Normal Cerebral Microdialysis Values
While the available guidelines do not provide specific numerical reference ranges for normal cerebral microdialysis values, the most rigorous recent study using zero-flow methodology establishes reference intervals for healthy brain tissue: glucose 1.15-4.13 mmol/L, lactate 1.30-5.31 mmol/L, pyruvate 54.4-197.0 µmol/L, and glycerol 23.6-227.3 µmol/L 1.
Key Reference Values from High-Quality Evidence
The 2017 study by Reinstrup et al. provides the most methodologically sound reference intervals using extrapolation to zero-flow methodology in 19 patients with normal white matter 1:
- Glucose: 1.57 mmol/L (median), range 1.15-4.13 mmol/L
- Lactate: 2.01 mmol/L (median), range 1.30-5.31 mmol/L
- Pyruvate: 80.0 µmol/L (median), range 54.4-197.0 µmol/L
- Glycerol: 49.9 µmol/L (median), range 23.6-227.3 µmol/L
Critical Context from Guidelines
The 2014 Neurocritical Care Society/European Society of Intensive Care Medicine consensus emphasizes that persistently low brain glucose and/or an elevated lactate/pyruvate ratio is a strong predictor of mortality and unfavorable outcome 2. However, these guidelines do not specify exact normal values, instead recommending that microdialysis be interpreted in combination with clinical indicators and other monitoring modalities.
Pathological Thresholds (For Context)
From SAH patient data comparing normal versus severely ischemic brain tissue 3:
Normal (non-ischemic) values:
- Glucose: 2.12 ± 0.15 mmol/L
- Pyruvate: 151 ± 11.5 µmol/L
- Lactate: 3.06 ± 0.32 mmol/L
- Glycerol: 81.6 ± 12.4 µmol/L
- Glutamate: 14 ± 3.33 µmol/L
Severe ischemia/brain death values:
- Glucose: 0 mmol/L
- Pyruvate: 0 µmol/L
- Lactate: 6.57 ± 1.07 mmol/L
- Glycerol: 639 ± 91 µmol/L
- Glutamate: 339 ± 53.4 µmol/L
Important Clinical Caveats
Methodological Considerations
Different thresholds apply based on patient state 1. The reference intervals are higher than traditionally used clinical thresholds, emphasizing the need to use different cutoffs for:
- Awake patients versus anesthetized patients
- Pathological brain tissue versus normal brain tissue
- Different probe locations (the technique provides focal measurements only) 2
Lactate/Pyruvate Ratio
While specific normal L/P ratio values are not provided in the guidelines, the consensus documents repeatedly emphasize that an elevated lactate/pyruvate ratio combined with low glucose predicts poor outcomes 2. Recent TBI data shows L/P ratio >25 is common (median prevalence 48.9%) and when consistently elevated with normal ICP, brain oxygenation, and perfusion, suggests mitochondrial dysfunction 4.
Glucose Management Context
The 2018 French guidelines on TBI management note that strict glycemic control (targeting serum glucose <6 mmol/L) was associated with decreased interstitial brain glucose and elevated lactate/pyruvate ratios suggesting cerebral energy crisis 5. This underscores that brain tissue glucose levels do not directly correlate with serum glucose and must be monitored independently.
Evidence Quality Limitations
A 2025 scoping review found substantial clinical and methodological heterogeneity across studies measuring microdialysis values in healthy brain tissue, with only 6 human studies (n=54 total patients) identified 6. The quality of several human studies was moderate at best, limiting definitive conclusions about absolute reference values.
The consensus recommendation is to focus on trends over time rather than absolute values when making clinical decisions 2, though the Reinstrup reference intervals 1 provide the best available benchmark for what constitutes normal brain tissue metabolism.