The diagnostic value of PCR assay for viral infections of the CNS will be improved now that a study has identified factors affecting test performance.

The positive predictive value was only 54% for infections judged clinically as likely CNS infections versus possible or unlikely infections when multiplex PCR results for various viral nucleic acids in patients’ CSF were matched with illness episodes. Twenty seven CSF samples for possible or unlikely CNS infections were positive for Epstein Barr virus. Most samples from episodes most likely to be CNS infections were positive for herpes simplex virus (HSV) and enterovirus. The negative predictive value was 83%, meaning that negative results can be interpreted as discounting viral infection with only moderate confidence. CSF samples obtained between three and 14 days after onset of neurological symptoms had the best chance of detecting viruses, as did those in which white cell count was raised.

PCR tests were done on 787 frozen samples collected over four years in a teaching hospital. Comparing results with episodes of possible viral CNS infections was possible for 494 episodes in 483 (66%) patients with detailed clinical data; 15% of patients were aged <1 year and 26% up to 16 years. Each sample was tested for human herpes virus types 1–6, human polyoma virus JC, enteroviruses, and Epstein Barr virus.

PCR assays have established that several known viral infections can include involvement of the CNS, but many have not been validated for CNS infections, and doctors are often unsure what positive or negative findings mean for patient management.