I had one a few months back....I think current terminology is "Lumbar Puncture". They took about 12cc of spinal fluid to analyze the proteins etc.
This test is really precise, if the person does not get a clean shot at the spinal cord, it ruins the test by getting blood in the spinal fluid. They had this scenario on me, and wanted to do a follow up in two weeks. My first EMG was done the following week and those results saved me from getting another lumbar puncture.
I had 5 MRI's, a Cat Scan, 2 EMG's, and about 6 blood tests. And I have to say the lumbar puncture was the biggest pain in the azz, especially because they make you lay flat for 48 hours after while the fluid builds back up.
I didn't like the MRI's either, because I am 6' 5" and go about 320 lbs. Talk about a case of claustrophobia, I'm talking if I weighed 5 lbs more they would not have been able to get me in the tube.....it sucked!