Investigations
1st investigations to order
blood gas (including lactate)
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. If there is evidence of shock or meningococcal disease, also check ionised calcium (Ca2+) on the blood gas.[28]
Result
metabolic acidosis; raised lactate; may show deranged calcium
blood glucose
Test
Identify and correct hypoglycaemia. Hyperglycaemia may also be present.
Result
hyper/hypoglycaemia
serum CRP, procalcitonin
Test
Measure C-reactive protein (CRP), and procalcitonin (if available). CRP and procalcitonin are usually elevated.[29][31][45] Markedly elevated levels may help to identify a bacterial infection from a viral infection.[29][31][45]
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][28] 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, 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][28] 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.[28] 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.[28] 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]
In children without petechial rash, the National Institute for Health and Care Excellence (NICE) in the UK recommends starting antibiotic treatment for bacterial meningitis if the CSF white blood cell count is abnormal:[2]
≥20 cells/microlitre in neonates (but continue to consider bacterial meningitis if <20 cells/microlitre and other symptoms and signs are present)
>5 cells/microlitre or >1 neutrophil/microlitre in older children and young people, regardless of other CSF variables.
If the cell count is lower, still consider bacterial meningitis if other symptoms and signs suggest the diagnosis, especially in neonates.
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]
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]
CSF glucose is low in bacterial meningitis, but the concentration is affected by the concomitant plasma glucose.[46] The CSF:plasma glucose ratio is therefore a more reliable marker.[47]
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.[48][49]
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
Order a CT scan to look for alternative intracranial pathology if the patient has reduced or fluctuating level of consciousness (Glasgow Coma Scale score <9 or drop of ≥3), or focal neurological signs; in children unable to give a verbal response (in practice, those under 2 years), use the Glasgow Coma Scale with modification for children.[2] Glasgow Coma Scale: modification for children Opens in new window [ Glasgow Coma Scale Opens in new window ]
Do not delay treatment to wait for a CT scan.[28]
Stabilise the patient clinically before CT scanning.[28]
Consult a senior emergency physician, anaesthetist, paediatrician, or intensivist if the child has any of the indications above for CT.[28]
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.[42][43]
Practical tip
Do not use CT to decide whether it is safe to perform a lumbar puncture; use your clinical assessment instead.[28] CT is unreliable for identifying raised intracranial pressure.[28] 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.[42][43]
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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