What Is Cerebrospinal Fluid?

Medically Reviewed by Sabrina Felson, MD on May 04, 2023
2 min read

Cerebrospinal fluid is the liquid around your brain and spinal cord. If a doctor thinks you have an illness that affects your nervous system, they might take a sample for testing.

The fluid is made by a group of cells, called the choroid plexus, that are deep inside your brain. Your body has about 150 milliliters of fluid -- roughly two-thirds of a cup.

As the colorless fluid goes around your brain and spinal cord, it cushions those organs, picks up needed supplies from your blood, and gets rid of waste products from brain cells.

Sometimes cerebrospinal fluid can have things in it that shouldn't be there, like bacteria or viruses that can attack your brain. With some illnesses, what's in that fluid can help your doctor figure out what's going on.

A sample of cerebrospinal fluid can be an important clue. It can tell your doctor if you have one of a number of conditions, such as:

  • Multiple sclerosis (when your body's immune system attacks your nerves) or other similar conditions known as autoimmune diseases
  • Myelitis: inflammation of your spinal cord
  • Encephalitis: inflammation of your brain cells
  • Meningitis: inflammation of the thin tissues that cover and protect your brain and spinal cord. This usually is caused by an infection in the cerebrospinal fluid.
  • A stroke or similar condition that causes bleeding around your brain
  • Leukemia: a kind of blood cancer
  • Dementia


Your doctor will use a procedure called a spinal tap or lumbar puncture. They'll take a sample of cerebrospinal fluid with a long, thin needle. You'll get a local anesthetic to numb the skin in the area, and the needle will go in between two of your vertebrae, the bones that surround your spinal cord and make up your spine. They'll take a tablespoon or two of the fluid for testing.

It usually takes about 45 minutes. You'll rest for a while afterward and may be told not to do anything strenuous for about a day. You may have a headache afterward, but tell your doctor if it lasts more than a few hours.

What's in your cerebrospinal fluid can help your doctor identify or rule out various diseases.

  • If you have high levels of a substance called immunoglobulin, which your body uses to fight disease, or other things related to your nerve cells, that could point to multiple sclerosis or another autoimmune disorder.
  • If your doctor thinks you have Alzheimer's disease or another kind of dementia, certain types of proteins linked to the disease may be in the fluid.
  • Discolored fluid might be a sign of a cerebral hemorrhage (bleeding in your brain) or stroke.
  • Signs of bacterial or viral infection could tell your doctor you have an illness like meningitis or encephalitis.