Investigations
1st investigations to order
blood gas (including lactate and ionised calcium)
Test
Order in any child who is considered to have a serious illness, such as bacterial meningitis. Blood gas is especially important if there are features of shock, or if there is a non-blanching rash suggestive of meningococcal disease.[2] Metabolic acidosis and raised lactate may indicate shock.
Result
metabolic acidosis; raised lactate; may show deranged calcium
blood glucose
Test
Identify and correct hypoglycaemia. Hyperglycaemia may also be present.
Result
hyper/hypoglycaemia
FBC
Test
Order in all patients.[2]
Result
leukocytosis
serum CRP, procalcitonin
Test
Measure C-reactive protein (CRP), and/or procalcitonin (if available). CRP and procalcitonin are usually elevated.[28][30][68] Markedly elevated CRP levels or a raised procalcitonin may help to identify a bacterial infection from a viral infection.[28][30][68]
However, do not use a normal CRP to rule out bacterial meningitis, particularly early in the course of the illness.[2] In practice, also do not exclude bacterial meningitis based on normal procalcitonin alone.
Result
usually elevated
coagulation screen
Test
Request a coagulation screen for all patients.[2] The coagulation screen may be deranged due to septicaemia.
The following coagulation abnormalities are relative contraindications to lumbar puncture:
Coagulation results (if obtained) outside the normal range
Platelet count <100 x 109/L
Receiving anticoagulant therapy.
Result
may be deranged
blood cultures
Test
Always take blood cultures as soon as possible (within 1 hour of arrival at hospital), and ideally before starting antibiotics if this will not delay treatment.
Result
positive
serum PCR for Neisseria meningitidis
Test
Always order whole-blood (EDTA) polymerase chain reaction (PCR) for N meningitidis.[2][4] See Meningococcal disease.
Result
molecular confirmation of specific pathogen
urea, electrolytes, and creatinine
Test
Check for:
Electrolyte disturbance, which may be present for various reasons, including shock and syndrome of inappropriate antidiuretic hormone secretion. If there is evidence of shock or meningococcal disease, also check ionised calcium (Ca2+) on a blood gas sample, and order serum magnesium (Mg2+), and phosphate (PO4-) on laboratory samples. These electrolytes may need replacement.
Renal impairment, which may be due to dehydration or shock.
Result
may show deranged electrolytes; elevated creatinine; elevated eGFR; reduced creatinine clearance
LFTs
Test
Order in all patients.[4]
Result
may be elevated
cross-match
Test
Request in all seriously unwell children, especially if there is evidence of shock or meningococcal disease. Blood products may be required.
Result
determination of blood group
CSF white blood cell count and examination
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[2] For more on lumbar puncture, see Diagnosis recommendations.
Request cerebrospinal fluid (CSF) results promptly.
CSF white blood cell counts, total protein, and glucose concentrations should be available within 4 hours to inform decision-making on adjunctive corticosteroid therapy.[2]
Interpret cerebrospinal fluid results using standard age-appropriate threshold values.
Normal thresholds for white cell count and protein may be higher in babies under 3 months.
Remember earlier antibiotics or immunodeficiency may reduce the diagnostic reliability of these investigations.
Red cells in the sample may suggest blood contamination or a different diagnosis.
In practice, the first dose of antibiotics should not be delayed by lumbar puncture or CSF results if there is clinical concern.
Result
leukocytosis
CSF total protein concentration
Test
Perform a lumbar puncture if you suspect bacterial meningitis, unless the procedure is contraindicated.[2] For more on lumbar puncture, see Diagnosis recommendations.
Request cerebrospinal fluid (CSF) results promptly.
CSF white blood cell counts, total protein, and glucose concentrations should be available within 4 hours to inform decision-making on adjunctive corticosteroid therapy.[2]
Interpret cerebrospinal fluid results using standard age-appropriate threshold values.
Normal thresholds for white cell count and protein may be higher in babies under 3 months.
Remember earlier antibiotics or immunodeficiency may reduce the diagnostic reliability of these investigations.
Red cells in the sample may suggest blood contamination or a different diagnosis.
In practice, the first dose of antibiotics should not be delayed by lumbar puncture or CSF results if there is clinical concern.
Result
typically elevated
CSF glucose concentration
Test
Perform a lumbar puncture if you suspect bacterial meningitis, unless the procedure is contraindicated.[2] For more on lumbar puncture, see Diagnosis recommendations.
Order a corresponding laboratory-determined blood glucose concentration.[2]
Request cerebrospinal fluid (CSF) results promptly.
CSF white blood cell counts, total protein, and glucose concentrations should be available within 4 hours to inform decision-making on adjunctive corticosteroid therapy.[2]
Interpret cerebrospinal fluid results using standard age-appropriate threshold values.
Normal thresholds for white cell count and protein may be higher in babies under 3 months.
Remember earlier antibiotics or immunodeficiency may reduce the diagnostic reliability of these investigations.
Red cells in the sample may suggest blood contamination or a different diagnosis.
CSF glucose is low in bacterial meningitis, but the concentration is affected by the concomitant plasma glucose.[69] The CSF:plasma glucose ratio is therefore a more reliable marker.[70]
In practice, the first dose of antibiotics should not be delayed by lumbar puncture or CSF results if there is clinical concern.
Result
CSF:blood glucose ratio low (<0.6)
CSF microscopy, Gram stain, culture, and sensitivities
Test
Perform a lumbar puncture if you suspect bacterial meningitis, unless the procedure is contraindicated.[2] For more on lumbar puncture, see Diagnosis recommendations.
Use microbiological culture and sensitivities to detect the causative organism.
Cerebrospinal fluid (CSF) culture is positive in 50% to 90% of patients with non-meningococcal bacterial meningitis, depending on the underlying causative organism; this is reduced by 10% to 20% if antibiotics have already been given prior to obtaining the CSF culture.[71][72]
Result
organisms seen on microscopy and grown in cultures evident on culture medium
CSF PCR for Neisseria meningitidis and Streptococcus pneumoniae
Test
Perform a lumbar puncture if you suspect bacterial meningitis, unless the procedure is contraindicated.[2] For more on lumbar puncture, see Diagnosis recommendations.
Submit cerebrospinal fluid (CSF) obtained during lumbar puncture to the laboratory to hold for polymerase chain reaction (PCR) testing for N meningitidis and S pneumoniae. Request PCR testing only if the CSF culture is negative.[2]
Result
N meningitidis or S pneumoniae DNA
Investigations to consider
cranial CT
Test
Do not routinely perform neuroimaging before lumbar puncture.
Only perform cranial imaging before lumbar puncture if there are:
Risk factors for an evolving space-occupying lesion[3][51][52] Children's Brain Tumour Research Centre. the brain pathways guideline Opens in new window
Any of the following clinical features, which might indicate raised intracranial pressure
New focal neurological features (including seizures or posturing)[2]
Abnormal pupillary reactions or papilloedema[50]
A Glasgow Coma Scale (GCS) score of 9 or less, or a progressive and sustained or rapid fall in level of consciousness.[2] Glasgow Coma Scale: modification for children Opens in new window [ Glasgow Coma Scale Opens in new window ]
Do not perform a lumbar puncture until these factors have been addressed.[2]
Neuroimaging is not routinely indicated in the initial investigation of a child with suspected meningitis, unless you suspect alternative pathology (e.g., intracranial abscess, space occupying lesion, neurovascular event, head trauma).[50] Consider neuroimaging before lumbar puncture in children with a CSF shunt in place or a history of hydrocephalus.
Consider CT or magnetic resonance imaging (MRI) in patients with a history of trauma, recent neurosurgery, rhinorrhoea, or otorrhoea to identify any source of CSF leak and source of contiguous spread of infection to the meninges.[53][54]
Practical tip
Do not use CT to decide whether it is safe to perform a lumbar puncture; use your clinical assessment instead. CT scan will not pick up raised intracranial pressure per se, but it may identify conditions associated with raised ICP such as a tumour or brain abscess, or meningitis-associated complications such as brain infarction, generalised cerebral oedema, or hydrocephalus.[3][55] However, if a CT scan has been performed and shows radiological evidence of raised intracranial pressure, do not proceed with a lumbar puncture.
Result
alternative intracranial pathology
MRI
Test
Consider CT or MRI in patients with a history of trauma, recent neurosurgery, rhinorrhoea, or otorrhoea to identify any source of cerebrospinal fluid leak and source of contiguous spread of infection to the meninges.[53][54]
Result
may identify any source of cerebrospinal fluid leak and source of contiguous spread of infection to the meninges
immune testing
Test
In practice, consider assessment of immune function for children who have more than one episode of bacterial meningitis, or who develop bacterial meningitis with a bacterial serotype that is covered by a vaccination schedule for which they have completed a primary course. This is particularly important if there are additional concerning features in the history or physical examination (e.g., recurrent infections, poor growth).
Always discuss immune testing with a specialist in infectious disease or immunology.
For information on immune testing for children with meningococcal disease, see Meningococcal disease.
Result
may be positive
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