A healthy person with a blood glucose of 80 mg/dL would have a simultaneously determined cerebrospinal fluid glucose value of which?

• A. 25 mg/dL (1.4 mmol/L)
• B. 50 mg/dL (2.3 mmol/L)
• C. 100 mg/dL (5.5 mmol/L)
• D. 150 mg/dL (8.3 mmol/L)

Answer: (B)

Glucose enters the CSF from blood mainly by facilitated diffusion, so CSF glucose tracks plasma glucose but sits at a lower level. In healthy individuals, CSF glucose is typically about two-thirds of the plasma glucose. With a plasma glucose of 80 mg/dL, the CSF glucose would be around 50 mg/dL (80 × ~0.66). That makes 50 mg/dL the closest, given normal physiology. Values as low as 25 mg/dL would suggest much lower diffusion or pathology, while 100 or 150 mg/dL would exceed what plasma glucose provides in a healthy state.