JC virus (JCV) seropositivity is a risk factor for progressive multifocal leukoencephalopathy (PML) in patients on natalizumab. Accordingly, the JCV serological antibody test is of paramount importance in determining disease risk.


We tested the accuracy of the JCV serum antibody test by comparing the results of JCV serology to JCV viruria and viremia in 67 patients enrolled in a single-center, retrospective cohort study. Bodily fluids (urine and blood) were assessed for JCV DNA by real time quantitative polymerase chain reaction 6 to 47 months (mean = 26.1 months) before JCV antibody testing. In 10 individuals, blood and urine samples were obtained on 2 separate occasions at 6-month intervals.


Forty (59.7%) of the 67 patients were JCV seropositive. Of 27 JCV seronegative patients, 10 (37%) had JCV viruria. Urine JCV DNA copy numbers were significantly higher in the seropositive group (mean log copy number = 5.93, range = 1.85–9.21) than the seronegative group (mean log copy number = 2.41, range = 1.85–5.43; p = 0.0026). Considering all body fluid test results, 50 (74.6%) of the 67 patients were previously infected with JCV.


The false-negative rate of the JCV serology in this study was 37%; therefore, JCV serostatus does not appear to identify all patients infected with JCV. Thus, a negative JCV antibody result should not be conflated with absence of JCV infection. This discordance may be important in understanding JCV biology, risk for PML, and PML pathogenesis. Ann Neurol 2013;74:84–90