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:

• CSF colour, clarity and pressure during collection
• CSF protein
• CSF glucose
• CSF cell count (total number of cells present)
• CSF white blood cell differential (numbers of different types of cells present)
• 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 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:

• 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 ultrafiltrate of the blood, (all of the large molecules such as proteins are let 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 done to detect and identify microorganisms if an infection is suspected.

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:

• Infectious diseases such as meningtis and encephalitis- testing is used to determine if infection is caused by bacteria, viruses or, less commonly, by Mycobacterium tuberculosis, fungi or parasites, and to distinguish them from other conditions; may also be used to detect infections of or near the spinal cord or to investigate a fever of unknown origin.
• Bleeding (haemorrhaging) within the brain or skull
• Diseases that cause inflammation or other immune responses such as antibodies - these may include autoimmune disorders such as Guillain-Barré syndrome or sarcoidosis or diseases that cause the destruction of myelin such as multiple sclerosis
• Tumours located within the CNS either primary tumours or secondary metastatic tumour.

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:

• changes in mental status and consciousness
• confusion, hallucinations or seizures
• muscle weakness or lethargy, fatigue
• nausea (feeling sick)
• flu-like symptoms that intensify over a few hours to a few days
• fever or rash
• sudden, severe or persistent headache or a stiff neck
• sensitivity to light
• numbness or tremor
• dizziness
• difficulties with speech
• difficulty walking, lack of coordination
• mood swings, depression
  infants may be irritable, cry when they are held, have body stiffness, refuse food, and have bulging fontanelles (the soft spots on the top of the head)

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:

• CSF physical characteristics
• CSF chemical tests
• CSF microscopic examination
• CSF infectious disease tests

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.

Common questions

• What is a lumbar puncture (spinal tap) and how is it performed?

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.

• Why do I need a spinal tap - why can't my blood or urine be tested?

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.

• What other tests may be done in addition to a CSF examination?

Other laboratory testing that may be requested along with or following CSF testing includes:

• Blood culture to detect and identify bacteria in the blood
• Cultures of other parts of the body - to detect the source of the infection that led to meningitis or encephalitis
• Blood glucose, total protein, bilirubin - to compare with the concentration of CSF glucose, protein and bilirubin
• FBC (full blood count) - to evaluate cell counts in blood
• Antibodies for a variety of germs
• ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein) - which are markers in the blood indicating inflammation

More information

RCPA Manual: Cerebrospinal fluid cytology

RCPA Manual: Cerebrospinal fluid examination

Healthdirect Australia: Brain diseases