Biochemistry composition of csf
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cerebrospinal fluid normal composition glucose protein electrolytes reference values
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Biochemical composition of normal cerebrospinal fluid (CSF)
CSF is a clear, colorless fluid formed mainly by the choroid plexus. It is isotonic with plasma, but its composition differs because of the blood-CSF barrier and active transport.
| Constituent | Normal CSF value / relation to plasma |
|---|---|
| Osmolality | Approximately equal to plasma, about 295 mOsm/L |
| Sodium (Na⁺) | Approximately equal to plasma, about 138 mEq/L |
| Chloride (Cl⁻) | Higher than plasma, about 119 mEq/L or roughly 15% higher |
| Potassium (K⁺) | Lower than plasma, about 2.8 mEq/L |
| Calcium (Ca²⁺) | Lower than plasma, about 2.1 mEq/L |
| Magnesium (Mg²⁺) | Slightly higher than plasma, about 2.3 mEq/L |
| Bicarbonate (HCO₃⁻) | Approximately equal or slightly lower, about 23 mEq/L |
| Glucose | 50-80 mg/dL, usually about 60%-70% of simultaneous blood glucose |
| Protein | Very low: usually 15-45 mg/dL in lumbar CSF; predominantly albumin, with small amounts of immunoglobulins and transthyretin |
| pH | Slightly more acidic than arterial blood: about 7.31-7.33 |
| PCO₂ | Higher than arterial blood, about 48 mm Hg |
| Lactate | About 1-3 mmol/L |
| Urea, creatinine, uric acid, amino acids | Lower than plasma |
| Cholesterol and lipoproteins | Negligible or absent |
| Cells | Normally acellular or only 0-5 lymphocytes/µL; no red cells |
Key comparison with plasma
- Same or nearly same: Na⁺, HCO₃⁻, osmolality
- Higher in CSF: Cl⁻, Mg²⁺
- Lower in CSF: K⁺, Ca²⁺, glucose, protein, amino acids, cholesterol, pH
- Minimal/absent: large proteins, lipoproteins, fibrinogen, cholesterol
The low protein and cholesterol content reflect the selective blood-CSF barrier. Active transport by choroid plexus epithelial cells secretes Na⁺, Cl⁻, bicarbonate, and water, while K⁺ is transported out of CSF. Costanzo Physiology, 7th ed., p. 120. Guyton reports that CSF chloride is about 15% above plasma, whereas potassium and glucose are about 40% and 30% below plasma, respectively. Guyton and Hall Textbook of Medical Physiology, p. 771.
For routine interpretation, normal CSF glucose and protein are commonly cited as 50-80 mg/dL and 15-45 mg/dL, respectively, in StatPearls' CSF overview.
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