The Test Sample
What is being tested?
Cerebrospinal fluid (CSF) is a clear watery liquid that 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. CSF flows around the brain and spinal cord, surrounding and protecting them. It is continually produced, circulated, and then absorbed into the blood system. 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 CSF analysis is a group of tests that evaluate substances present in CSF in order to diagnose conditions affecting the central nervous system.
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 symptoms.
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. Immune 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 CSF white blood cells, and cancerous tumors can result in the presence of abnormal cells. These changes from normal CSF constituents make the examination of cerebrospinal fluid valuable as a diagnostic tool.
CSF analysis usually involves an initial basic set of tests performed when CSF analysis is requested:
- CSF color, clarity and pressure during collection
- CSF protein
- CSF glucose
- CSF cell count (total number of cells present)
- CSF differential cell count (numbers of different types of cells present)
- If infection is suspected, CSF gram stain and culture
A wide variety of other tests may be ordered as follow-up depending on the results of the first set of tests. The specific tests that are ordered may also depend on the signs and symptoms a person has and the disease the doctor suspects may be the cause. Each of these tests can be grouped according to the type of exam that is performed:
- Physical characteristics —includes measurement of the pressure during sample collection and the appearance of the CSF.
- Chemical tests —this group refers to those tests that detect or measure the chemical substances found in spinal fluid. Many of the substances in CSF are also in blood and the relative amounts in CSF and blood are often compared. Normally, levels of certain constituents of CSF, such as protein and glucose, are reflective of their concentration in the blood.
- Microscopic examination (Cell count and differential)—any cells that may be present are counted and identified by cell type under a microscope.
- Infectious disease tests —numerous tests can be done to detect and identify microorganisms if an infection is suspected.
How is the sample collected for testing?
A sample of cerebrospinal fluid (CSF) is collected by a doctor 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 test and to lie quietly for a time period after the collection.