Cerebrospinal fluid is a clear, physiological saline body fluid with low protein content derived from ultrafiltration of the blood. It fills the space between the skull, brain and ventricles and surrounds the spinal cord. It serves as a ‛cushion’ for the brain and spinal cord, transports hormones and neurotransmitters, helps to remove toxic metabolites and ensures the brain has a constant milieu. A healthy adult usually has around 100 to 150 mL of this special fluid, and daily production is normally around 500 mL.

Cellular analysis of cerebrospinal fluid is requested to detect or exclude diseases affecting the central nervous system: infection by bacteria, viruses, fungi, or protozoans; inflammation, e.g. multiple sclerosis, or Guillain-Barré syndrome; leptomeningeal spread, i.e. cells from a peripheral tumour.

Samples are mostly taken by lumbar puncture and occasionally by cisternal puncture. In patients with a ventricular shunt, e.g. after surgery or during treatment of a hydrocephalus, samples can also be obtained from the shunt. Although a routine procedure, CSF sampling still bears some risks and makes CSF a precious material.

The counting and differentiation of cells found in cerebrospinal fluid supports distinguishing between different types of diseases. As a rule of thumb, more than 5 white blood cells/µL are considered pathological in adults, while in neonates up to 30 white blood cells/µL can occur in normal samples. A strongly elevated WBC count together with an increased number of polymorphonuclear cells is often indicative of bacterial infection. The predominance of mononucleated cells rather hints towards viral, tuberculous or fungal meningitides or inflammatory processes.

Pleural fluid accumulates between the two pleural layers, the space surrounding the lungs known as the pleural cavity. The physiological volume of pleural fluid is no more than 15 mL. Excessive amounts of this fluid (pleural effusion) are considered pathological. There are various reasons for such accumulation that range from congestive heart failure (most frequent reason) to pneumonia, pulmonary emboli or tuberculosis, etc.

In combination with biochemical parameters, the counting and differentiation of cells in pleural fluid can contribute to identifying the cause of the pleural effusion. Large numbers of neutrophils may, for example, suggest an infection, but even non-infectious pleural effusions may contain considerable numbers of white blood cells, although there may be more mononucleated cells in this case. In addition, mesothelial cells are commonly seen, and with an underlying oncological disease one can find tumour cells. If the fluid is bloody, this is most frequently due to trauma or cancer invasion.