Is a test needed for diagnosis, or can it be diagnosed from just the symptoms ?

I am assuming you are asking about MG since you did not specify. Since their are definite tests that can show the diagnosis, IMHO, a good doctor would put you through most if not all of them in order to rule it in or out or decide which varient or as it could be something else. and decide which treatement(s) are best.

However, not having the antibodies in not definitive - you have still have MG and be what is called 'sero-negative'. You can be given a trial of the medications and if you respond, that is also a round about test of it as well.

There are a lot of good sites posted up at the first thread there - why not read up?