In the context of glucose testing, which statement is true about CSF glucose compared with simultaneous blood glucose?

• A. CSF glucose equals blood glucose
• B. CSF glucose is higher than blood glucose
• C. CSF glucose is lower than blood glucose
• D. CSF glucose is unrelated to blood glucose

Answer: (C)

Glucose in CSF is lower than in blood because it has to diffuse across the blood–brain barrier and is continually consumed by brain tissue. The transport into CSF is limited, so CSF glucose runs below plasma glucose; in healthy individuals it’s typically about 60% of plasma glucose. For example, if plasma glucose is around 90 mg/dL, CSF glucose is usually in the roughly 50–60 mg/dL range. This makes the true statement that CSF glucose is lower than blood glucose. If CSF glucose were equal to or higher than plasma, that would not fit normal physiology.