Hi and welcome,
Technically the spinal tap isn't a necessary test in the dx of MS, but from my understanding the markers they look for with MS (obands) are permanent and do not disappear or only ever appear when a person with MS is a relapse.
"Cerebrospinal Fluid Analysis
Analysis of the cerebrospinal fluid, which is sampled by a spinal tap, detects the levels of certain immune system proteins and the presence of oligoclonal bands. These bands, which indicate an immune response within the CNS, are found in the spinal fluid of about 90-95% of people with MS. But because they are present in other diseases as well, oligoclonal bands cannot be relied on as positive proof of MS." (national ms society)
The same basically applies if there are no obands unique to the spinal fluid, their absence doesn't rule out MS. In MS meeting the requirements of Mcdonald Criteria is needed, a persons clinical signs and sx's consistent with MS (eg Optic Neuritis etc) are very important but the MRI evidence is more heavily weighted.
This also varies by which article you're reading. JJ's refers to 90-95% of people with MS, other articles cite as low as 70% positive for O bands. I was diagnosed with a negative LP, and I know quite a few others here were, as well.
Even the MRI, while important, is just a clue in making the diagnosis.
My symptoms were not particularly active when I had a spinal tap done, and I've never seen anything that would suggest a spinal tap would be less useful during remission.
A spinal tap is an invasive procedure with potential side effects. The prospect of having one understandably makes people nervous. It made me nervous. However, mine went very smoothly.
In my case, my neurologist deemed the test useful and necessary for diagnosing me with MS. Many medical professionals consider it a necessary test, and it is entirely between you and your doctor(s) to determine if it is necessary for you.
Thank you so much for taking the time to provide this information. I deeply appreciate it. It seems as if the LP might be more predictive of MS if such a high percentage of people with the condition have the obands.-junicat