A spinal tap is done to collect a sample of the cerebrospinal fluid that surrounds your spinal cord and brain. The doctor gives a shot of local anesthetic in your lower back and then slides a needle into the spinal column. Fluid is removed through the needle. While the needle is in, the doctor also checks the pressure of your spinal fluid.

Spinal fluid is usually clear, with no red blood cells and few white blood cells. It contains proteins, sugar, and other body chemicals. Analyzing the content of your spinal fluid can help detect many conditions. Too much protein might mean infection, tumors, or a hemorrhage. A high number of the infection-fighting white blood cells suggests an infection. If red blood cells are present, there is bleeding somewhere in the spinal column.