Clot formation is always abnormal and is often due to increased levels of protein, especially fibrinogen. When the protein level is 1000 mg/dL, clot formation will most likely occur. However, clots may also form at protein levels below 1000 mg/dL.
Some clots may be very fine and appear as a thin membrane or "scum" on the surface of the CSF specimen. This type of clot is referred to as a pellicle. Pellicles are composed of fibrinogen and white blood cells.
The type of clot formed may give some specific information about the disease state. Table 3 provides some examples.
Table 3. CSF Specimen Clots.| Example of Condition | Type of Clot |
| Bacterial meningitis | Pellicle forms in a short time; large clot formation follows |
| TB meningitis | Web-like clot (pellicle) after 12–24 hours (enhanced by refrigeration) |
| Paresis | Incomplete clot |
| Blockage of CSF circulation | Completely clotted due to presence of high levels of protein |
