This test is an analysis of cerebral spinal fluid (CSF). CSF bathes, cushions, and protects the brain and spinal cord. It flows around the skull and spine in the subarachnoid space (the area immediately surrounding the brain and spinal cord).

CSF chemistry test identifies chemicals such as proteins and glucose levels, which can help diagnose certain disorders and diseases.

A lumbar puncture (spinal tap) is the most common means of CSF collection. Generally, this is performed as follows: You are positioned on your side with your knees curled up to your abdomen and your chin tucked in to your chest. Local anesthetic is injected over the lower spine. The spinal needle is inserted, usually between the 3rd and 4th lumbar vertebrae, and fluid is withdrawn for testing. You will be asked to remain flat, or nearly flat, for 6 to 8 hours after the test.

Alternative methods of obtaining CSF are rarely used, but they may be recommended if you have a problem such as lumbar deformity or infection, which would make lumbar puncture impossible or unreliable:

• Cisternal puncture involves insertion of a needle below the occipital bone (back of the skull). This procedure can be hazardous because the needle is inserted close to the brain stem.
• Ventricular puncture is even more rare, but may be indicated when sampling of CSF is necessary in people with possible impending brain herniation. It is usually performed in the operating room. A hole is drilled in the skull and a needle is inserted directly into the lateral ventricle of the brain.

The CSF is then sent to the laboratory for evaluation.

Be prepared to sign a consent form and to remain in the hospital for at least 6 to 8 hours after the test.

The curled position may be uncomfortable but is necessary to avoid moving the needle and possibly injuring the spinal cord.

The anesthetic will sting or burn when first injected. There will be a hard pressure sensation when the needle is inserted, and there is usually some brief pain when the needle goes through the meninges. (See CSF collection.)

Overall, discomfort is minimal to moderate. The entire procedure usually takes about 30 minutes. The actual fluid collection only takes a few minutes.