Cell Count, Serous Fluid

Serous fluid

1 to 2 mL

0.5 mL (Note: This volume does not allow for repeat testing.)

Lavender-top (EDTA) tube

This test is used to evaluate serous fluid for the presence of RBCs (semiquantitative), WBCs, differential and for physical appearance.

Total leukocyte counts are of limited value in the differential diagnosis of pleural and pericardial effusions.^1,2 For completely clotted serous fluid specimens, actual cell counts will not be performed. Morphology will still be assessed by cytocentrifuged, stained slides. Routine cytological evaluations would be indicated to confirm or rule out suspected neoplastic or tumor cells, malignancy.¹ The total leukocyte count may be useful in distinguishing between peritoneal transudates (eg, in uncomplicated cirrhosis) from spontaneous bacterial peritonitis (SBP) caused by passage of bacteria from blood to ascitic fluid.¹

Microscopic and macroscopic examination

To minimize unnecessary laboratory testing it is important to have a clear differential diagnosis in mind. Results of these studies should always be correlated with clinical findings and any other ancillary studies to determine an accurate diagnosis.¹

Gross examination can play a vital role in determining the pathogenesis of the effusion. Transudates are usually clear and pale yellow and do not clot.¹

Pleural or pericardial fluids: Exudates are cloudy to purulent and often clot while standing because of the presence of fibrinogen. Exudative fluid usually contains large numbers of leukocytes and elevated protein content. Occasionally, exudates may be straw-colored, similar to transudates. A cloudy, purulent fluid is usually associated with an infectious process.¹

Peritoneal fluid: Exudates are cloudy or turbid due to large numbers of leukocytes, elevated protein concentrations, and/or microörganisms. Such fluids may be seen in peritonitis, perforated or infracted intestine, and pancreatitis.¹

Cell types seen on examination of a cytoprep-stained slide include mesothelial cells, which form the lining of pleural, pericardial, and peritoneal cavities. Sometimes, it is impossible to differentiate mesothelial cells from malignant cells. Mesothelial cells are seen in variable numbers in most effusions and are increased in sterile inflammations caused by such conditions as pneumonia, pulmonary infarction, and malignant disorders. Mononuclear phagocytes (monocytes, histocytes, and macrophages) are usually seen in variable numbers in pleural, pericardial, and peritoneal effusions. Since both monocytes and mesothelial cells may be transformed into macrophages, the distinction between them is not always obvious. Lymphocytes are seen in variable numbers in most serous effusions. Plasma cells may be seen in fluid specimens of patients with rheumatoid arthritis, malignant disorders, tuberculosis, and other conditions associated with lymphocytosis. Neutrophils may vary in number, but a predominance suggests a bacterial pneumonia, pulmonary infarction, or pancreatitis and bacterial peritonitis. Eosinophils can be seen in a wide variety of disorders, including pneumothorax, idiopathic effusions, infections, neoplasms, chronic peritoneal dialysis, congestive heart failure, vasculitis, and malignant lymphomas. Basophils are unusual in serous fluid, but usually accompany eosinophils.¹