What is the most likely diagnosis for a patient presenting with altered mental status, headaches, and a history of fever, with lab results showing anemia, leukocytosis, hyperglycemia, turbid cerebrospinal fluid, elevated total protein, low glucose, and lymphocytic pleocytosis?

Tubercular Meningitis (Option B)

The most likely diagnosis is tubercular meningitis based on the subacute presentation (fever for one month, headaches for 5 days), lymphocytic predominance (73%), elevated CSF protein (3.6 g/L), and critically low CSF glucose (2.7 mmol/L) with a CSF/plasma glucose ratio of 0.37, which falls into the characteristic range for TB meningitis. 1

Diagnostic Reasoning Based on CSF Analysis

CSF Glucose: The Critical Discriminator

• The CSF/plasma glucose ratio is 0.37 (2.7 mmol/L CSF glucose ÷ 7.22 mmol/L serum glucose), which is highly suggestive of TB meningitis where ratios are typically <0.5 1
• The absolute CSF glucose of 2.7 mmol/L is below the TB meningitis threshold of <2.2 mmol/L (sensitivity 68%, specificity 96%) 1
• This ratio excludes viral meningoencephalitis, where the CSF/plasma glucose ratio remains >0.36 1
• The ratio is not low enough (<0.36) to suggest acute bacterial meningitis 1

Lymphocytic Predominance: Timing Matters

• The 73% lymphocyte predominance is characteristic of TB meningitis, though neutrophils may predominate early in the disease course 1
• Acute bacterial meningitis typically shows 80-95% neutrophil predominance, which is absent here 1
• The 23% neutrophils present are consistent with the subacute phase of TB meningitis transitioning from early neutrophilic response 1

Protein Elevation: Markedly High

• CSF protein of 3.6 g/L is markedly elevated (normal 0.22-0.33 g/L), which is typical for TB meningitis where protein is typically >1 g/L 1
• If CSF protein is <0.6 g/L, bacterial meningitis is unlikely, further supporting TB meningitis with this degree of elevation 1

Clinical Presentation Supports TB Meningitis

Subacute Timeline

• The one-month history of fever followed by 5 days of headaches represents the classic subacute presentation of TB meningitis 1, 2
• A clinical history of more than 5 days is independently predictive of TB meningitis with 93% sensitivity 1
• A subacute course of more than 3 weeks strongly favors TB meningitis over acute bacterial causes 1

Classic Meningeal Signs Present

• Nuchal rigidity with altered sensorium and headache represents the classic triad, though this triad is present in only 41-51% of bacterial meningitis cases 3
• The presence of these signs does not rule in or out any specific etiology, as sensitivity of neck stiffness is only 31% in adults 3

Why Other Options Are Less Likely

Option A (Septicemia): Excluded

• Septicemia would not explain the specific CSF findings with lymphocytic predominance and low CSF/plasma glucose ratio 4
• No evidence of septic shock or systemic bacterial infection beyond the CNS 4

Option C (Pyogenic/Bacterial Meningitis): Excluded

• Bacterial meningitis would show neutrophil predominance (80-95%), not the 73% lymphocytic predominance seen here 1, 4
• The CSF/plasma glucose ratio of 0.37 is too high for bacterial meningitis, which typically shows ratios <0.36 1
• The subacute one-month presentation is inconsistent with acute bacterial meningitis 1

Option D (Viral Meningoencephalitis): Excluded

• Viral meningitis presents with normal or only slightly low CSF glucose, with CSF/plasma glucose ratio remaining >0.36 1
• The ratio of 0.37 is at the borderline but combined with the markedly elevated protein (3.6 g/L) and subacute presentation makes viral etiology unlikely 1
• Viral meningitis would not typically cause such profound protein elevation 1

Critical Clinical Pitfalls to Avoid

• Do not rely on absolute CSF glucose values alone when serum glucose is abnormal—always calculate the CSF/plasma glucose ratio 1
• Do not wait for microbiological confirmation (acid-fast bacilli smear, culture, or PCR) before initiating treatment, as these tests are frequently negative even in confirmed TB meningitis 2, 5, 6
• The delay in starting antitubercular therapy significantly increases mortality and neurological morbidity 2, 7
• Treatment should be initiated based on clinical suspicion supported by CSF findings, not microbiological proof 2, 5, 8

Immediate Management Approach

• Initiate empiric four-drug antitubercular therapy immediately with isoniazid, rifampin, pyrazinamide, and either streptomycin or ethambutol 5, 8
• Add adjunctive corticosteroids, which have been shown to improve mortality in TB meningitis 5, 8
• Total treatment duration should be 12 months (four drugs for 2 months, then two drugs for 10 months) 8