Introduction
Lesions involving the skin and subcutaneous tissues are the most common samples collected for cytologic evaluation from companion animals. This is likely due to the fact that the lesions are clearly visible or palpable, so they are observed by owners and noted during routine physical examination.
Their location makes them easy to aspirate or fenestrate, using a needle (i.e., fine needle aspirate or biopsy; henceforth referred to as FNA), without the need for more invasive techniques. Surface lesions of the skin also can be sampled for cytology using scrape, impression, hair-pluck, and tape collection techniques. This chapter will focus on FNA of skin and subcutaneous lesions and highlight some of the common/important superficial skin cytology lesions. More information about these types of collection methods and diseases associated with lesions on the surface of the skin can be found in Chapters 1 and 18 (‘Sample Acquisition and Preparation’ and ‘Cytology of the Ear’, respectively).Cytology of cutaneous and subcutaneous lesions often provides a fast, accurate analysis of the underlying pathogenesis of disease. In one study, FNA cytologic samples were diagnostic in 83.2% (243/292) of cases from dogs and cats with cutaneous and subcutaneous masses (Ghisleni et al., 2006). When compared with histologic diagnosis (gold standard), the sensitivity and specificity of cytology were extremely good (89.3% and 97.9%, respectively) and the positive predictive value was excellent (99.4%). The negative predictive value was somewhat low (68.7%), as expected from a technique that utilizes such a small amount of starting material, may not be representative of the entire lesion, and lacks tissue architecture. This study indicates that the limitations of cytology are minor for most lesions in the skin and subcutaneous tissues. Indeed, accurate cytologic information is often instrumental in guiding diagnostic planning and treatment strategies.