To characterize atypical cerebrospinal fluid profiles (CSF) by historical definition and compare length of hospital stay (LOS) and time to definitive diagnosis (TTD) against typical profiles.

Research has identified typical CSF profiles for infectious meningitis/encephalitis with thresholds for “atypical” profiles. Limited research has analyzed these atypical profiles and their impact on diagnosis and management.

A retrospective study of patients (age ≥18) with confirmed infectious meningitis/encephalitis admitted between 2010-2021 was performed. Cases were identified via ICD-9/10 codes and confirmed via DNA/RNA CSF analysis or culture. Rstudio was used for descriptive and inferential analyses. Logistic Linear regression analyzed age, sex, immune status and positive antimicrobials before lumbar puncture as predictors of “atypical CSF profiles.” Atypical profiles were classified as those occurring in 1-10% of reference papers/texts and defined as - Bacterial: wbc <100 , protein <45mg/dl or CSF:serum glucose >40%; Viral: corrected wbc count <5; Fungal: CSF:serum glucose >40%. Mann Whitney U tests with bootstrapping for 95% confidence intervals compared LOS (days) and TTD (hours).

Out of 52 bacterial, 54 fungal and 85 viral cases, 21, 17 and 5 cases respectively had atypical profiles. LOS (W=1663, p<0.001) and time to definitive diagnosis (W=1957, p<0.001) were significantly different between atypical and typical profiles. The median LOS was 18 (95% CI 12,31) and 6 (95% CI 5,8) for atypical versus typical profiles. The median TTD was 6.5 (95% CI 5,15) and 4 (95% CI 3,4) for atypical versus typical profiles.

Bacterial and fungal meningitis/encephalitis cases had higher rates of atypical profiles. Regardless of pathogen type, atypical profiles were associated with longer hospitalizations and increased time until diagnosis. These results suggest atypical profiles significantly impact the ability to diagnose and manage patients.