lumbar puncture 
noun Medicine, British
The procedure of taking fluid from the spine in the lower back through a hollow needle, usually done for diagnostic purposes: clear colourless fluid was obtained by lumbar puncture [count noun]: repeated lumbar punctures may be required...
- All patients had a lumbar puncture to obtain cerebrospinal fluid, to exclude other causes of coma.
- Reducing the failure rate in practice is therefore crucial if patients are to benefit from lumbar punctures with atraumatic needles.
- So I did the lumbar puncture to get spinal fluid.