Investigations
1st investigations to order
blood gases (including lactate and ionised calcium)
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.[48][49][51][82] 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
FBC
CRP (and/or procalcitonin)
coagulation profile
Test
Request a coagulation screen for all patients.[49][51]
Coagulopathy is common in severe meningococcal infections.
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.[48][49]
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
Test
Key tests for children and adults.[49]
Patients with severe meningococcal infections often have metabolic abnormalities.
Result
acidosis, hypokalaemia,; elevated creatinine; elevated eGFR; reduced creatinine clearance
LFTs
cross-match (children)
Test
Cross-matching is essential if the patient is a child.
Result
variable
CSF PCR for Neisseria meningitidis and Streptococcus pneumoniae
Test
Perform a lumbar puncture if you suspect bacterial meningitis unless the procedure is contraindicated.[48][49][51] For more information on lumbar puncture, see Diagnosis recommendations.
Submit cerebrospinal fluid (CSF) for PCR testing for relevant pathogens including N meningitidis and S pneumoniae.
Result
N meningitidis or S pneumoniae DNA
PCR for Neisseria meningitidis and Streptococcus pneumoniae
Test
Perform whole blood real-time PCR testing (EDTA sample) for N meningitidis and S pneumoniae.[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 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.[48][49] For more information on lumbar puncture, see Diagnosis recommendations.
CSF white blood cell counts, total protein, and glucose concentrations should be available within 4 hours to inform decision making on adjunctive corticosteroid therapy.[48]
Interpret cerebrospinal fluid results using standard age-appropriate threshold values.
Normal thresholds for white cell count and protein may be higher in babies aged 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.
If cerebrospinal fluid results are abnormal, consider alternative viral, mycobacterial, fungal or non-infectious causes as well as bacterial meningitis.[48]
Store remaining cerebrospinal fluid in case more tests are needed.[48]
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.[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.[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.[95] 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 accident and emergency department).[49][96]
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.[48][49][97] 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
throat swab for culture
Test
Take a throat swab for meningococcal culture.[49]
Meningococci can be isolated from the nasopharynx in up to 50% of patients with meningococcal disease.
Result
positive for Neisseria meningitidis
Investigations to consider
cranial CT
Test
Only perform cranial imaging before lumbar puncture if there are:[48][49][52][83][84]
Risk factors for an evolving space-occupying lesion.
Any of the following clinical features, which might indicate raised intracranial pressure.
new focal neurological features (including seizures or posturing)
abnormal pupillary reactions or papilloedema
a Glasgow Coma Scale (GCS) score of 9 or less, or a progressive and sustained or rapid fall in level of consciousness. 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.
Do not routinely perform neuroimaging before lumbar puncture.[48]
Neuroimaging is not indicated unless you suspect alternative pathology (i.e., because there are signs indicating risk of cerebral herniation).[49]
Consider CT or 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.[85][86]
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.[52][87] However, if a CT scan has been performed and shows radiological evidence of raised intracranial pressure, do not proceed with a lumbar puncture.
Result
intracranial pathology
serum HIV
Test
Test for HIV in all patients with bacterial meningitis as a screen for predisposition to meningitis.[48][49]
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).[49]
Consider testing for HIV in babies, children and young people if they have signs of immunodeficiency or risk factors for HIV.[48]
Result
positive; may be negative in seroconversion illness
screening for predisposing factors
Test
Consider further investigations for patients with recurrent meningococcal disease; arrange review with a paediatric immunology and infectious disease specialist or an adult infection specialist or immunologist (as appropriate).[48]
Result
Primary or secondary immunodeficiency (e.g., complement deficiency)
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