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Body fluid cell count
Body fluid cell count testing performed on an automated hematology analyzer
Purpose	Analyzing white and red blood cells in body fluids

A body fluid cell count is a medical laboratory test involving the analysis of white and red blood cells in samples of serous fluid, synovial fluid and cerebrospinal fluid. The white and red blood cell counts may be determined by manual or automated methods. A body fluid differential count, which enumerates the different types of white blood cells in the fluid, can also be performed. The cell count and differential can help determine the cause of an effusion (an abnormal collection of serous or synovial fluid), or, in the case of cerebrospinal fluid, help to diagnose conditions like meningitis or brain hemorrhage.

Medical uses

Cell counts are typically performed on serous fluids—a classification that includes peritoneal fluid, which is found in peritoneal cavity in the abdomen; pleural fluid, which lines the pleural space that surrounds the lungs; and pericardial fluid, which lines the pericardial cavity surrounding the heart—as well as synovial fluid (found in joint cavities) and cerebrospinal fluid. Except for cerebrospinal fluid, these fluids are not present in volumes large enough to sample in healthy people. An increased level of serous or synovial fluid is referred to as an effusion (or ascites in the case of peritoneal fluid) and indicates a pathological process. When an effusion is present, cell count testing can provide information about the underlying medical condition.

Cerebrospinal fluid cell counts are usually performed when a patient has symptoms that suggest meningitis, encephalitis, intracranial hemorrhage, neurological disorders like multiple sclerosis, or malignancies involving the central nervous system.

Procedure

Cell count

Left: filling a hemocytometer; right: microscopic view of manual cell count

Body fluid cell counts can be performed manually, using a hemocytometer to count the cells under a microscope, or automatically, using an automated analyzer designed for complete blood count analysis. Because the number of cells in body fluids is much lower than in blood, automated analyzers have special modes for body fluid analysis that increase the volume of fluid analyzed in order to improve accuracy and precision. Most hematology analyzers are not suitable for the analysis of cerebrospinal fluid, as cell counts in CSF are typically extremely low; however, some specialized analyzers offer this capability.

The analysis begins with visual examination of the fluid. The colour and clarity of the body fluid, which provides information about the cell counts, is recorded. A white blood cell count greater than 200 per microlitre, or a red cell count greater than 400 per microlitre, can cause the fluid to be slightly cloudy. If the fluid is grossly bloody or cloudy, it can be diluted to make manual counting easier. Synovial fluid is naturally viscous due to the presence of hyaluronic acid, so hyaluronidase must be added to the sample to liquefy it in preparation for testing.

Differential

For differential counts on blood samples, see White blood cell differential.

Cytocentrifuge slide of peritoneal fluid, showing monocytes, lymphocytes, neutrophils and mesothelial cell

A differential count identifies and enumerates the types of white blood cells found in the fluid. Body fluid differential counts are performed by using a specialized centrifuge (a cytocentrifuge) to concentrate the cells on a microscope slide, then staining the slide with Wright's stain or Wright-Giemsa so that the cells can be identified. If the cell count is very high, the sample can be diluted before making a cytocentrifuge slide so that the cells are not distorted by crowding. A technologist counts 100 cells on the slide, then multiplies the resulting percentages by the total white blood cell count to determine the absolute number of each white blood cell type in the fluid. Some analyzers can provide an automated differential count, but it is limited to two types of cells – mononuclear and polymorphonuclear cells - while the manual method can differentiate neutrophils, lymphocytes, monocytes, eosinophils, basophils, mesothelial cells, malignant cells and more.

Interpretation of results

Serous and synovial fluids

In serous fluids, a white blood cell count above 1000/μL is one of the criteria that suggest a fluid may be an exudate, meaning that it is caused by an inflammatory process, as opposed to a transudate, which is generally caused by increased blood pressure in the capillaries or decreased oncotic pressure.

A white blood cell count greater than 1000/μL in pericardial fluid can suggest pericarditis. In peritoneal fluids, a neutrophil count above 500/μL is suggestive of bacterial peritonitis, which is a medical emergency.

An elevated RBC count in pericardial and pleural fluids may indicate a hemorrhagic effusion. The pleural fluid RBC count may also be elevated in cases of pleural neoplasms or trauma. However, red blood cell counts of serous and synovial fluids offer little clinical significance beyond what can be ascertained from visual inspection of the sample.

Cerebrospinal fluid

Further information: Cerebrospinal fluid § Clinical significance

Four vials of human cerebrospinal fluid

The white blood cell count in cerebrospinal fluid is normally up to 5/μL in adults and up to 30/μL in neonates. A markedly increased WBC count in CSF is termed pleocytosis and can be caused by a variety of conditions. CSF white blood cell counts above 1000/μL are suggestive of bacterial or fungal meningitis.

The presence of red blood cells in CSF is abnormal, but usually signifies contamination of the sample with blood during the lumbar puncture rather than any actual pathology. In rare cases, RBCs may be present due to a recent subarachnoid or cerebral hemorrhage. A contaminated sample may also have a falsely elevated WBC count, as white blood cells from peripheral blood will be present in the CSF specimen. To help differentiate a contaminated draw from a hemorrhage, the amount of blood in each CSF tube can be examined. In a traumatic draw, the amount of blood in the CSF often decreases from the first tube drawn to the last, while in a hemorrhage, the amount of blood is often constant. However, this method is not always reliable.

In cerebrospinal fluid, the white blood cell differential is useful in determining the cause of an elevated WBC count. A CSF differential that predominantly contains neutrophils is associated with bacterial meningitis, brain abscess, and hemorrhage, while one that predominantly contains lymphocytes is associated with other infectious conditions like viral, tubercular, fungal and syphilitic meningitis, as well as neurological disorders like multiple sclerosis and Guillain–Barré syndrome. Increased monocytes can be seen in tubercular and fungal meningitis as well as in response to brain trauma or drug treatment. Eosinophils may be increased in parasitic and fungal infections, idiopathic eosinophilic meningitis, and allergic reactions (e.g. to cerebral shunts or intrathecally administered drugs).

Malignant cells may be observed in serous and cerebrospinal fluid, and rarely in synovial fluid. Tumour cells in body fluids indicate the presence of a primary or metastatic tumour. In lymphoma and leukemia with central nervous system involvement, blast cells can be present in the CSF.

Reference ranges

Reference ranges for body fluids

Fluid type	WBC reference range	RBC reference range	Appearance
Cerebrospinal	≤5/μL (adults); ≤30/μL (neonates)	≤1/μL (adults); ≤3/μL (neonates)	Clear and colourless
Synovial	≤200/μL	≤1/μL	Clear and pale yellow
Serous (peritoneal, pleural, pericardial)	≤200/μL	≤1/μL	Clear and pale yellow