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 but clots may also form at lower levels of protein.
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. Some examples are provided in the following table:
| 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 (type of neurosyphilis) | incomplete clot |
| blockage of CSF circulation | completely clotted due to protein |
