Diagnostic procedures to predict the prognosis of acute meningitis are of paramount importance in order to choose the appropriate level of further surveillance. The aim of this study was to evaluate the predictive power of IgG–index as CSF
biomarker for disease prognosis in patients with acute meningitis. This is a prospective study done on forty patients; group I: Twenty patients with the clinical diagnosis and CSF analysis of acute bacterial meningitis and group II: Twenty patients with the clinical diagnosis and CSF analysis of aseptic meningitis. All the patients were subjected to routine clinical and laboratory evaluation and complete CSF analysis. Intrathecal IgG synthesis was measured using radial immunodiffusion (RID) technique. Glasgow outcome scale (GOS) was done at discharge .The duration of hospital stay was recorded. The IgG-index was the only independent predictor for unfavorable outcome (GOS < 5) in patients' groups' especially aseptic group. The best cut off value of IgG index for early prediction of unfavorable outcome (GOS < 5) in bacterial meningitis group was ≥6.75 with AUC of 0.922 and 95% CI of 0.769-1.07 and sensitivity of 75% and specificity of 93.7%. While, in aseptic meningitis group was ≥7.9 with AUC of 1 and 95% CI of 1.00-1.00 and sensitivity of 100% and specificity of 100%.