What Is a Cerebrospinal Fluid Analysis?

A cerebrospinal fluid (CSF) analysis is a medical examination that helps diagnose an array of conditions and diseases. Cerebrospinal fluid is fluid that surrounds the brain, as well as the spinal cord; it helps protect both from injury. For a cerebrospinal fluid analysis, a doctor collects a sample of cerebrospinal fluid from the patient and sends it for a laboratory examination, where an analysis of the fluid determines the presence of any abnormalities. Abnormalities are used to make a diagnosis. While there are several different methods of collection for a cerebrospinal fluid analysis, the test most commonly used is a lumbar puncture, also known as a spinal tap.

A doctor might order a cerebrospinal fluid analysis when a patient exhibits signs and symptoms that indicate a condition that involves the central nervous system (CNS). Diseases, infection and inflammation are some conditions that might produce abnormal results in a cerebrospinal fluid analysis. Examples of medical conditions that this analysis might help diagnose include encephalitis, meningitis and metastatic cancer. In addition, the analysis can also diagnose multiple sclerosis, sarcoidosis and tumors.

Usually, collection of cerebrospinal fluid is done through a procedure called a lumbar puncture. During a lumbar puncture, the patient lies down or sits up with his back curved. If he was lying down, his knees would be pulled in towards his chest and his chin would be tucked inwards as well, while if he was sitting up, the patient would be bent forward. The doctor inserts a needle into a numbed area of the back, usually the lower back, to collect a sample of the fluid. Sometimes, the doctor will use an x-ray to help position the needle during a lumbar puncture; this is known as a fluoroscopy.

Other methods of collection are cisternal puncture and ventricular puncture and through a tube placed in the fluid. These methods are not as commonly used because they are seen as more dangerous, but in certain cases, such as if the patient has a back deformity, they might be preferred over a lumbar puncture. A cisternal puncture, always done in combination with a fluoroscopy, involves inserting the needle below the back of the skull, which is a risky procedure due to its close proximity to the brain stem. A ventricular puncture, usually performed in an operating room, involves drilling a hole into the skull and inserting the needle into a brain ventricle. With collection through a tube, the fluid is acquired through a tube that was put into the fluid.