Lymph node cytology

In clinical practice, animals with lymphadenopathy are eminently suitable candidates for cytology sample collection by FNAB from several enlarged nodes; or, if surgical biopsies are made, imprint smears from the tissue may yield diagnostically useful information to supplement the histological findings. Cytology may reveal the lesion to be reactive, inflammatory, or neoplastic. Cytologically, reactive nodes will contain increased numbers of plasma cells, possibly with some inflammatory cells, along with the resident lymphoid population. If inflammation is present, neutrophils and macrophages also will be found and the type of inflammation may be classified. Any infectious agent such as fungal hyphae, yeasts, bacteria, and protozoa also may be demonstrated. Aspirates may be cultured directly onto blood agar plates or transported in nutrient broth for culture at a referral laboratory. In chronic dermatopathic nodes, a mixed inflammatory cell infiltrate is expected, and in pruritic skin disorders, eosinophils usually are plentiful in node aspirates. Increased numbers of eosinophils also may be found in dogs that are microfilaremic with heartworm infection. Background debris of hemosiderin and melanin pigment and other fine particles may occur in some chronic inflammatory lymphadenopathies. Metastatic lesions are identified by the presence of foreign neoplastic cells, but this diagnosis may be missed in early metastatic spread or if the aspirate is not sufficiently cellular. A cytological guide to the classification of the more common diffuse canine lymphomas is provided but full characterization of the lymphoma type may require histology and immunocytochemistry. In practice, a simple differential Romanowsky stain such as Diff Quik is suitable for most purposes. Supplementary stains using 1 per cent toluidine blue may increase the detection of mast cells. Aspirates also may be transferred into suitable media for transport to a referral diagnostic laboratory for cytocentrifugation or further tests such as electron microscopy, immunocytochemistry, flow cytometry, and culture. Although definitive diagnosis by histopathology and other tests still may be required, in many routine cases, diagnoses can be achieved expediently in clinical practice by aspiration cytology.