Investigations
1st investigations to order
blood gases (including lactate)
Test
Order a venous blood gas (bicarbonate, base deficit, ionised calcium [Ca2+]) in any child or young person with an unexplained petechial rash and fever (or history of fever), particularly if there are features of shock.[9][48][50] Blood gases are also a key investigation in adults.
Patients with severe meningococcal infections often have metabolic abnormalities.
Result
metabolic acidosis, raised lactate, may show deranged calcium
full blood count
procalcitonin (or CRP)
Test
Measure serum PCT if available.[48][49] Procalcitonin may be helpful to differentiate between bacterial and viral infections.
Raised CRP indicates an increased risk of having meningococcal disease but the CRP may be normal or low even in severe disease.
A normal CRP does not rule out bacterial meningitis.[9]
Result
may be elevated
coagulation profile
Test
Request a coagulation screen for all patients.[9][48][49][50]
Coagulopathy is common in severe meningococcal infections.[9][50]
Disseminated intravascular coagulation (DIC) is caused by acquired deficiencies of protein C, protein S, and antithrombin III, increases in plasminogen activator inhibitor and thrombin-activatable fibrinolysis inhibitor, and reduced activation of protein C on endothelial cells.
Result
may show evidence of disseminated intravascular coagulation (prolonged thrombin time, elevated fibrin degradation products or D-dimer, low fibrinogen or antithrombin levels)
blood cultures
PCR for Neisseria meningitidis
Test
Always perform whole blood real-time PCR testing (EDTA sample) for N meningitidis to confirm a diagnosis of meningococcal disease.[9][48]
Take the blood sample for PCR testing as soon as possible because early samples are more likely to be positive.
Use PCR testing of blood samples from other hospital laboratories if available, to avoid repeating the test.
A negative blood PCR test result for N meningitidis does not rule out meningococcal disease.
Result
N meningitidis DNA
urea, electrolytes and creatinine, serum calcium, ionised magnesium (Mg2+), ionised phosphate (PO4-)
liver function tests
cross-match (children)
Test
Cross-matching is essential if the patient is a child.[50]
Result
variable
CFS PCR for Neisseria meningitidis and Streptococcus pneumoniae
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
Submit cerebrospinal fluid (CSF) obtained during LP to the laboratory to hold for PCR testing for N meningitidis and S pneumoniae. Request PCR testing only if the CSF culture is negative.[9]
CSF samples taken up to 96 hours after admission to hospital may give useful results.
Result
N meningitidis or S pneumoniae DNA
PCR for Streptococcus pneumoniae (adults)
Test
If the patient is an adult, always perform whole blood real-time PCR testing (EDTA sample) for S pneumoniae.[48]
Result
N meningitidis or S pneumoniae DNA
CSF white blood cell count and examination
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
CSF WBC count can be fewer than 0.1 x 109/L or normal in the early phase of bacterial meningitis.[48]
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:[9]
≥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.
If the CRP and/or white blood cell count is raised and there is non-specifically abnormal CSF (e.g.,consistent with viral meningitis), treat as bacterial meningitis.[9]
If no CSF is available for examination or the CSF findings are uninterpretable, manage as if a diagnosis of meningitis is confirmed.[9]
In practice the first dose of antibiotics should not be delayed by lumbar puncture or CSF results if there is clinical concern.
Consider alternative diagnoses if the patient is significantly ill and has CSF variables within accepted normal ranges.[9]
Result
leukocytosis
CSF total protein concentration
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
Result
may be elevated
CSF glucose concentration
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
Request a corresponding laboratory-determined blood glucose concentration.
CSF glucose is low in bacterial meningitis, but the concentration is affected by the concomitant plasma glucose.[75] The CSF:plasma glucose ratio is therefore a more reliable marker, with a cut-off of 0.36 having a high sensitivity and specificity (sensitivity 93% and specificity 93% in one single-centre retrospective review of medical records of 15 adults with bacterial meningitis and 129 adults with aseptic meningitis even after administration of antimicrobials prior to examination in the A&E department).[48][76]
Result
CSF:blood ratio low
CSF microscopy, Gram stain, culture and sensitivities
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
Gram-negative diplococci may be present in patients with meningococcal disease. Use microbiological culture and sensitivities to check for the causative organism Neisseria meningitidis.
Result
organisms seen on microscopy and cultures evident on culture medium
CSF lactate (adults)
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[9][48][49] For more information on lumbar puncture, see Diagnosis recommendations.
CSF lactate has a high sensitivity and specificity for distinguishing between bacterial and viral meningitis in adults if antibiotics have not been given.[9]
Result
high
Investigations to consider
cranial CT
Test
Order a CT scan to look for alternative intracranial pathology in children and young people with suspected bacterial meningitis and reduced or fluctuating level of consciousness (GCS <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, or assess using focal neurological signs.[9] Glasgow Coma Scale: modification for children Opens in new window
Do not delay treatment to carry out a CT scan.
Stabilise the patient clinically before CT scanning.
Consult a senior emergency physician, anaesthetist, paediatrician, or intensivist if performing a CT scan
If the patient is an adult with suspected meningitis, only arrange a CT scan (once stabilised) if there are signs suggestive of shift of brain compartments secondary to raised intracranial pressure:[48]
Focal neurological signs
Presence of papilloedema
Continuous or uncontrolled seizures
GCS ≤12; before brain scan, arrange assessment by a critical care physician. [ Glasgow Coma Scale Opens in new window ] Glasgow Coma Scale: modification for children Opens in new window
Result
intracranial pathology
complement deficiency (children)
Test
Test children and young people for complement deficiency if they have had any of the following:[9]
More than one episode of meningococcal disease
One episode of meningococcal disease caused by serogroups other than B (e.g., A, C, Y, W135, X, 29E)
Meningococcal disease caused by any serogroup and a history of other recurrent or serious bacterial infections.
Discuss appropriate testing for complement deficiency with local immunology laboratory staff.[9]
Result
complement deficiency
serum HIV (adults)
Test
Get consent and check HIV status in all patients with bacterial meningitis as a screen for predisposition to meningitis.[48]
HIV can cause meningitis directly or indirectly via opportunistic infections.
HIV antibody tests may be negative in the early phase of the illness (during seroconversion).[48]
Result
positive; may be negative in seroconversion illness
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