A case of natalizumab-associated progressive multifocal leukoencephalopathy with repeated negative CSF JCV testing.

The development of progressive multifocal leukoencephalopathy (PML) in patients treated with natalizumab is a well-known potential risk. Diagnosis of PML can be confounded in patients with multiple sclerosis (MS) if new demyelinating lesions develop, and the sensitivity of existing diagnostic tests is less than ideal. In the case presented here, four samples of cerebrospinal fluid tested negative for John Cunningham virus (JCV) DNA by polymerase chain reaction, yet brain biopsy eventually proved positive by immunohistochemistry.