Cerebrospinal fluid analysis is a test used to diagnose conditions that affect the central nervous system.

Cerebrospinal fluid (CSF) is a clear liquid that is produced mostly in the ventricles of the brain. It surrounds and protects the brain and the spinal cord, which make up the central nervous system (CNS).

A doctor will collect a sample of fluid from a person's lower back during a procedure called a lumbar puncture, which is also known as a spinal tap.

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Cerebrospinal fluid protects the brain and spinal cord.

Autoimmune conditions, such as multiple sclerosis (MS) and Guillain-Barre syndrome, can lead to an inflammatory response, which can be detected by the presence of autoantibodies in the CSF.

Doctors can also identify certain cancers, including leukemia, as the CSF analysis test may reveal increased numbers of white blood cells. Doctors also use the CSF analysis test to check for primary and metastatic cancerous tumors in the CNS.

The CSF analysis test can also diagnose Alzheimer's disease, by measuring levels of amyloid beta 1-42 (Aß1-42) and p- and t- tau proteins.

In cases of infectious diseases causing meningitis and encephalitis, doctors use a CSF test to find out if the cause is viral, fungal, bacterial, or due to a parasite, which will influence the course of treatment.

The blood-brain barrier

The brain and the bloodstream are separated by the blood-brain barrier. This barrier stops large molecules, toxins, and most blood cells traveling from the blood into the brain. The spinal cord and bloodstream also have a blood-spinal cord barrier.

If a person has a condition that affects the CNS, it will disrupt the blood-brain barrier, which can usually be detected by changes in the composition of the spinal fluid.

What is CSF?

CSF surrounds and protects the brain and the spinal cord. About 500 to 600 mL of clear, watery CSF is formed in the ventricles of the brain every day. The fluid is continually circulated and then absorbed into the blood.

This fluid has several functions. It protects the brain by acting as a cushion and can remove harmful substances, including drugs.

CSF helps keep the pressure in the skull cavity and spinal canal at constant levels. It also transports hormones from where they are produced to parts of the brain where they are needed.

If a doctor suspects that a person has a condition affecting the CNS, or that cancer that may have spread to the CNS, the person may be sent for a lumbar puncture.

The signs and symptoms of conditions that affect the CNS include:

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A lumbar puncture is used to collect samples of CSF.
Image credit: Brainhell, 2006
  • changes in consciousness and mental state
  • sudden, severe, or persistent headaches
  • confusion, hallucinations, or seizures
  • nausea
  • sensitivity to light
  • numbness or tremor
  • dizziness
  • speech difficulties
  • walking difficulties
  • mood swings and depression

What happens during a lumbar puncture

Doctors usually perform a lumbar puncture on a person who is lying still in a fetal position. The doctor will clean the skin of the person's lower back and inject a local anesthetic.

Once the back is numb, the doctor will insert a hollow needle into the spinal canal (containing the spinal cord) between two vertebrae. This part of the procedure is called the "opening."

The doctor will collect samples of CSF in sterile vials before withdrawing the needle; they will then apply a sterile dressing and pressure as part of the "closing."

The procedure usually takes about 30 minutes and may be uncomfortable.

Some people experience a headache between 24 and 48 hours after the procedure. People have described the pain as a dull or throbbing pain at the front of the head, sometimes spreading to the neck and shoulders.

Over-the-counter medications can help alleviate soreness, but if the pain is severe or is accompanied by sickness and vomiting, it is important to seek medical attention.

To reduce the risk of developing a post-lumbar puncture headache, a person will be asked to lie on their back quietly and not lift their head for 1 to 2 hours afterward the procedure.

The lumbar puncture is performed below the end of the spinal cord where there are small strands of nerves. Sometimes, the needle can make contact with a small vein, causing a traumatic tap.

If this happens, a small amount of blood may leak into one or more of the samples, which may impact the results.

Some people experience lower back pain in the area where the procedure took place, while others may also feel pain in the back of their legs. Doctors usually recommend over-the-counter pain relievers, and the pain usually subsides in a few days.

A person may also experience some bruising and swelling related to a small amount of leaking fluid collecting under the skin. This tends to go away without treatment.

The CSF that is collected during a lumbar puncture will contain protein and glucose and may also contain white blood cells. It will be examined to detect any disruption to the normal flow of CSF or damage to the blood-brain barrier.

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CSF will be examined to determine if there is any damage to the blood-brain barrier.

Initial tests performed on the CSF look at:

  • color, clarity, and pressure during collection
  • protein levels
  • glucose levels
  • total number of cells present (cell count)
  • numbers of different types of cells present (differential cell count)
  • gram stain and culture, if an infection is suspected

Depending on the results of the test and the signs and symptoms the individual experienced, a doctor may perform further tests.

These fall into four broad categories:

  • Physical characteristics: These include measuring the pressure of the CSF flow, and checking whether the color and consistency are normal.
  • Chemical tests: These detect or measure the chemical substances in the CSF, including levels of protein and glucose, which usually correlate with their concentration in the blood.
  • Cell count and differential: Any cells present may be counted and identified under a microscope.
  • Infectious disease tests: Numerous tests can identify microorganisms if an infection is suspected.

If analysis of the CSF identifies any irregularities, a doctor will recommend further tests to help make a diagnosis.