Investigations

If the diagnosis of meningitis is suspected it is of the utmost importance that the cause is established – pyogenic meningitis has a high mortality without treatment.

The following clinical signs should be recorded in all patients with suspected bacterial meningitis or meningococcal disease:

• heart rate
• respiratory rate
• blood pressure
• temperature
• capillary refill time
• oxygen saturation measurement.

Perform a neurological assessment using the AVPU (1):

• Alert? (even an alert child may be very ill with septicaemia)
• Responds to Voice?
• Responds to Pain?
• Unresponsive?

Laboratory investigations in patients with suspected meningitis include:

• blood cultures – ideally should be done before antibiotics
• full blood count, urea, creatinine, electrolytes, liver function tests and clotting screen
• procalcitonin (or CRP if unavailable)
  • useful in differentiating bacterial and viral infection
  • there is insufficient evidence for routine use
• meningococcal and pneumococcal PCR
• serology sample
• glucose
• a swab of the posterior nasopharyngeal wall – for meningococcal culture (2)
• HIV test test (5)
  • for HIV in adults with bacterial meningitis or meningococcal disease
  • consider testing for HIV in babies, children and young people with bacterial meningitis or meningococcal disease, if they have signs of immunodeficiency or risk factors for HIV
• lumbar puncture
  • this is mandatory in any patient in whom bacterial meningitis is suspected (3)
  • this should be performed unless specifically contraindicated
  • either cranial computed tomography (CT) or magnetic resonance imaging (MRI) is recommended as a precaution in selected patients before lumbar puncture to detect brain shift (3,4)
    • NICE states that should not routinely perform neuroimaging before lumbar puncture (5):
      • perform imaging if the person has:
        • risk factors for an evolving space-occupying lesion or
        • any of these symptoms or signs, which might indicate raised intracranial pressure:
          • new focal neurological features (including seizures or posturing)
          • abnormal pupillary reactions
          • a Glasgow Coma Scale (GCS) score of 9 or less, or a progressive and sustained or rapid fall in level of consciousness

• do not perform a lumbar puncture until these factors have been resolved

• European guidance also notes as additional indications for neuroimaging before lumbar puncture(4):
  • immunocompromised state (AIDS, immunosuppressive therapy, or after transplantation)
  • history of CNS disease (mass lesion, stroke, or focal infection)
  • papilloedema

• diagnosis of bacterial meningitis depends on CSF examination performed after lumbar puncture
• CSF should be sent for:
  • opening pressure
  • gram stain, culture and sensitivity
  • cell count
  • biochemistry – glucose, protein, lactate
  • meningococcal and pneumococcal PCR
    • antibiotics should be given as a priority and should not be delayed because a lumbar puncture has not been performed.

In children and young people with suspected bacterial meningitis, perform a CRP and white blood cell count (6):

• if the CRP and/or white blood cell count is raised and there is a non-specifically abnormal cerebrospinal fluid (CSF) (for example consistent with viral meningitis), treat as bacterial meningitis
• be aware that a normal CRP and white blood cell count does not rule out bacterial meningitis
• regardless of the CRP and white blood cell count, if no CSF is available for examination or if the CSF findings are uninterpretable, manage as if the diagnosis of meningitis is confirmed

If a child or young person has an unexplained petechial rash and fever (or history of fever), carry out the following investigations (6):

• full blood count
• C-reactive protein (CRP)
• coagulation screen
• blood culture
• whole-blood polymerase chain reaction (PCR) for N meningitidis
• blood glucose
• blood gas.

Reference:

1. Meningitis Research Foundation 2018. Meningococcal Meningitis and Sepsis. Guidance notes. Diagnosis and treatment in general practice
2. McGill F et al. The UK joint specialist societies guideline on the diagnosis and management of acute meningitis and meningococcal sepsis in immunocompetent adults. J Infect. 2016;72(4):405-38.
3. van de Beek D, et al. Community-Acquired Bacterial Meningitis in Adults. N Engl J Med 2006;354:44
4. van de Beek D et al. ESCMID guideline: diagnosis and treatment of acute bacterial meningitis. Clin Microbiol Infect. 2016;22 Suppl 3:S37-62.
5. NICE (March 2024).Meningitis (bacterial) and meningococcal disease: recognition, diagnosis and management
6. NICE (June 2010). Meningitis (bacterial) and meningococcal septicaemia in under 16s: recognition, diagnosis and management.