Investigation of CSF samples for bacterial and fungal infection
Meningitis is an infection of the meninges, which are a system of membranes that envelops the central nervous system, Bacterial meningitis, is often a life-threatening infection, whereas viral meningitis tends to be less severe. However, exceptions do exist. The species of infecting organisms tend to be characteristically associated with age or status of the patient.
Diagnosis of meningitis is best established by laboratory examination of the CSF. This is usually obtained by lumbar puncture, although ventricular, cisternal or fontanelle taps may also be used. Lumbar puncture may cause cerebral herniation, therefore in patients where there is a risk of increased intracranial pressure CT scanning is advised prior to the procedure. In some cases, the patient is too unstable or has a bleeding diathesis as a result of sepsis syndrome and cannot undergo immediate lumbar puncture. Blood cultures and pharyngeal swabs may be useful in addition to CSF examination in the diagnosis of meningococcal meningitis and serology may allow retrospective diagnosis on acute and convalescent sera. In patients for whom lumbar puncture is contraindicated, every effort must be made to establish a microbiological diagnosis by other means. This is desirable both for epidemiological purposes and for the appropriate management of contacts of cases.
The diagnosis of meningitis from the examination of CSF includes the following:
- Complete cell count
- Differential leucocyte count
- Examination of Gram-stained smear
- Determination of glucose and protein concentrations (performed by clinical biochemistry departments)
- PCR (on request only)
Intra-cerebral or sub-arachnoid haemorrhage or a traumatic spinal tap can result in the presence of RBC. Several sequential samples from one lumbar puncture are examined. Uniform blood staining of all samples suggests previous haemorrhage, whereas reducing counts in later obtained samples suggest bleeding induced by the tap procedure.
For CSF cell counts:
|Neonates||less than 28 days||0-30 cells x 106/L|
|Infants||1 to 12 months||0-15 cells x 106/L|
|Children/Adults||1 year +||0-5 cells x 106/L|
No RBCs should be present in normal CSF
|Neonates||less than 28 days||1.94-5.55 mmol/L|
|Infants||29 to 58 days|
2 to 12 months
|Children/Adults||1 year +||2.22-4.44 mmol/L|
|Neonates||less than 28 days||0.65-1.5 g/L|
|Infants||29 to 56 days||0.5-0.9 g/L|
|Children||2 months to 18 years||0.05-0.6 g/L|
|Adults||18 to 60|
CSF should be collected sequentially into 3 or more separate containers together with a fluoride sample for glucose estimation. Each container must be numbered indicating the order of collection.
The first and the third specimens are used for microbiological examination and the second specimen for biochemical analysis. Aseptic technique should be employed.
- White top sterile universal
- Specimens 1 and 3 should be sent for Microbiology investigation
Limitations and restrictions
Samples sent to the laboratory 2 or more hours following collection risk disintegration of cells within the sample and therefore results will not be accurate.
Samples containing blood clots will not have a cell count performed or reported. Clotted samples will only be investigated with a Gram’s stain and culture.
- Microscopy: 2 hours
- Culture: 49 hours
Microbiology Lab, James Cook University Hospital, Marton Road, TS4 3BW
The microbiology laboratory must be contacted by telephone to inform them of the subsequent arrival of a CSF sample. The requested should provide the laboratory with:
- Patient name
- Hospital number
- Date of birth
- Tests required
- Requestors name and role
- Contact number for the microscopy result to be telephoned back to
CSF samples are not routinely testing for fungal pathogens. If fungal investigation is required, please contact the Consultant Microbiologist to discuss this prior to sending samples to the lab.
Additional tests may be requested for Mycobacteria sp. and bacterial PCR. These tests should be requested on a separate request form and a separate sample provided where possible.
Routinely only sample 3 will have a cell count performed. If sub-arachnoid haemorrhage (SAH) is suspected, this must be stated on the form to allow for both samples 1 and 3 to have a cell count performed to aid diagnosis.