Diagnostic accuracy of cerebrospinal fluid gram stain for bacterial meningitis in patients with a suspected central nervous system infection.

OBJECTIVES: Bacterial meningitis is a life-threatening disease requiring rapid pathogen identification and prompt initiation of treatment. Cerebrospinal fluid (CSF) Gram stain is a widely recommended, rapid diagnostic test, yet its diagnostic accuracy in unselected patients with suspected central nervous system (CNS) infection remains insufficiently defined. METHODS: We assessed the diagnostic accuracy of CSF Gram stain for bacterial meningitis in patients aged ≥16 years old who underwent a lumbar puncture for a suspected CNS infection. The reference standards were microbiologically confirmed bacterial meningitis, and a clinical diagnosis of bacterial meningitis as scored by two physicians. RESULTS: Among 1637 episodes, CSF Gram stain was performed in 1226 (75%), in which bacterial meningitis was microbiologically confirmed in 135 (11%) cases and clinically diagnosed in an additional 53 episodes (4%). The sensitivity of CSF Gram stain was 53% (95% confidence interval (CI): 42-59) for microbiologically confirmed meningitis and 38% (95% CI: 31-46) for clinically diagnosed meningitis, while specificity was 100% in both groups. In 16 CSF culture-negative cases, Gram stain identified a bacterial pathogen in CSF. Antibiotic treatment was adjusted within 24 hours more frequently in bacterial meningitis patients with a positive CSF Gram stain compared to those with a negative results (20 of 68 [29%] vs. 11 of 114 [10%], P<0.001). CONCLUSION: CSF Gram stain has excellent specificity but low sensitivity for the diagnosis of bacterial meningitis in patients with a suspected CNS infection. While positive results reliably confirm disease, the impact on clinical management is modest.