The Test Sample
What is being tested?
Cerebrospinal fluid (CSF) is a clear, watery liquid that flows around the brain and spinal cord, surrounding and protecting them. A CSF analysis is a group of tests that evaluate substances in CSF in order to diagnose conditions affecting the brain and spinal cord (central nervous system).
CSF is formed and secreted by the choroid plexus, a special tissue that has many blood vessels and that lines the small cavities or chambers (ventricles) in the brain. It is continually produced, circulated, and then absorbed into the blood. About 17 ounces (500 mL) are produced each day. This rate of production means that all of the CSF is replaced every few hours.
A protective blood-brain barrier separates the brain from the bloodstream and regulates the distribution of substances between the blood and the CSF. It helps keep large molecules, toxins, and most blood cells away from the brain. Any condition that disrupts this protective barrier may result in a change in the normal level or type of constituents of CSF. Because CSF surrounds the brain and spinal cord, testing a sample of CSF can be very valuable in diagnosing a variety of conditions affecting the central nervous system.
Though a sample of CSF may be more difficult to obtain than, for example, urine or blood, the results may reveal more directly the cause of central nervous system conditions.
- Infections and inflammation in the meninges, the layers of tissue that surround the spinal cord and brain, can disrupt the blood-brain barrier and allow white blood cells (WBCs) and red blood cells (RBCs) and increased amounts of protein into the CSF. Meningitis, an infection in the meninges, and encephalitis, an infection in the brain, can also lead to the production of antibodies, which can be detected in the CSF.
- Autoimmune diseases that affect the central nervous system, such as Guillain-Barré syndrome and multiple sclerosis, can also produce antibodies that can be found in the CSF.
- Cancers such as leukemia can lead to an increase in white blood cells in the CSF and cancerous tumors can result in the presence of abnormal cells.
- Alzheimer disease is an irreversible form of dementia. Measuring amyloid beta 42 (Aß42) and tau protein in CSF may help establish a diagnosis for this disease.
These changes from normal CSF constituents make the examination of cerebrospinal fluid valuable as a diagnostic tool.
For more on CSF tests, see the "How is it used?" section.
How is the sample collected for testing?
A sample of cerebrospinal fluid (CSF) is collected by a health practitioner from the lower back using a procedure called a lumbar puncture or spinal tap. Often, three or more separate tubes of CSF are collected, and multiple tests may be run on the different samples.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
The person being tested should empty their bladder and bowels prior to the sample collection. It will be necessary to lie still in a curled-up fetal position during the collection and to lie flat and still for a time period after the collection.