--are rarely a diagnostic clincher in and of themselves, unless one has some sort of central nervous system infection that is picked up there (i.e., meningitis or encephalitis), but often the protein levels, and the kinds of protein that are found, can be a clue to processes going on in the body, particularly autoimmune ones, and that's why they are often performed in more ambiguous cases for which other tests are not clinching a diagnosis (such as early MS or immune neuropathies).