You can get an MS diagnosis without a spinal tap. 15% of MSers who have had a spinal tap have had negative results. The spinal tap is not a specific test for MS nor should it be the decision maker for MS. As MS22 has pointed out, there are other tests which need to be taken. I have had 2 spinal taps, once in 1982 when I was diagnosed and the second in 2012 when my MS worsened. Both of my spinal taps were negative.
There are also a number of disease specific tests which are normally run in order to screen out those diseases which can mimic MS symptoms.
You need not worry about a normal (negative) spinal tap.