A CSF/serum glucose ratio of <0.4 is highly suggestive of:
Now, when does CSF glucose decrease? The main reason I can think of is bacterial meningitis. Bacteria in the CSF consume glucose more rapidly than normal, leading to lower levels. In contrast, viral meningitis usually doesn't cause such a drastic drop because the glucose is still around normal. So a ratio <0.4 is a red flag for bacterial infection. Wait, are there other conditions that could cause low CSF glucose? Maybe something like a fungal infection or even a tumor? But I think bacterial meningitis is the most common and classic cause tested here. The other options might include viral, tuberculous, or maybe something else. Let me think. If the options were A. Viral meningitis, B. Bacterial meningitis, C. Subarachnoid hemorrhage, D. Multiple sclerosis, then B would be correct. But the options are not given here, so I need to make sure my explanation covers the correct answer even without the options. Since the correct answer is likely bacterial meningitis, I should explain why that's the case. So, the core concept is that a low CSF/serum glucose ratio is a hallmark of bacterial meningitis. Then, explaining why other options are wrong would involve pointing out that viral meningitis doesn't lower glucose much, and other conditions like tumors or TB might have different ratios. The clinical pearl is to remember the ratio as a key diagnostic clue for bacterial vs. viral.
**Core Concept**
The cerebrospinal fluid (CSF)/serum glucose ratio reflects glucose metabolism in the central nervous system. A ratio 0.6.
**Option B:** Subarachnoid hemorrhage may elevate CSF protein but spares glucose levels.
**Option C:** Neurosyphilis (e.g., tuberculous meningitis) can lower glucose, but the ratio is usually >0.4, and other CSF findings (e.g., lymphocytic pleocytosis) dominate.
**Clinical Pearl / High-Yield Fact**
Memorize the "classic