What investigations are essential for confirming a diagnosis of bacterial meningitis in a primary care setting?

In a primary care setting, confirming a diagnosis of bacterial meningitis relies primarily on clinical assessment and urgent referral rather than performing definitive diagnostic investigations locally.

Essential investigations to confirm bacterial meningitis include blood tests such as blood culture, white blood cell count (including neutrophils), C-reactive protein (CRP) or procalcitonin (PCT), blood glucose, and whole-blood PCR for relevant pathogens including meningococcal and pneumococcal species. A bacterial throat swab for meningococcal culture is also recommended if possible before antibiotics are started.

The definitive diagnostic investigation is a lumbar puncture to obtain cerebrospinal fluid (CSF) for analysis, including red and white cell counts with differential, total protein, glucose concentration (to calculate CSF to blood glucose ratio), microscopy with Gram stain, microbiological culture and sensitivities, and PCR for relevant pathogens.

However, lumbar puncture and these detailed investigations are typically performed in secondary care after urgent hospital referral, not in primary care. Primary care clinicians should focus on early recognition, starting antibiotics if there will be a delay in hospital transfer, and arranging emergency transfer to hospital for these investigations and management.