The results of the procedure can help doctors see if your immune system is attacking itself, which is what happens in MS. If you have MS, your spinal fluid will have more of certain proteins. But if someone’s spinal fluid doesn’t have these proteins, they might still have MS -- 5% to 10% of people with the condition never show signs in the fluid. These signs can show up in other diseases, too. So a spinal tap can't confirm or rule out MS. It must be part of overall testing for the disease.