An effusion from a patient with tuberculosis is composed predominantly of lymphocytes and rare mesothelial cells. A similar pattern may be observed in effusions from patients with lymphoma/leukemia, recent cardiac surgery, or sarcoidosis. Clinical history and ancillary tests are helpful is determining the etiology of the lymphocytes.
Carcinomas in fluids will occur as clusters and single cells. A second cell population that does not resemble mesothelial cells or histiocytes can be helpful in identifying adenocarcinoma. Features helpful in making a diagnosis of malignancy include: cells with high nuclear/cytoplasmic ratios, prominent nucleoli, enlarged cells, and irregular nuclear contours. Poorly-differentiated carcinomas may have all of these features, whereas well-differentiated carcinomas may lack some of these characteristics.
Most benign effusions are composed of mesothelial cells and histiocytes, which tend to occur as single cells. In contrast to mesothelial cells, histiocytes have more vacuolated cytoplasm, folded or bean-shaped nuclei, and no “windows.”
Lymphomas present as single cells, and the type of lymphoma dictates other cytological features. For example, an effusion caused by a diffuse large B-cell lymphoma has large lymphoid cells with prominent nucleoli. In contrast, one caused by follicular lymphoma is composed of relatively smaller cells with cleaved nuclei.
Mesothelial cells tend to have dense cytoplasm. When they do aggregate, mesothelial cells have a small space or window between them resulting from intervening long microvilli.
