From the Guidelines
In bacterial encephalitis, CSF glucose levels are typically low, often less than 40 mg/dL or less than 50% of the simultaneous blood glucose level, as supported by the most recent and highest quality study 1. This hypoglycorrhachia occurs because the bacteria consume glucose and interfere with glucose transport across the blood-brain barrier. Normal CSF glucose ranges from 50-80 mg/dL or about two-thirds of blood glucose. When evaluating a patient with suspected bacterial encephalitis, CSF should be collected before antibiotics are administered if possible, though treatment should not be delayed if the patient is critically ill, as emphasized in the ESCMID guideline 1. The CSF findings in bacterial encephalitis typically include:
- Elevated protein (>100 mg/dL)
- Pleocytosis with neutrophil predominance (100-10,000 cells/mm³)
- Decreased glucose levels mentioned above These findings help differentiate bacterial from viral or fungal causes of encephalitis, which generally have normal or only slightly decreased CSF glucose. The glucose ratio (CSF:serum) is more reliable than the absolute CSF glucose value, as it accounts for variations in blood glucose levels, and a ratio below 0.4-0.5 strongly suggests bacterial infection in the appropriate clinical context, as reported in the study 1. It is essential to note that the diagnosis of bacterial meningitis is primarily based on CSF analysis, and the ESCMID guideline recommends striving for treatment of all patients with suspected bacterial meningitis within one hour, irrespective of cranial imaging before the lumbar puncture 1. In addition, the study 1 highlights the importance of CSF lactate concentration as a diagnostic test, with a sensitivity of 98% in patients who did not receive antibiotic pretreatment. However, the glucose ratio remains a crucial diagnostic tool in differentiating bacterial from other causes of encephalitis.
From the Research
Bacterial Encephalitis CSF Glucose Results
- The normal CSF glucose level is approximately two-thirds of the blood glucose level 2.
- A decreased glucose level in CSF can predict the presence of bacterial meningitis, which can be related to bacterial encephalitis 2.
- The CSF/blood glucose ratio is a more precise indicator for bacterial meningitis, with an optimal cut-off of 0.36, sensitivity of 92.9%, specificity of 92.9%, and area under the curve of 0.97 2.
- Bacterial meningitis has characteristic effects on CSF white blood cell counts, CSF protein levels, and the CSF:serum glucose ratio, which can be useful in the evaluation of bacterial encephalitis 3.
Diagnostic Criteria
- A CSF-blood glucose ratio of 0.4 or less is a strong indicator of bacterial meningitis, which can be related to bacterial encephalitis 4.
- A CSF white blood cell count of 500/muL or higher and a CSF lactate level of 31.53 mg/dL or more can also accurately diagnose bacterial meningitis 4.
- These diagnostic criteria can be useful in the evaluation of bacterial encephalitis, but it is essential to consider the clinical context and other diagnostic tests, such as imaging and electroencephalography (EEG) 5.
Lumbar Puncture and CSF Analysis
- Lumbar puncture is a crucial diagnostic tool for suspected central nervous system infections, including bacterial encephalitis 5, 6.
- CSF analysis can provide valuable information about the presence of infection, inflammation, or other conditions affecting the central nervous system 3, 6.
- The introduction of a simple lumbar puncture pack can improve the diagnosis of central nervous system infections, including bacterial encephalitis, by reducing errors in CSF sample collection and analysis 6.