Pleural, Pericardial, and Peritoneal Fluid Analysis

It is important to note that both monocytes and mesothelial cells can transform into macrophages. During this process, the visual appearance of intermediate “transitioning” cells can be very difficult to specifically identify (Figs. 10.3 and 10.4). Fortunately, because there is usually no clinical value in differentiating these three cell types (monocytes, macrophages, and mesothelial cells), they are often counted together in a single group or category.² Despite this, when performing the nucleated cell count, some institutions choose to identify and report these cell types in two categories: monocytes/macrophages and mesothelial cells. Table 10.5 is provided as a guide for differentiating monocytes, macrophages, and mesothelial cells.

Monocytes move into body fluids from the bloodstream in response to inflammation and infection. These round or slightly irregular-shaped mononuclear cells are typically 12 to 20 μm in diameter (Fig. 10.3). The smooth-bordered nucleus is kidney bean or horseshoe–shaped and it can have delicate, brain-like folds. The nuclear chromatin appears loose with a lacy or streaked (i.e., raked, combed) appearance. The cytoplasm of monocytes is pale blue-gray and azurophilic granules may be present, although few in number. Monocytes can demonstrate blebs or pseudopods on their cytoplasmic borders. They may also have a few vacuoles or phagocytic remnants, but they do not form cell clumps.

In serous body fluids, macrophages derive from either monocytes or mesothelial cells. They appear in a variety of irregular shapes (i.e., pleomorphic) and their size is highly variable, ranging from 12 to 85 μm in diameter (Figs. 10.3 and 10.4). Their oval or round nucleus is eccentric with a smooth border; it can also be lobular or bean-shaped. The nuclear chromatin can vary from fine to dense and may be coarsely clumped. The pale blue-gray cytoplasm often appears cloudy or foamy and azure granules can be present. These actively phagocytic cells have numerous vacuoles of varying sizes, and their cytoplasmic borders often have blebs or pseudopods present. At times during cell transformation to a macrophage, it is difficult to distinguish whether the cell is a monocyte or a macrophage, in which case they are identified and enumerated in a “monocyte-macrophage” category.

Macrophages can also be descriptively named based on the type of inclusions within them (as discussed in Chapter 9, section Cerebrospinal Fluid). Erythrophages (macrophages with RBC inclusions) and siderophages (macrophages with hemosiderin inclusions) are associated with hemorrhage into a body cavity (see Figs. 9.10–9.12). When numerous neutrophils are present in a body fluid, macrophages can ingest them. These neutrophages will contain one or several neutrophils in various stages of degeneration and are associated with acute inflammation. Although lipophages (macrophages with numerous small, clear, lipid-containing vacuoles) may be seen in pleural and pericardial fluids, they are more often seen in the cerebrospinal fluid (see Fig. 9.13) and synovial fluids.

In a macrophage, several vacuoles can merge together to form a single large vacuole. When this occurs, the nucleus becomes pressed against the cell membrane and because of their appearance, these cells are sometimes called “signet ring” macrophages (Fig. 10.5). However, it is important that these macrophages are not confused with signet ring carcinoma, an adenocarcinoma of glandular epithelial cells most often of the stomach. Therefore, to reduce confusion for clinicians, the term signet ring should not be used when reporting macrophages in serous fluids.

A detailed discussion of the formation of these various macrophages is provided in Chapter 9, section Macrophages. Note that because macrophages are counted collectively in a monocyte/macrophage/mesothelial cell category, some institutions append a comment to the report that indicates the descriptive type of macrophages present, for example, “many erythrophages present, rare siderophage present.”

Mesothelial cells are the lining cells of serous membranes and are routinely sloughed off. Hence, they are often seen in effusions. These cells are 20 to 50 μm in diameter, with light gray to deep blue staining cytoplasm that can appear grainy (Fig. 10.6). Their nuclei may be eccentric with smooth, regular borders and they can be multinucleated (see Fig. 10.3). The nuclear chromatin is evenly distributed and can be fine or dense; one to three nucleoli may be present. Because mesothelial cells can show a perinuclear halo, they sometimes resemble plasma cells (Fig. 10.7), which can also be in pleural and peritoneal fluids.

Mesothelial cells have ruffled or fringed cytoplasmic borders that can form blebs or pseudopods (see Fig. 10.3). They can have a few small vacuoles which, when present, are usually located at the cell’s periphery. In serous body fluids, mesothelial cells appear singly and as cell clumps. Therefore, it is imperative that when cell clumps are present they are correctly identified and distinguished from malignant cells. Note that as mesothelial cells transform into macrophages, their cytoplasm becomes less basophilic and more vacuolated, giving it a foamy appearance.

Conditions associated with an increase in granulocytes, lymphocytes, and plasma cells in serous fluids are varied and briefly listed in Table 10.6. Neutrophils, when present in large numbers, suggest an infection but they could originate from blood contamination. Infections include peritonitis, pericarditis, bacterial pneumonia, and fungal infections. Although rare, lupus erythematosus (LE) cells (i.e., neutrophils with a phagocytized homogeneous nucleus) may be present in fluids from patients with systemic lupus erythematosus, a chronic autoimmune disorder (Fig. 10.8).

Eosinophilia, when the eosinophil count is greater than 10%, can accompany a variety of conditions, such as parasitic infections, foreign body response (e.g., shunts, chest tubes, peritoneal dialysis), allergies, air in the body cavity (e.g., pneumothorax), leukemias, lymphoma, and others (Fig. 10.9).

Basophils and mast cells can look similar but are actually derived from different cell precursors. A mast cell is usually larger than a basophil and its nucleus is round compared with the segmented nucleus of a basophil. Both cells have numerous blue-black cytoplasmic granules that can fill the cell such that it is difficult to see the nucleus. There is a greater density of these granules in a mast cell compared with a basophil. The peritoneal fluid in Fig. 10.4 shows a mast cell based on its round, eccentric nucleus. It is unusual to see basophils or mast cells in serous fluids. However, they often accompany eosinophilia (e.g., parasitic infections), are associated with a foreign body response (e.g., peritoneal dialysis), and may be encountered in other inflammatory conditions.

Lymphocytes can be seen in all body fluids. They often vary both in the number present and in their size. Note that lymphocytes transform in response to various stimuli. Consequently, a spectrum of different types of lymphocytes can be encountered in a single body fluid. Small lymphocytes (7–12 μm) have a small to moderate amount of light blue cytoplasm; a round to oval nucleus; and condensed, dark-staining chromatin (Figs. 10.2, 10.4, 10.7, and 10.9). A few small azurophilic granules may be present in the cytoplasm. In comparison, large lymphocytes are 12 to 18 μm in diameter, primarily due to a larger amount of cytoplasm. Reactive lymphocytes are usually the largest lymphocytes, typically 10 to 30 μm in diameter, with moderate to abundant cytoplasm. The cytoplasm can be pale to dark blue, and in some cells the cytoplasm stains darker blue at the periphery with a light area or zone near the nucleus, resembling a plasma cell. The nucleus of reactive lymphocytes is larger and the nuclear chromatin less basophilic compared with smaller lymphocytes, and several nucleoli may be prominent. Reactive lymphocytes accompany other inflammatory cells and are frequently observed molding around RBCs and other cells in the cytocentrifuged smear.

Note that lymphocytes also transform under various stimuli to form mature plasma cells. As this process occurs, various intermediate cell types are formed: reactive lymphocytes, plasmacytoid lymphocytes, and immunoblasts. When mature, a plasma cell is 8 to 20 μm in diameter and has an eccentric round or oval nucleus with no nucleoli and coarsely clumped chromatin. Sometimes the chromatin takes on a “clock-face” pattern. Plasma cells can be binucleated and rarely multinucleated. Their moderate to abundant cytoplasm has no granulation and stains moderate to dark blue with a light area or clear zone called a “hof” next to the nucleus (Fig. 10.7). Vacoules may or may not be present.

In response to infections, inflammation, or malignancy, plasma cells can produce antibodies. These plasma cells, also called “Mott” cells, have vacuoles that contain immunoglobulin proteins. Their cytoplasm is usually dark blue with a tinge of pink (Fig. 10.10).

The morphology of malignant cells can be divided into two types: monomorphic (i.e., cells that show no variation in morphology; all cells look alike) and pleomorphic (numerous variations in cellular morphology). A variety of morphologic features are associated with malignant cells, and the most common are listed in Box 10.2. At times, the changes observed can be obvious, with cells looking extremely bizarre and atypical; other times, the changes may not be as recognizable. Malignant characteristics include a high nucleus-to-cytoplasmic (N:C) ratio and changes in the nucleus size, shape, and appearance. Nucleoli are atypically large and irregular in shape, and there are changes in the chromatin density and distribution. Cells can be oddly multinucleated, with the various nuclei differing in size and shape within a single cell. Nuclear molding, where the nucleus of one cell indents the nucleus of another cell, can produce unusual cell clusters that are usually detectable using low-power magnification. Lastly, a feature of malignancy that may be observed is cytoplasmic granules, that is, the cells have large clumps of prominent granules that are normally not present. It can be very difficult to discern between normal and malignant cells; therefore always consult a pathologist whenever suspicious cells are observed in a body fluid. This professional interaction provides an opportunity for learning and team building in the laboratory. For example, reactive and malignant mesothelial cells can be very difficult to differentiate in cases of malignant mesothelioma.

Most malignant effusions are caused by metastatic adenocarcinomas (Fig. 10.11). However, any of the blood cancers, leukemias, or lymphomas can infiltrate body cavities to cause an effusion. In these cases, the use of immunocytochemistry and flow cytometry is valuable in determining an accurate diagnosis. In summary, proper identification of malignant cells in body fluids by a skilled professional (e.g., pathologist, cytologist) is crucial and is performed when a smear is forwarded for review or when a cytologic examination is specifically requested.