Dr Leigh Martens

General Practitioner

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Release Version: Beta

Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

CSF Analysis


At a Glance

Why Get Tested?

To diagnose a disease or condition affecting the central nervous system such as infection, bleeding within the brain or skull, cancer, or autoimmune disorder

When to Get Tested?

When your healthcare provider suspects that your symptoms are due to a condition or disease involving your central nervous system

Sample Required?

A sample of cerebrospinal fluid (CSF) collected by a health practitioner from the lower back using a procedure called a lumbar puncture or spinal tap

Test Preparation Needed?

You will be instructed to empty your bladder and bowels prior to 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.

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. Thumbnail diagram of the brain

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.

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.

The Test

How is it used?

Cerebrospinal fluid (CSF) analysis may be used to help diagnose a wide variety of diseases and conditions affecting the brain and spinal cord (central nervous system). They may be divided into four main categories:

  • Infectious diseases such as meningitis 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 (hemorrhaging) within the brain or skull
  • Autoimmune disorders, such as Guillain-Barré syndrome, sarcoidosis or multiple sclerosis 
  • Tumors located within the central nervous system (primary) or that spread to the central nervous system (metastatic cancer)

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 health practitioner 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.

When is it ordered?

CSF analysis may be ordered when a health practitioner suspects that a person has a condition or disease involving their central nervous system. A person's medical history may prompt the request for CSF analysis. It may be ordered when someone has suffered trauma to the brain or spinal cord, has been diagnosed with cancer that may have spread into the central nervous system, or has signs or symptoms suggestive of central nervous system involvement.

The signs and symptoms of central nervous system 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 central nervous system bleeding or infection, or may be slow to develop, indicating a chronic disease, such as multiple sclerosis or Alzheimer disease.

Depending on a person's history, a healthcare provider may order CSF analysis when some combination of the following signs and symptoms appear, especially when accompanied by flu-like symptoms that intensify over a few hours to a few days and fever:

  • Changes in mental status and consciousness
  • Sudden, severe or persistent headache or a stiff neck
  • Confusion, hallucinations or seizures
  • Muscle weakness or lethargy, fatigue
  • Nausea (severe or prolonged)
  • 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 fontanels (the soft spots on the top of the head)

What does the test result mean?

CSF usually contains a small amount of protein and glucose and may have a few white blood cells.

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 test 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?

Bacterial and parasitic (such as amebic) meningitis are medical emergencies. A healthcare provider must rapidly distinguish between these conditions, viral meningitis, which is typically milder, and other conditions with similar symptoms. Because prompt treatment is crucial, the health practitioner may start the person affected on a broad-spectrum antibiotic before the diagnosis has been definitely determined.

To help diagnose a central nervous system-related illness, a healthcare provider may want to know about recent vaccinations, sickness, contact with others who are ill, places a person has traveled to, what symptoms a person is experiencing, and their duration.