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CSF: composition and meningitis

What is in cerebrospinal fluid and how it differs from plasma, then the four-column table that separates bacterial, viral and tuberculous meningitis.

Production, circulation, absorption

Only 150 ml exists at any moment, but 500 ml is made each day — so the whole volume turns over three to four times daily.

150 ml

total volume

500 ml/day

production

5–15 mmHg

normal pressure

1.003–1.008

specific gravity

The pathway in words

Produced by ependymal cells of the choroid plexus → lateral ventricles → foramen of Monro → third ventricle → aqueduct of Sylvius → fourth ventricle → foramina of Luschka and Magendie → subarachnoid space → reabsorbed by arachnoid granulations into the superior sagittal sinus.
Luschka = Lateral (two of them). Magendie = Median (one, in the midline). That pairing is the whole memory problem solved.
Absorption is passive and pressure-dependent through one-way valves — flow only occurs when CSF pressure exceeds venous sinus pressure. Production, by contrast, is active and largely pressure-independent, which is why CSF keeps being made even when ICP is dangerously high.

Where obstruction causes hydrocephalus

Non-communicating (obstructive) — a blockage anywhere within the ventricular system. The aqueduct of Sylvius is the commonest site, being the narrowest point in the pathway. Also at the foramen of Monro or the fourth ventricular outlets.
Communicating — flow through the ventricles is intact but absorption at the arachnoid granulations fails. Classically after subarachnoid haemorrhage or meningitis, where blood or exudate blocks the villi.
Practical point: in obstructive hydrocephalus, lumbar puncture risks coning — CSF is removed below a blockage while pressure above it stays high. Image first.

Why specific gravity matters to you

CSF specific gravity 1.003–1.008 is the reference against which spinal solutions are called hyperbaric, isobaric or hypobaric.
Heavy bupivacaine (with 8% dextrose) sinks in CSF; plain bupivacaine is roughly isobaric at body temperature. This — with patient position — is the main determinant of block spread.

CSF versus plasma

CSF is an active secretion, not a filtrate — which is why it is not simply dilute plasma. The differences that get examined are protein, glucose, potassium and chloride.

	CSF	Plasma	Direction
Protein	0.15–0.45 g/L	60–80 g/L	↓↓↓ ~200× lower
Glucose	2.5–4.5 mmol/L	3.5–5.5 mmol/L	↓ ratio ≈ 0.6
Potassium	2.8–3.0 mmol/L	3.5–5.0 mmol/L	↓
Calcium	1.1–1.3 mmol/L	2.2–2.6 mmol/L	↓ total (ionised similar)
Chloride	115–125 mmol/L	95–105 mmol/L	↑
Magnesium	1.1–1.2 mmol/L	0.8–1.0 mmol/L	↑
Sodium	140–145 mmol/L	135–145 mmol/L	≈ same
Osmolality	~295 mosmol/kg	~295 mosmol/kg	iso-osmolar
pH	7.33	7.40	↓ slightly acidic
PCO₂	6.6 kPa (50 mmHg)	5.3 kPa (40 mmHg)	↑
Cells	0–5 lymphocytes/mm³	—	no polymorphs, no RBC

The four that carry the marks

Protein is the headline — roughly 200 times lower than plasma. The blood–brain barrier excludes it, so a raised CSF protein means barrier breakdown.
Glucose sits at about 60% of plasma — which is why a CSF glucose is meaningless without a paired plasma sample. Always take them together.
Potassium is lower and tightly regulated, staying near 2.9 mmol/L even when plasma potassium swings — active transport, not filtration.
Chloride is higher than plasma, balancing the low protein to preserve electroneutrality.

Why CSF pH matters for ventilation

CO₂ crosses the blood–brain barrier freely; H⁺ and HCO₃⁻ do not. So an acute rise in PaCO₂ acidifies CSF quickly and stimulates the central chemoreceptors on the ventral medulla.
Because CSF has almost no protein buffer, its pH shifts more for a given CO₂ change than blood does — making it a sensitive ventilatory sensor.
Over 24–48 hours HCO₃⁻ is transported into CSF to normalise pH — the basis of the blunted CO₂ drive in chronic CO₂ retention.

CSF in meningitis

The single most examined table in neuro-infection. Read it in this order: cell type → glucose → protein. Those three almost always give you the answer.

	Normal	Bacterial	Viral	Tuberculous
Appearance	Clear, colourless	Turbid, purulent	Clear	Clear or slightly cloudy; fibrin web on standing
Opening pressure	5–15 mmHg	↑↑	Normal or mildly ↑	↑↑
White cell count	0–5 /mm³	1000–5000+ /mm³	50–1000 /mm³	50–500 /mm³
Predominant cell	Lymphocytes	Neutrophils	Lymphocytes	Lymphocytes (neutrophils early)
Protein	0.15–0.45 g/L	↑↑ >1 g/L	Normal or mildly ↑ (<1)	↑↑↑ 1–5 g/L (highest)
Glucose (CSF:plasma)	>0.6	↓↓ <0.4	Normal	↓↓ <0.4
Lactate	<2 mmol/L	>3.5 mmol/L	Normal	↑
Microbiology	Sterile	Gram stain & culture; antigen testing	Viral PCR (enterovirus, HSV, VZV)	ZN stain (low yield), culture 6 wks, PCR/GeneXpert, ADA
Onset	—	Hours	Days	Weeks — insidious

The discriminators

Glucose separates viral from the rest. Viral meningitis characteristically leaves glucose NORMAL; bacterial, TB and fungal all consume it.
Cell type separates bacterial from TB/viral. Neutrophils point bacterial; lymphocytes point viral or TB — though TB and early bacterial meningitis can both show neutrophils.
Protein separates TB. A very high protein (1–5 g/L) with lymphocytes, low glucose and a subacute history is TB until proven otherwise. The fibrin web on standing is classic.
Partially treated bacterial meningitis is the great mimic — prior antibiotics shift it towards lymphocytes with a negative Gram stain, resembling viral or TB. Always ask what antibiotics were given.

Two patterns worth knowing alongside

Guillain–Barré syndrome: albuminocytological dissociation — high protein with a NORMAL cell count. Appears after about a week.
Subarachnoid haemorrhage: xanthochromia (bilirubin from lysed red cells), best detected by spectrophotometry at least 12 hours after onset. A traumatic tap clears across successive bottles; SAH does not.

Lumbar puncture — the safety points

Contraindications

Raised ICP with mass effect or focal signs — risk of coning. Image first if there are focal neurological signs, seizures, reduced or fluctuating consciousness, papilloedema or immunosuppression.
Coagulopathy or therapeutic anticoagulation, and platelets below roughly 50 ×10⁹/L.
Local infection at the puncture site, and cardiovascular instability.

Never delay antibiotics for the LP. In suspected bacterial meningitis, take blood cultures and give antibiotics immediately; CSF cell counts, protein, glucose and PCR remain informative for hours afterwards even as culture yield falls.
Adult cord ends at L1–L2, so use L3–4 or L4–5. Tuffier's line marks approximately L4 but is unreliable and often reads a space too high.
Layers traversed: skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → CSF.