A 58-year-old man presents with acute headache, vomiting, decreased level of consciousness (Glasgow Coma Scale 10), stable vital signs, markedly elevated procalcitonin, white‑blood‑cell count 22,000/µL with 93% neutrophils, normal routine labs and a normal non‑contrast head computed tomography—what is the most likely diagnosis and should I perform a lumbar puncture now?

Diagnosis: Bacterial Meningitis

This patient requires immediate lumbar puncture and empiric antibiotic therapy for presumed bacterial meningitis. The combination of headache, vomiting, decreased sensorium (GCS 10/15), markedly elevated procalcitonin (13.34 ng/mL), and leukocytosis with left shift (WBC 22,000 with 93% neutrophils) in the setting of a normal CT scan strongly indicates bacterial CNS infection requiring urgent intervention. 1, 2

Immediate Management Algorithm

Step 1: Proceed with Lumbar Puncture NOW

• Lumbar puncture is mandatory and should be performed immediately. 1, 2
• The normal CT scan does NOT exclude meningitis and actually clears the patient for safe LP. 1
• GCS of 10 represents moderate impairment of consciousness, which is an indication for CT before LP (which you've already done), but once CT is normal, LP should proceed without delay. 1
• The guideline threshold for absolute contraindication varies (GCS <8 to <13), but with a normal CT scan, LP remains essential for diagnosis even at GCS 10. 1

Step 2: Obtain CSF Analysis

Look for the following parameters that confirm bacterial meningitis: 1, 2

• Opening pressure (typically elevated >180 mm H₂O)
• Cell count (expect hundreds to thousands of WBCs with neutrophil predominance)
• Protein (typically >150 mg/dL, often much higher)
• Glucose (typically <40 mg/dL or <50% of serum glucose)
• Gram stain and culture (positive in 60-90% of untreated bacterial meningitis)
• CSF procalcitonin if available (elevated in bacterial vs. viral)

Step 3: Initiate Empiric Antibiotics Immediately

• Do NOT delay antibiotics waiting for LP if there is any procedural delay. 2
• For a 58-year-old, empiric coverage should include:
  • Ceftriaxone 2g IV q12h (covers Streptococcus pneumoniae and Neisseria meningitidis)
  • Vancomycin 15-20 mg/kg IV q8-12h (covers resistant pneumococcus)
  • Ampicillin 2g IV q4h (covers Listeria monocytogenes, critical in patients >50 years)
  • Dexamethasone 10mg IV q6h (start before or with first antibiotic dose, improves outcomes in pneumococcal meningitis)

Differential Diagnosis

Primary Diagnosis: Bacterial Meningitis (Most Likely)

• Procalcitonin >10 ng/mL is highly specific for severe bacterial infection. 3
• In bacterial sepsis, procalcitonin ranges from 6-53 ng/mL at diagnosis, with values >10 ng/mL indicating severe invasive bacterial disease. 3
• The marked leukocytosis (22,000) with 93% neutrophils represents a profound left shift characteristic of bacterial infection. 2
• The triad of headache, altered mental status, and fever is present in <50% of bacterial meningitis cases, so absence of documented fever does NOT exclude the diagnosis. 1, 4

Alternative Considerations (Less Likely but Must Exclude)

Viral Encephalitis (HSV):

• Would typically show procalcitonin <1.5 ng/mL, as viral infections rarely elevate procalcitonin significantly. 3
• CSF would show lymphocytic pleocytosis rather than neutrophilic
• Requires HSV PCR on CSF and empiric acyclovir if LP delayed 1

Subarachnoid Hemorrhage:

• Excluded by normal CT scan performed acutely 1
• CT sensitivity is 98-100% within 12 hours of symptom onset 1, 5
• Would not explain the markedly elevated procalcitonin or leukocytosis 1

Brain Abscess:

• Would typically show focal findings on CT scan 1
• Normal CT makes this unlikely but not impossible (early abscess may not be visible)
• Would still require LP to exclude concurrent meningitis 1

Critical Interpretation of Laboratory Findings

Procalcitonin 13.34 ng/mL

• This level is diagnostic of severe bacterial infection requiring immediate antibiotics. 3
• Values >6 ng/mL indicate severe bacterial sepsis with high specificity. 3
• Procalcitonin decreases rapidly with appropriate antibiotic therapy, making it useful for monitoring response. 3

Important caveat: Procalcitonin can be falsely elevated in medullary thyroid cancer with liver metastases (secreting PCT from tumor cells), but this is rare and would not explain the acute presentation with altered mental status. 6

WBC 22,000 with 93% Neutrophils

• Profound neutrophilia indicates acute bacterial infection rather than viral (which typically shows lymphocyte predominance or normal WBC). 2, 3
• This degree of left shift strongly supports bacterial meningitis over viral encephalitis. 3

Common Pitfalls to Avoid

1. Delaying LP because of decreased consciousness: With a normal CT scan, LP is safe and essential even at GCS 10. 1

2. Waiting for fever to diagnose meningitis: Fever may be absent in up to 50% of bacterial meningitis cases, especially in elderly or immunocompromised patients. 1, 4

3. Assuming normal CT excludes CNS infection: CT is normal in the majority of bacterial meningitis cases; it only excludes mass effect and hemorrhage. 1, 2

4. Delaying antibiotics for LP: If LP cannot be performed within 30-60 minutes, start antibiotics immediately and perform LP afterward (cultures may still be positive for 24-48 hours after antibiotic initiation). 2

5. Misinterpreting procalcitonin: While procalcitonin >10 ng/mL is highly specific for bacterial infection, rare causes like medullary thyroid cancer can elevate it without infection—but this would not explain acute neurological deterioration. 6, 3

Disposition and Monitoring

• Admit to intensive care unit for close neurological monitoring and hemodynamic support. 2
• Repeat neurological examination every 1-2 hours initially. 2
• Monitor for complications: seizures, increased intracranial pressure, hydrocephalus, cerebral edema. 1
• Repeat imaging if clinical deterioration occurs despite appropriate antibiotics. 1
• Follow procalcitonin levels to assess treatment response (should decrease by 50% within 24-48 hours with appropriate therapy). 3