An inpatient diagnostic laboratory is correlating cerebrospinal fluid (CSF) analysis patte… — USMLE Step 3

An inpatient diagnostic laboratory is correlating cerebrospinal fluid (CSF) analysis patterns from multiple lumbar punctures with their corresponding clinical etiologies. Assume a concurrent normal serum glucose of $90\text{ mg/dL}$ for all patients. Match each distinct CSF laboratory profile to the most likely associated diagnosis.

Acute bacterial meningitisWBCs: $1,800/\mu L$ ( $92\%$ neutrophils); Glucose: $18\text{ mg/dL}$ ; Protein: $175\text{ mg/dL}$
Viral (aseptic) meningitisWBCs: $140/\mu L$ ( $85\%$ lymphocytes); Glucose: $68\text{ mg/dL}$ ; Protein: $55\text{ mg/dL}$
Cryptococcal meningitisWBCs: $80/\mu L$ ( $80\%$ lymphocytes); Glucose: $30\text{ mg/dL}$ ; Protein: $140\text{ mg/dL}$ ; Opening pressure: $290\text{ mm H}_2\text{O}$
Guillain-Barré syndromeWBCs: $2/\mu L$ ; Glucose: $70\text{ mg/dL}$ ; Protein: $210\text{ mg/dL}$

Answer

Acute bacterial meningitis matches the profile with neutrophilic pleocytosis and low glucose; Viral meningitis matches the profile with lymphocytic pleocytosis and normal glucose; Cryptococcal meningitis matches the profile with lymphocytic pleocytosis, low glucose, and high opening pressure; Guillain-Barré syndrome matches the profile with elevated protein but a normal cell count.

Each laboratory pattern uniquely identifies a specific pathophysiologic state. Bacterial infections consume glucose and rapidly recruit neutrophils. Viral infections elicit a lymphocytic response without affecting glucose transport. Fungal infections elicit a lymphocytic response while consuming glucose and often raising intracranial pressure. Immune-mediated demyelination in Guillain-Barré syndrome increases blood-brain barrier permeability to proteins without recruiting leukocytes into the CSF.

Step-by-Step Solution

Analyze the leukocyte count and differential.

Identifies the predominant inflammatory response: neutrophils suggest bacterial infection, lymphocytes suggest viral or fungal infection, and a normal count suggests a non-infectious etiology.

Different pathogens and pathophysiologic states trigger distinct cellular recruitment patterns across the blood-brain barrier.

Evaluate the CSF glucose concentration relative to serum glucose.

Differentiates lymphocytic processes: normal glucose strongly favors viral meningitis, whereas low glucose (hypoglycorrhachia) points to fungal (e.g., Cryptococcus) or tuberculous meningitis.

Bacteria, fungi, and marked meningeal inflammation alter glucose transport and consume glucose, whereas typical viral infections do not.

Assess protein concentration and opening pressure.

Identifies specific diagnostic hallmarks, such as albuminocytologic dissociation (high protein, no cells) in Guillain-Barré syndrome or severely elevated intracranial pressure in cryptococcal infection.

Provides the final confirming evidence for etiologies that might otherwise share overlapping cell count patterns.

Key Concept

Interpreting cerebrospinal fluid parameters (cell count, differential, glucose, and protein) to differentiate infectious and neuropathic etiologies.

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