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CSF interpretation

  • See also

     Meningitis 
     Lumbar Puncture

    This guideline is to help with the interpretation of CSF results for the purpose of diagnosing or excluding meningitis. The use of CSF for other purposes (including the diagnosis of specific neurological conditions, subarachnoid haemorrhage or malignancy) is outside the scope of this guideline.

    Common clinical questions

    Normal values

     

    White cell count

    Biochemistry

    Neutrophils

    (x 106 /L)

    Lymphocytes

    (x 106/L)

    Protein

    (g/L)

    Glucose

    (CSF:blood ratio)

    Normal

    (>1 month of age)

    0

      ≤ 5

    < 0.4

    ≥ 0.6 (or ≥ 2.5 mmol/L)

    Normal neonate

    ( <1 month of age)

    0

      < 20

    <1.0

    ≥ 0.6 (or ≥ 2.5 mmol/L)

    The presence of any neutrophils in the CSF is unusual in normal children and should raise concern about bacterial meningitis

    Meningitis can occur in children with normal CSF microscopy.

    If it is clinically indicated, children who have a 'normal' CSF should still be treated with IV antibiotics pending cultures.

    CSF white cell count and protein level are higher at birth than in later infancy and fall fairly rapidly in the first 2 weeks of life. In the first week, 90% of normal neonates have a white cell count less than 18, and a protein level < 1.0 g/L.

    Interpretation of abnormal results

     

    White cell count

    Biochemistry

    Neutrophils

    (x 106 /L)

    Lymphocytes

    (x 106/L)

    Protein

    (g/L)

    Glucose

    (CSF:blood ratio)

    Normal

    (>1 month of age)

    0

    ≤ 5

    < 0.4

    ≥ 0.6 (or ≥ 2.5 mmol/L)

    Normal term

    neonate

    0*

      < 20

    < 1.0

    ≥ 0.6 (or ≥ 2.5 mmol/L)

    Bacterial meningitis

    100-10,000

    (but may be normal)

    Usually < 100

    > 1.0

    (but may be normal)

    < 0.4

    (but may be normal)

    Viral meningitis

    Usually <100

    10-1000

    (but may be normal)

    0.4-1

    (but may be normal)

    Usually normal

    TB meningitis

    Usually <100

    50-1000

    (but may be normal)

    1-5

    (but may be normal)

    < 0.3

    (but may be normal)

    * Some studies have found up to 5% of white cells in neonates without meningitis comprise neutrophils

    • Gram stain may be negative in up to 60% of cases of bacterial meningitis even without prior antibiotics.
    • Neither a normal Gram stain, nor a lymphocytosis excludes bacterial meningitis.
    • Neutrophils may predominate in viral meningitis even after the first 24 hours.
    • CSF findings in bacterial meningitis may mimic those found in viral meningitis (particularly early on). It may be possible with modest accuracy to judge whether bacterial or viral is more likely based on CSF parameters. However if the CSF is abnormal the safest course is to treat as if it is bacterial meningitis.

    Other factors affecting results

    Antibiotics prior to lumbar puncture

    • Prior antibiotics usually prevent the culture of bacteria from the CSF.
    • Antibiotics are unlikely to significantly affect the CSF cell count or biochemistry in samples taken <24 hours after antibiotics.

    Seizures

    • Recent studies do not support the earlier belief that seizures can increase cell counts in the absence of meningitis.
    • It is safest to assume that seizures do not cause an increased CSF cell count.

    Traumatic tap

    • Some guidelines suggest that in traumatic taps you can allow 1 white blood cell for every 500 to 700 red blood cells and 0.01g/L protein for every 1000 red cells. However rules based on a 'predicted' white cell count in the CSF are not reliable.
    • In order not to miss any patients with meningitis, guidelines relating to decisions about who not to treat for possible meningitis need to be conservative. The safest interpretation of a traumatic tap is to count the total number of white cells, and disregard the red cell count. If there are more white cells than the normal range for age, then the safest option is to treat.

    Additional tests

    PCR

    • PCR is routinely available for Neisseria meningitidis, Herpes Simplex and Enterovirus.
    • As results are not immediately available, they will only help with decisions concerning discontinuing treatment.
    • Enterovirus PCR should be requested on CSF from patients with clinical and/or CSF features of viral meningitis.
    • HSV PCR should be requested for patients with clinical features of encephalitis.
    • Meningococcal PCR is particularly useful in patients with a clinical picture consistent with meningococcal meningitis, but who have received prior antibiotics.

    Bacterial antigens

    • CSF bacterial antigen tests have low sensitivity and specificity.
    • They should therefore never influence treatment decisions and have little role if any in current management.