Reading the CSF: bacterial, viral, fungal or bleeding?

Interpretation

The fluid around the brain is tested for cells, sugar and protein; the pattern shows whether infection or bleeding is likely and what kind.

Updated Nov 13, 2026

What it is

Cerebrospinal fluid (CSF) is made mainly by the choroid plexus, bathes the brain and spinal cord and is normally clear, almost free of cells, low in protein and with a glucose about two thirds of the blood value. A lumbar puncture gives five readings that are always interpreted together: the opening pressure, the appearance, the cell count with its differential, the protein and the glucose compared with a blood glucose drawn at the same time. Bacteria provoke a neutrophil response, break down the blood–brain barrier (protein rises) and consume glucose while blocking its transport (glucose falls). Viruses cause a milder lymphocytic reaction with normal glucose. Tuberculosis and fungi such as Cryptococcus give a lymphocytic fluid with high protein and low glucose and a slower course. Blood in the sample means either a traumatic tap or a subarachnoid haemorrhage, separated by watching the red cells across the tubes and looking for xanthochromia. Gram stain, culture, antigen tests and PCR then name the organism. The patterns overlap, early or partially treated infections can mislead, and in suspected bacterial meningitis antibiotics are never delayed for the puncture.

Reference values and concepts

Usual adult values (vary by laboratory): opening pressure about 10–20 cmH2O lying on the side; clear and colourless.
White cells 0–5 cells/µL, all mononuclear; no red cells in an atraumatic tap.
Protein about 15–45 mg/dL; glucose about 40–80 mg/dL, a CSF:serum ratio of about 0.6.
Draw blood glucose at the time of the puncture: a «normal» CSF glucose can be low for a hyperglycaemic patient.
Raised CSF lactate favours bacterial over viral meningitis, especially before antibiotics.
Xanthochromia (yellow supernatant from haemoglobin breakdown) takes several hours to appear after a haemorrhage.
Albuminocytological dissociation (high protein, normal cells) suggests Guillain–Barré syndrome; oligoclonal bands support multiple sclerosis.

Reading it step by step

Check the opening pressure first — very high pressure points to cryptococcus, bacterial meningitis or raised intracranial pressure.
Look at the fluid — turbid suggests many cells (bacterial); bloody needs the tube test; yellow suggests old blood or very high protein.
Read the white cell count and differential — neutrophils point to bacteria; lymphocytes to viruses, tuberculosis or fungi.
Compare CSF glucose with blood glucose — a ratio under 0.4 means consumption: bacteria, tuberculosis or fungi, not viruses.
Read the protein — very high protein with low glucose is bacterial or tuberculous; high protein with normal cells suggests Guillain–Barré.
If blood is present, compare red cells in tubes 1 and 4 and check for xanthochromia — this separates a traumatic tap from a bleed.
Correct white cells in a bloody tap — about one white cell is expected for every 500–1000 red cells from blood alone.
Read the Gram stain, culture, antigen and PCR results — they confirm the organism and guide treatment; a negative Gram stain does not exclude bacteria.
Fit the result to the patient — age, immune status, prior antibiotics and time since onset change the expected pattern.

Typical CSF patterns in adults (values overlap; ranges vary by laboratory).

Pattern	Opening pressure	White cells	Protein	Glucose (CSF:serum)	Other clues
Normal	About 10–20 cmH2O	0–5 cells/µL, mononuclear	15–45 mg/dL	About 0.6	Clear, colourless
Bacterial	Raised	Usually over 1000/µL, neutrophils	High, often over 100 mg/dL	Low, under 0.4	Turbid; Gram stain, culture, high lactate
Viral	Normal or slightly raised	Tens to a few hundred/µL, lymphocytes	Normal or mildly raised	Normal	Clear; PCR for enterovirus or herpes simplex
Tuberculous	Raised	Tens to hundreds/µL, lymphocytes	Very high	Low	Subacute; acid-fast stain, PCR, culture
Fungal (cryptococcal)	Often very high	Few to hundreds/µL, lymphocytes	High	Low	Cryptococcal antigen, India ink; may be bland in HIV
Subarachnoid haemorrhage	Often raised	In proportion to red cells	Raised	Normal	Red cells stay high in every tube; xanthochromia
Traumatic tap	Normal	In proportion to red cells	Slightly raised	Normal	Red cells fall from tube 1 to tube 4; no xanthochromia

Patterns and common mistakes

Early viral meningitis may show neutrophils in the first day; a repeat puncture often shows the shift to lymphocytes.
Antibiotics before the puncture make culture negative and may soften the pattern, but glucose often stays low and protein high.
Listeria may give a mixed or lymphocytic fluid; think of it in infants, older adults, pregnancy and immunosuppression.
Cryptococcal meningitis in advanced HIV can show few cells: ask for cryptococcal antigen and measure the opening pressure.
Reading CSF glucose alone without a paired blood glucose; diabetics and patients on glucose infusions mislead.
Calling a bloody tap traumatic without checking clearance across the tubes and xanthochromia, and missing a subarachnoid haemorrhage.

Next step

If bacterial meningitis is suspected, take blood cultures and start empirical antibiotics with corticosteroid at once; do not wait for the puncture or the scan.
Scan the head before the puncture when there are focal signs, papilloedema, seizures, reduced consciousness or immunosuppression.
Call a senior or intensive care for reduced consciousness, shock, purpura or raised intracranial pressure.
Ask for PCR (herpes simplex, enterovirus), cryptococcal antigen or tuberculosis tests according to the pattern; add aciclovir if encephalitis is possible.
Report meningococcal and other notifiable meningitis to public health (SIVIGILA in Colombia) so close contacts receive prophylaxis.
Recheck the patient and the results within 24–48 hours; repeat the puncture if the pattern was unclear or the patient does not improve.

Test yourself

Think of the answer, then tap the question to see it.

A 20-year-old with fever and neck stiffness has turbid CSF: 2500 cells/µL, 90% neutrophils, protein 250 mg/dL, glucose 20 mg/dL with blood glucose 100 mg/dL. Likely cause?

Bacterial meningitis: neutrophils, high protein and a CSF:serum glucose ratio of 0.2. Antibiotics and corticosteroid should already be running; Gram stain and culture name the organism.

CSF shows 150 cells/µL, mostly lymphocytes, protein 60 mg/dL and a normal glucose ratio in a well-looking young adult. Most likely pattern?

Viral (aseptic) meningitis, often enterovirus. Confirm with PCR; check herpes simplex PCR if there is confusion or focal signs, and review if antibiotics were given before the puncture.

How do you tell a traumatic tap from a subarachnoid haemorrhage?

In a traumatic tap the red cell count falls clearly from the first to the last tube and there is no xanthochromia. In a haemorrhage the count stays high and the supernatant turns yellow after several hours.

A patient with advanced HIV has headache, opening pressure 35 cmH2O, few lymphocytes and low glucose. Which test is key?

Cryptococcal antigen in CSF (and blood), with fungal culture. Cryptococcal meningitis often gives a bland fluid with a very high opening pressure, which itself needs treatment.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Bacterial Meningitis (StatPearls (NCBI Bookshelf), 2023); Viral Meningitis (StatPearls (NCBI Bookshelf), 2026); Meningitis (StatPearls (NCBI Bookshelf), 2026).

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