What needs to be present for the diagnosis of pml?

MRI. There are characteristic MRI findings. Certain test can be done on spinal fluid too.
Immune suppression. The jc virus is present in at least 60% of normal individuals and truly does little thruout our lifetimes. Pts with HIV infections, or individuals being treated with cancer chemotherapies, or other immune-suppressants are also at risk. Tysabri (natalizumab) for ms is also a provocateur. Testing of blood or spinal fluid can be confirmatory, and a brain MRI can be characteristic.