What is being tested?
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. Autoimmune 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 examination usually involves an initial basic set of tests performed when the test is requested:
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 ordered 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:
How is it used?
Cerebrospinal fluid (CSF) examination may be used to help diagnose a wide variety of diseases and conditions affecting the central nervous system (CNS). They may be divided into four main categories:
When is it requested?
CSF examination may be requested when a doctor suspects that a patient has a condition or disease involving their CNS. A patient's medical history may prompt the request for CSF examination. It may be ordered when a patient has suffered trauma to the brain or spinal cord, has been diagnosed with cancer that may have spread (metastatic) into the CNS or has signs or symptoms suggestive of a condition involving their CNS.
The signs and symptoms of CNS conditions can vary widely and many overlap with a variety of diseases and disorders. They may have sudden onset, suggesting an acute condition such as CNS bleeding or infection or may be slow to develop, indicating a chronic disease such as cancer or multiple sclerosis.
Depending on a patient's history, doctors may order a CSF examination when some combination of the following signs and symptoms appear:
What does the result mean?
CSF usually contains a small amount of protein and glucose and may have a few white blood cells (WBCs).
Any condition that disrupts the normal pressure or flow of CSF or the protective ability of the blood/brain barrier can result in abnormal results of CSF testing. For detailed explanations of what various tests results may mean, see the sections on:
Is there anything else I should know?
Multiple tubes of CSF are often collected during a lumbar puncture (see Common questions) to ensure the quality of samples for testing.
Meningitis due to infective causes is a medical emergency. Your doctor must rapidly distinguish between this and other causes. Because prompt treatment is crucial, your doctor may start you on a broad-spectrum antibiotic before the diagnosis has been definitely determined.
To help diagnose your illness your doctor may want to know what recent illnesses and previous vaccinations you may have had, what symptoms you are experiencing, whether you have been in contact with any ill people, and to what places you have recently travelled.
The lumbar puncture is a special but relatively routine procedure. It is usually performed while you are lying on your side in a curled up foetal position, but may sometimes be performed in a sitting position. It is crucial that you remain still during the procedure. Once you are in the correct position, your back is cleaned with an antiseptic and a local anaesthetic is injected under the skin. When the area has become numb, a special needle is inserted through the skin, between two vertebrae, and into your spinal canal. An “opening” or initial pressure reading of the CSF is obtained. The doctor then collects a small amount of CSF in multiple sterile vials. A “closing” pressure is obtained, the needle is withdrawn, and a sterile dressing and pressure are applied to the puncture site. You may be asked to lie quietly in a flat position, without lifting your head, for one or more hours to avoid a potential post-test spinal headache.
The lumbar puncture procedure usually takes less than half an hour. For most patients it is a moderately uncomfortable to somewhat painful procedure. The most common sensation is a feeling of pressure when the needle is introduced. Let your doctor know if you experience a headache or any abnormal sensations, such as pain, numbness, or tingling in your legs, or pain at the puncture site.
The lumbar puncture is performed low in the back, well below the end of the spinal cord - usually between lumbar (L) vertebrae L4 and L5. There are spinal nerves in the location sampled, but they have room to move away from the needle. There is the potential for the needle to contact a small vein on the way in. This can cause a “traumatic tap,” which just means that a small amount of blood may leak into one or more of the samples collected. While this is not ideal, it may occur a certain percentage of the time. The evaluation of your results will take this into account.
Blood from the collection procedure may contaminate the first portion of CSF sample that is collected. However, there are usually three or more separate tubes used to collect CSF samples during one spinal tap procedure. The last tube that is collected during a spinal tap is least likely to have blood cells present due to the procedure and is usually the sample used to test for the presence of blood cells in the CSF. Likewise, the last sample collected is used for infectious disease testing since it will not be contaminated with microorganisms from inserting the needle through the skin.
Spinal fluid, obtained during a spinal tap, is often the best sample to use for conditions affecting your central nervous system because your CSF surrounds your brain and spinal cord. Changes in the elements of your CSF due to CNS diseases or other serious conditions are often first and most easily detected in a sample of your spinal fluid. Tests on blood and urine may be used in conjunction with a CSF examination to evaluate your condition.
Other laboratory testing that may be requested along with or following CSF testing includes:
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