One of the most devastating central nervous system infections, progressive multifocal leukoencephalopathy (PML), is caused by the JC virus (JCV). It often presents in immunosuppressed patients, specifically in those treated with natalizumab, rituximab, or other immunomodulatory agents. A brain biopsy is rarely required for establishing a diagnosis and a clinical diagnosis is also possible, the latter based on evolving symptoms and MRI data. The cornerstone of diagnosis of PML is the detection of JCV DNA in the cerebrospinal fluid (CSF) by polymerase chain reaction (PCR). However, an alarming and under-recognized vulnerability exists in the way JCV PCR results are reported - most laboratories provide a binary result (”positive” or ”negative”) while omitting critical assay performance metrics such as the LOQ and LOD data. This practice creates a false sense of diagnostic certainty and may lead to missed or delayed diagnoses of early-stage PML, where viral loads are often low. This systemic reporting failure is best understood as a diagnostic shortcoming with consequences for timely care. I argue for mandatory inclusion of LOQ and LOD across all JCV PCR reports to restore diagnostic transparency and prevent diagnostic delay. Since most clinical laboratories do not disclose the analytical sensitivity of their assays, a negative JCV PCR result may be falsely reassuring. Unless clinicians recognize this possibility, diagnosis of PML can be delayed—or missed entirely. Because limits of quantitation (LOQ) and detection (LOD) are pivotal for establishing PML, laboratories that fail to report them risk misleading clinicians, potentially provoking legal consequences. Transparent disclosure of assay performance is essential for accurate decision-making and timely intervention.
