This is what it says in the McDonald criteria.
To diagnose RRMS, there needs to be…
“2 attacks; objective clinical evidence of 2 lesions or objective clinical evidence of 1 lesion with reasonable historical
evidence of a prior attack”
If this is satisfied then…
“No additional tests are required. However, it is desirable that any diagnosis of MS be made with access to imaging based on these criteria. If imaging or other tests (for instance, CSF) are undertaken and are negative, extreme caution needs to be taken before making a diagnosis of MS, and alternative diagnoses must be considered. There must be no better explanation for the clinical presentation, and objective evidence must be present to support a diagnosis of MS.”
Objective clinical evidence basically means symptoms/signs that have been observed by a medical professional and that can’t be faked; having confirmation from a machine confirms objective evidence too (e.g. scan results). So, e.g., optic neuritis diagnosed by an ophthalmologist is objective clinical evidence, but tinnitus isn’t because it can be faked.
When they say “reasonable historical evidence” they are normally talking about something that is in your medical records (i.e. observed by a medical professional), but if the rest of the evidence is strong, they can take the patient’s word for it.
Assuming there is clinical evidence, there is no need to do MRI, LPs or anything else, BUT, it clearly states that “imaging” (i.e. MRI) is preferable. If the MRI or LP is negative, then they have to be really sure before they should diagnose MS.
There is a heavy (over!)reliance on MRI these days, but ultimately RRMS is diagnosed CLINICALLY: two attacks and two areas of the nervous system with no better explanation = MS. (That “no better explanation” normally means loads of tests though!)
The criteria are different for PPMS. For a diagnosis of PPMS, the patient needs to have been getting gradually worse over a year (or less than this, but the consultant is confident that the progression is going to continue) plus have two of three criteria:
So, for PPMS, a positive LP can be very important, but there is no need for one if the MRI is conclusive.
If you want to read the full McDonald criteria paper, google Polman et al, Diagnostic Criteria for Multiple Sclerosis: 2010 Revisions to the McDonald Criteria, ANN NEUROL 2011;69:292–302.