No, doctors won't normally order a spinal tap, as it's a very invasive and sometimes painful procedure. But if other signs are "mixed" -- the patient has some clinical signs (objective, testable, demonstrable signs, not subjective symptoms) but other tests are coming back negative,
meaning the doctor doesn't know what is affecting the patient, then he might order a spinal tap, which will it isn't a specific test for MS, CAN show lots of other disease mechanisms in the body, that might cause those signs.
You said in another thread that you have fibro - -which has some definite specific tests that can be run to determine that, and specific pressure points, where..if the doctor presses on those points, you experience pain. That sort of thing doesn't occur in MS, usually. If you're uncomfortable with that diagnosis of fibro, you should go back to the doctor, report your new or increased symptoms, and ask for further testing.