Clinical and Interpretive

This assay is used to detect increased permeability of the blood-brain barrier to plasma proteins and to detect increased intrathecal production of immunoglobulins.

Cerebrospinal fluid (CSF) is secreted by the choroid plexuses, around the cerebral vessels, and along the walls of the ventricles of the brain. It fills the ventricles and cisternae, bathes the spinal cord, and is reabsorbed into the blood through the arachnoid villi. CSF turnover is rapid, exchanging about four times per day. More than 80% of CSF protein content originates from plasma by ultrafiltration through the walls of capillaries in the meninges and choroid plexuses; the remainder originates from intrathecal synthesis. Because CSF is mainly an ultrafiltrate of plasma, low-molecular plasma proteins such as prealbumin, albumin, and transferrin predominate. No protein with a molecular weight greater than that of IgG is present in sufficient concentration to be visible on electrophoresis.

The permeability of the blood-brain barrier to plasma proteins is increased by high intracranial pressure due to brain tumor; intracerebral hemorrhage; traumatic injury; or by inflammation due to bacterial or viral meningitis, encephalitis, or poliomyelitis.Increased intrathecal synthesis of immunoglobulins, particularly IgG, is seen in demyelinating diseases of the central nervous system (CNS), especially multiple sclerosis. Increased immunoglobulins are also seen in other chronic inflammatory diseases of the CNS such as chronic meningoencephalitis due to bacteria, viruses, fungi or parasites; subacute sclerosing panencephalitis; and Guillian-Barre syndrome.

Striking elevations of cerebrospinal fluid (CSF) total protein are noted in bacterial meningitis; smaller elevations occur in the other inflammatory diseases and with tumor or hemorrhage. The effect of any of these conditions is that the proportions of specific proteins in CSF increasingly resemble serum.

In order to assess increased permeability or increased intrathecal production of proteins, simultaneous serum specimen and CSF specimens should be taken.

Specimens should be collected prior to the intrathecal administration of contrast media. Significant positive bias can occur when cerebrospinal fluid (CSF) contains contrast media. If possible, the patient should be recumbent for about one hour before the specimen is drawn. Erect posture causes a redistribution of the body fluid, increasing total serum protein concentration.

Blood in the CSF specimen invalidates the protein value.