Doctors do a spinal tap to get a sample of cerebrospinal fluid (the fluid that surrounds the spinal cord). A spinal tap checks the pressure and content of the fluid. It is usually very clear. It contains proteins, sugar, and other substances that can be found in blood. It typically does not contain red blood cells or many white blood cells. Signs of problems include evidence of bleeding, an increase in white blood cells (infection fighting cells), an increase in protein levels, or inflammation. This could mean there is an infection, tumor, or hemorrhage around the brain or spinal cord.
To obtain the fluid sample, a needle will be inserted into the spinal canal in the low back. The needle is inserted through the back, usually between two of the vertebrae. You may be asked to sit and lean over or to lie on your side. Before the spinal needle is inserted, the skin will be cleansed with an antiseptic and numbed with lidocaine. The procedure usually takes only 10-15 minutes.
A spinal tap has more risks associated with it than most other tests. This is one reason that doctors prefer to use "noninvasive" tests first, such as the MRI and CT scan. The risks associated with a spinal tap include meningitis (infection of the spinal fluid) and the possibility of developing a spinal headache. There is also a very small chance that the needle will cause bleeding around the spinal sac. This is more of a risk if you are on medications that thin the blood and interfere with blood clotting. If possible, you should not take aspirin or ibuprofen for five days before having a spinal tap. Make sure to tell your doctor if you are taking any type of blood thinners.