To check for JC virus, get a neurological exam and brain MRI; if suspicious, perform lumbar puncture for CSF PCR to detect JC virus DNA (most definitive for PML). Blood testing for anti-JC virus antibodies assesses prior exposure and risk (used before some immunotherapies). Urine or plasma PCR can show viral shedding but aren’t diagnostic of brain disease. Discuss results with your neurologist.

JC virus is primarily detected by PCR testing for viral DNA in cerebrospinal fluid, the main diagnostic test for suspected central nervous system infection. Brain MRI supports diagnosis by showing characteristic white‑matter lesions. Anti‑JC virus antibody testing in blood or urine indicates prior exposure but not active disease. If PCR is negative yet suspicion remains, a brain biopsy may be needed for definitive diagnosis.

A positive JC virus (JCV) test means you've been exposed and have antibodies—common and usually latent in healthy people. It does not indicate active infection, but in people on certain immunosuppressive treatments (for example, some MS drugs) a positive result signals increased risk for progressive multifocal leukoencephalopathy (PML). Clinicians use antibody status and index values to guide treatment choices and monitoring.

Test by performing a lumbar puncture to collect CSF and running PCR for JC virus (JCV) DNA—qualitative or quantitative assays detect viral load. PCR is highly specific; repeat testing may be needed if the initial result is negative but clinical suspicion remains. Results typically take a few days. If PCR is inconclusive, correlate with clinical findings and MRI, and consider brain biopsy in select cases.

A JCV antibody test is a blood test that detects antibodies to the John Cunningham virus, indicating prior exposure. It’s primarily used before therapies such as natalizumab to help estimate the risk of progressive multifocal leukoencephalopathy (PML). A positive result suggests past infection and higher PML risk; a negative result indicates lower risk but can change over time, so periodic retesting may be recommended.

You can sometimes go from JCV antibody–positive to negative, but true reversal is uncommon. Antibody levels can fluctuate and occasionally fall below the test’s detection threshold, producing a negative result. Seroconversion (negative → positive) is more common. Because assays vary and results affect treatment decisions, repeat testing at recommended intervals and discussion with your clinician are advised.