In a 19-year-old man with cerebrospinal fluid showing 290 white blood cells per mm³ with 81% lymphocytes, normal glucose (55 mg/dL) and mildly elevated protein (100 mg/dL), which organism is the most likely cause of meningitis?

Coxsackievirus (Enterovirus) is the Most Likely Causative Agent

In this 19-year-old man with lymphocytic-predominant CSF pleocytosis (81% lymphocytes), normal glucose (55 mg/dL), and mildly elevated protein (100 mg/dL), coxsackievirus—an enterovirus—is the most likely diagnosis, representing the most common cause of viral (aseptic) meningitis in young adults. 1, 2

Key Diagnostic Features Supporting Viral Meningitis

The CSF profile is classic for viral meningitis:

• Lymphocytic predominance (81%) with moderate pleocytosis (290 WBC/mm³) is characteristic of viral CNS infections, which typically produce tens to hundreds of lymphocytes 3, 4
• Normal CSF glucose (55 mg/dL) effectively excludes bacterial, tuberculous, and fungal meningitis, which typically present with CSF:plasma glucose ratio <0.5 5, 4
• Mildly elevated protein (100 mg/dL) aligns with viral meningitis, whereas bacterial and tuberculous meningitis usually show protein >100-200 mg/dL 5, 4

Why Coxsackievirus (Enterovirus) is Most Likely

• Enteroviruses (coxsackievirus and echovirus) account for the bulk of aseptic meningitis cases reported annually in the United States 1
• The patient's age (19 years) and the CSF profile are typical for enteroviral infection 1, 2
• Enteroviruses are detected in approximately 90% of viral CNS infections when PCR testing is performed 4

Why Other Options Are Less Likely

Cryptococcus neoformans

• Would typically show low CSF glucose (<40 mg/dL) and **much higher protein** (>100 mg/dL) 4
• Primarily affects immunocompromised patients (HIV, transplant recipients), not healthy 19-year-olds 4

Haemophilus influenzae & Streptococcus pneumoniae

• Bacterial meningitis typically presents with neutrophilic predominance (>50% neutrophils), not 81% lymphocytes 5, 6
• While CSF lymphocytosis can occur in bacterial meningitis, it accounts for only 32% of cases with WBC ≤1,000/mm³ and is significantly less common at this cell count (290/mm³) 6
• Bacterial meningitis usually shows CSF:plasma glucose ratio <0.36** and **protein >0.6 g/L 5
• The 18% neutrophils present is insufficient to suggest bacterial etiology 5

Mycobacterium tuberculosis

• TB meningitis presents with subacute course (>3 weeks), low CSF glucose (<0.5 × serum)**, and **very high protein (>100-200 mg/dL) 4
• The normal glucose (55 mg/dL) essentially excludes this diagnosis 4

Critical Clinical Caveat

• Early viral meningitis (especially enteroviral) can show neutrophil predominance initially, but total CSF WBC is unlikely to exceed 2,000 cells/mm³ 5
• If bacterial meningitis cannot be definitively excluded clinically, empiric antibiotics (ceftriaxone + vancomycin + ampicillin if >50 years or immunocompromised) should be started immediately while awaiting PCR results 4
• Repeat lumbar puncture at 12-24 hours would show a shift from neutrophils to lymphocytes if viral etiology, confirming the diagnosis 1

Recommended Diagnostic Workup

• CSF PCR for enteroviruses (coxsackievirus/echovirus), HSV-1/2, and VZV has >95% sensitivity and should be sent immediately 5, 4
• CSF bacterial culture and Gram stain to definitively exclude bacterial causes 4
• In this case with classic viral CSF profile in an immunocompetent young adult, antibiotics can likely be withheld if clinical presentation is consistent with viral meningitis and close observation is maintained 1