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 scrapingsWhen 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.
Cytology is your friend! In order to make this sampling technique useful, apply a few rules.
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).
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.