How is CSF glucose interpreted in CNS infections?

• A. Reduced in bacterial meningitis relative to serum glucose; preserved in most viral infections
• B. Elevated in bacterial meningitis
• C. Unchanged in bacterial meningitis but reduced in viral infections
• D. Not useful for differentiating infections

Answer: (A)

CSF glucose patterns help distinguish bacterial from viral CNS infections. In bacterial meningitis, bacteria and inflammatory cells use a lot of glucose, and inflammation can impair glucose transport from the blood into the CSF, so the CSF glucose level falls relative to serum glucose. In most viral infections, glucose consumption is less and the transport barrier remains more intact, so CSF glucose stays normal or near normal compared with serum. That combination—low CSF glucose compared with serum in bacterial meningitis and preserved CSF glucose in most viral infections—is why this option is best. Clinically, a CSF/serum glucose ratio below about 0.4–0.5 supports bacterial meningitis, while a normal ratio favors viral etiologies.