism or taking glucocorticoids.
Infections are rare.
Key Prognostic Points:
The final diagnosis is always made by the clinician not the pathologist
If you are submitting a skin biopsy for a tissue culture, do not use lidocaine local block and make sure to scrub the surface. Lidocaine will inhibit certain gram positive and negative bacteria, fungi and Mycobacteria, as does bicarbonate and epinephrine.7 Instead you can do a ring block or use general anesthesia.
Summary including 5 KEY “TAKE HOME” POINTS:
1. When selecting a biopsy site think about your differential diagnosis list or look for primary lesions.
2. Always take multiple specimens if you are able.
3. Select punch biopsy that just fits over lesion or consider excisional biopsy with scalpel.
4. Include thorough history including response to medication, other clinical signs, photos, time period etc.
5. Treat the clinical picture not the biopsy report.
1. Miller WH, Griffin CE, Campbell KL. Diagnostic Methods. In: Miller WH, Griffin CE, Campbell KL. Muller and Kirk’s Small Animal Dermatology 7th ed. St Louis, MO: Elsevier Mosby; 2013: 57-107.
2. Bettenay SV, Hargis AM. Biopsy Collection: Why, When, Where and How. In: Bettenay SV, Hargis AM. Practical Veterinary Dermatopathology. Jackson, WY: Teton NewMedia; 2006: 1-20.
3. Hnilica K. Diagnostic Techniques. In: Small Animal Dermatology A Colour Atlas and Therapeutic Guide 3rd ed. St Louis, MO: Elsevier Saunders; 2011: 22-36.
4. Henfrey JI, Thoday K, Head K. A comparison of three local anesthetic techniques for skin biopsy in dogs. Vet Dermatol 1991; 2: 21-27.
5. Pascuet E, Donnelly RF, Garceau D, Vaillancourt R. Buffered lidocaine hydrochloride solution with and without epinephrine: Stability in polypropylene syringes. Can J Hosp Pharm. 2009; 62: 375-380.
6. Dunstan RW. A User’s Guide to veterinary surgical pathology laboratories or, why do I still get a diagnosis of chronic dermatitis even when I take a perfect biopsy? Vet Clin North Am Small Anim Pract. 1990; 20: 1397-1417.
7. Williams BJ, Hanke CW, Bartlett M. Antimicrobial effects of lidocaine, bicarbonate, and epinephrine. J Am Acad Dermatol. 1997; 37: 662-664.