The spinal tap: a new look at an old test.

The spinal tap, or lumbar puncture, has indisputable value; opinions differ, however, on the amount of that value. The procedure has variable utility depending on the clinical indications and the results of tests on the cerebrospinal fluid. Its greatest value is in the evaluation of infectious or malignant meningitis; for most other diseases, it provides additional, but not essential, information. Because of the potential risk of the spinal tap, decisions about when to do the procedure must be made carefully. A probability analysis is provided to elucidate the usefulness of data from cerebrospinal fluid tests.