Cerebrospinal Fluid Culture

Cerebrospinal fluid (CSF) is a clear watery liquid filtrate that is formed by the choroid plexus, special tissue that has many blood vessels and lines the small spaces or chambers (ventricles) in the brain. CSF flows around the brain and spinal cord, surrounding and protecting them. CSF is continually produced, circulated, and then absorbed into the blood system. About 500 mL is 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 circulating blood and regulates the distribution of substances between the blood and the CSF. The barrier 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 diseases affecting the central nervous system (CNS). Although 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 brain-related symptoms.

For example, infections and inflammation in the meninges (called meningitis) or the brain (called encephalitis) can disrupt the blood-brain barrier and allow white and red blood cells and increased amounts of protein into the CSF. Meningitis and encephalitis can also lead to the production of antibodies. Immune diseases that affect the CNS, such as Guillain-Barré Syndrome, and multiple sclerosis can also produce antibodies that can be found in the CSF. Cancers such as leukaemia can lead to an increase in CSF white blood cells (WBCs), and cancerous tumours 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 colour, clarity and pressure during collection
• CSF protein
• CSF glucose
• CSF cell count
• CSF differential
• If infection is suspected, CSF gram stain and culture

A wide variety of other tests may be requested as follow-up depending on the results of the first set of tests. The specific tests that are requested may also depend on the signs and symptoms of the patient and the disease the doctor suspects is the cause. Each of these tests can be grouped according to the type of examination 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. CSF is basically an ultafiltrate of the blood (all of the large molecules such as proteins are left in the blood and do not normally pass across into the CSF), so it can also be affected by what is going on in the blood. Normally, certain constituents of CSF such as protein and glucose are a percentage of blood levels, so CSF levels are often evaluated in relation to blood levels.
• 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 performed to detect and identify microorganisms if an infection is suspected.

For detailed explanations of various type of examinations, please see Details on CSF Tests below.

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.

Is any test preparation needed to ensure the quality of the sample?

The patient should empty their bladder and bowels prior to the sample collection. It will be necessary to lie still in a curled-up foetal position during the test and to lie quietly for a time period after the collection.