Dermatologic diagnostics: maximizing results from skin scrapings to biopsies (Proceedings)


Dermatologic diagnostics: maximizing results from skin scrapings to biopsies (Proceedings)

Why should I perform dermatologic diagnostics?

There are many reasons to perform dermatologic diagnostics: to guide therapeutic choices, to make decisions regarding further diagnostic testing, to judge response to therapy as well as to actually make a diagnosis! Incorporating these tests into a busy practice schedule mandates that these tests be quick and easy to perform, useful information must be gained in an efficient manner and the results must be easily interpreted within the medical record. The basic and most commonly used tests include skin scrapings, surface cytology (ears, skin) and trichograms.

Skin scrapings

When appropriately performed, negative skin scrapings will rule out canine demodicosis. Skin scrapings for demodicosis are focused on lesional areas (select a minimum of 3 lesions consistent with folliculitis) and are deeper than those for surface dwelling parasites. In contrast, broad superficial scrapings are performed for sarcoptic mange; negative skin scrapings in this instance DO NOT rule out scabies as a differential diagnosis for intense pruritus. In order to maximize results when performing superficial skin scrapings to locate scabies mites, focus on heavily crusted areas such as elbows or ear margins.

Surface cytology

Cytology is your friend! In order to make this sampling technique useful, apply a few rules.
• Only sample areas where, depending upon the results, you will change your therapeutic plan.
• This technique is most useful for surveying areas for Malassezia spp. organisms. A negative surface cytology sample does not rule out a bacterial pyoderma since most organisms are below the surface layers of the skin.
• Use an adhesive technique to maximize organism recovery, such as clear acetate tape or Durotak® adhesive slides manufactured by Delasco. **These preparations are not placed into the fixative solution during the staining process. • If the area is moist or excessively greasy, firmly press a plain glass slide to the area and heat fix prior to staining with Diff Quik® stain.
• Roll out the swabs from both ears on a single slide to minimize staining and microscope time: place the sample from the left ear on the left side of the slide and from the right ear on the right side of the slide. Remember to heat fix these prior to examination under 1000x magnification with immersion oil.

Papule/ Pustule cytology

This technique is very useful when differentiating infectious causes for papules/ pustules from allergic papules and pustules caused by autoimmune disease (pemphigus foliaceus).
• Degenerate neutrophils with intracellular cocci = requires oral and/or topical antibiotic therapy.
• Nondegenerate neutrophils without bacteria, but with acantholytic keratinocytes = requires skin biopsy for histopathology and culture (aerobic and fungal).
• Eosinophils without evidence of organisms = most likely a lesion associated with an allergic response, such as insect exposure.
• Some neoplasias can appear papular prior to forming nodules—consider skin biopsy for histopathology if the cells do not appear consistent with an inflammatory response.
• Deeper lesions associated with a deep pyoderma, such as those that drain or bleed easily may reveal pyogranulomatous inflammation. Any cause of follicle rupture resulting in furunculosis can create this lesion. Both deep pyoderma and fungal kerions can be the cause for abundant neutrophils and macrophages found on cytology. If intracellular cocci and eosinophils are also present, this implies a foreign body reaction, such as the reaction to free hair and keratin found with interdigital lesions of pododermatitis, which will require weeks to months of antibiotic therapy. If bacteria are not identified, consider a fungal culture to rule out dermatophytosis as an infectious cause.

Nodule cytology

Every lump and bump deserves a fine needle aspirate. The majority of these lesions will also require biopsy for histopathology in order to make a final diagnosis, however it is helpful to be able to anticipate a diagnosis of neoplasia or something infectious or immune-mediated (sterile) in advance. A mast cell tumor is typically easily identifiable and proper surgical plans can be made prior to removal, whereas a sterile or infectious cause will require aerobic and fungal cultures in addition to histopathology to make the diagnosis. **Tip—a sterile disease cannot be declared sterile if cultures have not been performed.


This technique is as simple as pulling a few hairs from lesional sites, laying them out straight in a drop of mineral oil and covering with a cover slip. The hairs are then examined with the 10x objective, just like skin scrapings. Hairs with ectothrix spores, indicative of a dermatophyte infection (usually Microsporum canis) can be identified this way, indicating the need for fungal culture. In addition Demodex spp. mites can also be found clinging to the hairs in affected animals. This is very useful when the only lesions of folliculitis are present around the mouth or eyes where skin scrapings can be technically challenging.