Recurrent cerebrospinal fluid herpesvirus PCR positivity: a retrospective analysis of inter-positive intervals

Introduction. Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and varicella-zoster virus (VZV) are established causes of central nervous system infection, whereas human herpesvirus 6 (HHV-6) DNA may be detected in more heterogeneous contexts. However, longitudinal patterns of recurrent cerebrospinal fluid (CSF) PCR positivity remain poorly characterized. Hypothesis/Gap statement. When the same herpesvirus is detected in CSF on different dates, the meaning of the interval between positive results is uncertain. Its observed distribution may also depend on retesting opportunities and changes in testing practice. Aim. To describe inter-positive intervals across five herpesvirus targets and assess how their interpretation changes with operational cut-offs and calendar-period testing patterns. Methodology. We retrospectively analysed qualitative CSF PCR results for five herpesviruses from 16 March 2010 to 30 March 2026. Recurrent positivity required at least two distinct positive dates per patient–virus pair. First-to-last intervals were classified at 30 days, with exploratory cut-offs of 14, 60 and 90 days. Calendar-period analyses used FilmArray Meningitis/Encephalitis panel introduction on 10 November 2020 as the boundary and period-specific positive pairs as denominators. Results. Among 19,959 interpretable target-level results (994 positive), 33 recurrent pairs were identified: HHV-6 (18), VZV (7), HSV-1 (5), HSV-2 (2) and cytomegalovirus (1). The median first-to-last inter-positive interval was 11 days (interquartile range, 4–185; range, 1–864). Nineteen intervals were <30 days and 14 were ≥30 days; HHV-6 accounted for 10 longer pairs, all pre-implementation, limiting target-specific interpretation. The HHV-6 versus non-HHV-6 comparison was not significant (10/18 versus 4/15; two-sided Fisher’s exact P =0.158). Within-period recurrent positivity was 24/771 (3.1%) in the pre-implementation calendar period and 8/176 (4.5%) in the post-implementation period; corresponding HHV-6 recurrent positivity was 17/302 (5.6%) and 1/84 (1.2%). Conclusion. Recurrent CSF herpesvirus PCR positivity showed heterogeneous target composition and inter-positive intervals. These data cannot distinguish clustered retesting, persistence, reactivation or chromosomal integration. Interpretation requires prior testing history and linked clinical or quantitative virological data.