According to the Updated McDonald Criteria (2005) a spinal tap is not required for dx under certain circumstances.

http://www.mult-sclerosis.org/DiagnosticCriteria.html

If you have 2 clinical attacks and 2 clinically defined lesions, no further evidence is required.

If you have 2 clinical attacks, 1 lesion, then an MRI is required, OR positive CSF, etc.

This also states that the updated McDonald criteria using the advanced techniques of MRI are intended to replace the Poser criteria and the older Schumacher criteria for dx.

And yes, it does read Positive CSF as additional data for dx PPMS under the McDonald Criteria.