Accurate skin disease diagnosis relies on a systematic approach, which includes obtaining signalment and history, performing a complete dermatologic examination, and performing appropriate tests as indicated.
Determining degree of pruritus, types of lesions present, and pattern or distribution of lesions can help shorten the list of differential diagnoses. Primary lesions (eg, papules, vesicles, plaques, cysts) develop directly from disease processes. Secondary lesions (eg, lichenification, hyper- or hypopigmentation, scales, alopecia) develop from primary lesions and are often induced by the patient or the environment. In-house diagnostics often prove helpful for confirming diagnosis and monitoring patient response to therapy.
Skin scrapings are most often used in diagnosing parasitic infestations. Tape preparations are useful for detecting superficial parasites and organisms on dry, scaly skin. Impression smears allow for detection of organisms and help determine presence or absence of various cells; they are particularly useful for moist, greasy, or exudative lesions. Exudate preparations are used when direct impression smears are not possible (eg, for external ear canal, deep skin folds).
During cytologic examination, cell types, inflammatory response, presence or absence of organisms, and characterization and numbers of organisms present should be noted and recorded. Skin biopsies with histopathology should be conducted in patients that have persistent lesions, suspected neoplasia, undiagnosed alopecia, or vesicular or unusual dermatoses.
Bacterial culture is indicated when empiric antibiotic therapy fails to resolve infection; positive results should be interpreted carefully, as nonpathogenic contaminants may be present. Fungal cultures are also useful to diagnose dermatophytosis in patients with or without distinct lesions.—Werner A