What is pleocytosis and what does a neutrophil‑predominant cerebrospinal fluid pleocytosis indicate in a patient with a KPC‑producing Klebsiella pneumoniae brain abscess?

What is Pleocytosis?

Pleocytosis is an elevated white blood cell (WBC) count in the cerebrospinal fluid (CSF), defined as greater than 5 cells/µL in adults, with age-specific thresholds in infants (≥25 cells/mL for 0-28 days old, ≥10 cells/mL for 29-90 days old). 1, 2

Definition and Thresholds

• Pleocytosis represents an abnormal increase in CSF leukocytes, indicating inflammation or infection of the central nervous system 2
• The normal CSF contains fewer than 5 white blood cells per microliter in adults 2
• In neonates and young infants, higher thresholds apply due to physiologic differences: ≥25 cells/mL for those 0-28 days old and ≥10 cells/mL for those 29-90 days old 1

Types of Pleocytosis

Lymphocytic Pleocytosis

• Typically indicates viral CNS infections, autoimmune conditions, or chronic infections like tuberculosis 3
• Seen in approximately 76% of viral CNS infections 3
• Common causes include enterovirus (33% of lymphocytic cases) and herpes viruses (46% of lymphocytic cases) 3

Neutrophilic (Polymorphonuclear) Pleocytosis

• Suggests bacterial infection, early viral infection, or brain abscess 3, 4
• Found in 24.7% of confirmed viral CNS infections, particularly enterovirus (64% of neutrophilic cases) 3
• Strongly associated with bacterial meningitis when combined with other findings (CSF protein >100 mg/dL, CRP >5 mg/dL, altered mental status, nuchal rigidity) 2

Clinical Context for KPC-Producing Klebsiella pneumoniae Brain Abscess

In a patient with KPC-producing Klebsiella pneumoniae brain abscess, neutrophil-predominant CSF pleocytosis indicates active bacterial CNS infection requiring aggressive antimicrobial therapy. 5, 4

Key Clinical Features

• Klebsiella pneumoniae brain abscesses present with severe headache, fever, altered mental status, and focal neurological deficits 5
• CSF analysis typically shows neutrophilic pleocytosis with elevated protein (>100 mg/dL) and low glucose 4
• Next-generation sequencing or culture of CSF can identify the organism when traditional cultures may be negative 5

Diagnostic Implications

• Neutrophil predominance in CSF with brain abscess indicates ongoing suppurative infection requiring prolonged antibiotic therapy (typically 2 months or longer) 5, 4
• The combination of pleocytosis, elevated CSF protein, and systemic inflammatory markers (elevated CRP, WBC) strongly suggests bacterial etiology with sensitivity of 92.1% and specificity of 90.9% 2
• Marked pleocytosis (>50 cells/µL) should prompt consideration of bacterial, fungal, or neoplastic causes rather than simple viral infection 1

Common Etiologies of Pleocytosis

Infectious Causes (34% of all pleocytosis cases)

• CNS infections with identified pathogen (20%): bacterial meningitis, viral encephalitis, brain abscess 2
• CNS infections without identified pathogen (14%): presumed viral or culture-negative bacterial infections 2
• Median CSF cell count in infections with detected pathogen is 82 cells/µL, significantly higher than other causes 2

Non-Infectious Causes

• Autoimmune diseases (21%): multiple sclerosis, neurosarcoidosis, autoimmune encephalitis with median 11 cells/µL 2
• Neoplastic diseases (16%): leptomeningeal carcinomatosis, CNS lymphoma with median 19 cells/µL 2
• Post-seizure pleocytosis (30% of seizure patients): transient, often PMN-predominant (57%), mean 72 cells/mm³, resolves rapidly 6

Important Clinical Pitfalls

• Pleocytosis does not always equal bacterial meningitis: approximately 50% of enterovirus-positive infants have CSF pleocytosis without bacterial infection 1
• Normal CSF does not rule out serious infection: 10% of bacterial meningitis patients have <100 cells/mm³, and HSV encephalitis can initially show normal CSF in 5-10% of cases 7
• Traumatic lumbar puncture can falsely elevate WBC count: subtract 1 WBC for every 700 RBCs to correct, though this is a guide not a rule 7
• In HSV encephalitis, blood-stained CSF reflects hemorrhagic pathophysiology (50% of cases), not traumatic tap, especially if serial specimens remain bloody 1, 7
• Mild pleocytosis (10-50 cells/µL) in suspected GBS should prompt consideration of alternative diagnoses like infectious polyradiculitis, though it can occur in GBS 1