Five cases of natalizumab-associated progressive multifocal leukoencephalopathy (PML) with high CSFJC virus (JCV) antibody index (AIJCV) values prior to or at diagnosis of PML are shown in an x–y diagram, plotting the absolute arbitrary unit (AU) values in blood and cerebrospinal fluid (CSF; left) and the AIJCV values (right) over time. PLEX marks the start with plasma exchange or immune adsorption. I* indicates indeterminate polymerase chain reaction (PCR) result at a local virology laboratory, negative at the Institute for Virology, Düsseldorf. 14# indicates 14 copies/ml CSF at the NIH, negative at the Swedish reference laboratory for JCV testing. 25## indicates 25 copies/ml CSF at the NIH, indeterminate result (1 probe positive in 2 duplicates and repeated testing) at the Institute for Virology, Düsseldorf. Dashed lines indicate 95% confidence interval of antibody-positive natalizumab controls in blood (upper line) or CSF (lower line); continuous lines indicate AIJCV cutoff at 1.5. FTY = fingolimod.