Match each cerebrospinal fluid (CSF) laboratory profile to its most likely corresponding c… — USMLE Step 3

Match each cerebrospinal fluid (CSF) laboratory profile to its most likely corresponding clinical diagnosis. Assume a baseline normal serum glucose of $100 \text{ mg/dL}$ for all patients.

WBC: $3500/\mu\text{L}$ ( $92\%$ neutrophils), Glucose: $20 \text{ mg/dL}$ , Protein: $210 \text{ mg/dL}$ , Opening pressure: $280 \text{ mm H}_2\text{O}$ Acute bacterial meningitis
WBC: $180/\mu\text{L}$ ( $85\%$ lymphocytes), Glucose: $65 \text{ mg/dL}$ , Protein: $75 \text{ mg/dL}$ , Opening pressure: $160 \text{ mm H}_2\text{O}$ Viral (aseptic) meningitis
WBC: $400/\mu\text{L}$ ( $80\%$ lymphocytes), Glucose: $25 \text{ mg/dL}$ , Protein: $280 \text{ mg/dL}$ , Opening pressure: $300 \text{ mm H}_2\text{O}$ Tuberculous meningitis
WBC: $3/\mu\text{L}$ (normal), Glucose: $60 \text{ mg/dL}$ , Protein: $35 \text{ mg/dL}$ , CSF oligoclonal IgG bands positive (absent in serum)Multiple sclerosis

Answer

The correct matches align the neutrophil-predominant profile with low glucose to bacterial meningitis, the lymphocyte-predominant profile with normal glucose to viral meningitis, the lymphocyte-predominant profile with low glucose to tuberculous meningitis, and the normal profile with oligoclonal bands to multiple sclerosis.

Each diagnosis produces a distinct pathophysiological change in the CSF. Acute bacterial infections lead to rapid neutrophil recruitment and glucose consumption. Viral infections trigger a lymphocytic response without affecting glucose metabolism. Tuberculous infections cause chronic granulomatous inflammation, drawing lymphocytes while simultaneously consuming glucose and elevating protein. Autoimmune conditions like multiple sclerosis produce localized intrathecal IgG (oligoclonal bands) without significantly altering routine cellular or metabolic parameters.

Step-by-Step Solution

Analyze the white blood cell (WBC) count and differential.

Neutrophilic predominance strongly suggests a bacterial etiology, while a lymphocytic predominance suggests a viral, fungal, or mycobacterial etiology. Normal WBC counts indicate a non-infectious or distinct inflammatory process.

Different pathogens trigger specific immune cell recruitment pathways.

Evaluate the CSF glucose concentration.

Low glucose levels (

< 40 \text{ mg/dL}

< 40\%

of serum glucose) narrow the differential of a lymphocytic pleocytosis to tuberculous or fungal meningitis, ruling out most viral causes.

Bacteria and mycobacteria consume glucose in the subarachnoid space, whereas viruses do not.

Examine specialized tests (e.g., oligoclonal bands).

The presence of oligoclonal bands in the CSF, when absent in the serum, confirms localized central nervous system antibody production.

This finding is a hallmark of demyelinating diseases like multiple sclerosis and helps distinguish it from other pathologies.

Key Concept

Cerebrospinal fluid (CSF) analysis patterns are crucial for distinguishing between different infectious and inflammatory etiologies of central nervous system disease.

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