in 1997 i was dx with pml i could not walk, talk or write for 4 months i eventualy began speaking with a slur i could write the dx was because of mri scan.

i spoke with a doctor at johns hopkins who thought i did not have pml at all he referred me to a neuro-aids specialist in nyc at mount sinai he ordered a spinal tap which was neg for jc virus so we did not know what we were dealing with a brain biopsy was ordered that came back neg also so even with the biopsy i still have no real dx i only know i have a lesion on the cerebellum i now have poor balance, slurred speech, poor left sided coordination and some visual stuff (blurred, doubled) so even with biopsy you are not 100% guaranteed a definitive dx

Yes, that is true-- even a biopsy can fail to reveal the cause of a problem, although in general it's the best tool that we have. There are a number of reasons for this, including "sampling error"-- that is, the surgeon may not get the most representative piece of tissue. The disease itself can be spotty at a microscopic level even though an entire area may look abnormal at the scan level and to the surgeon's eyes. There are no guarantees in life or medicine.