From the Guidelines
Cerebrospinal fluid (CSF) analysis is the primary diagnostic tool to rule out meningitis.
Key Laboratory Tests
- CSF white blood cell count: A high count, typically >100 cells/μL, is indicative of bacterial meningitis 1
- CSF Gram stain: Can rapidly identify the causative bacterium in 60%-90% of patients with community-acquired bacterial meningitis, with a specificity of 97% 1
- CSF culture: The gold standard for diagnosing bacterial meningitis, with a diagnostic yield of 70%-85% in cases where prior antibiotics have not been given 1
- CSF protein and glucose levels: Can help differentiate between types of meningitis, with bacterial meningitis typically showing elevated protein and low glucose levels 1
Additional Tests
- Blood cultures: Should be performed in patients suspected of having bacterial meningitis, with at least 2-4 cultures recommended 1
- Molecular testing: May be used to diagnose enteroviral meningitis, but is not routinely relied upon for bacterial meningitis diagnosis 1
- Cryptococcal antigen test: Can be used to diagnose meningitis caused by Cryptococcus neoformans or Cryptococcus gattii, with a sensitivity and specificity of >90% 1
From the Research
Laboratory Tests for Meningitis
The following laboratory tests can help rule out meningitis:
- Cerebrospinal fluid (CSF) evaluation, including measurement of pressure, cell count and white cell differential, determination of glucose and protein levels, Gram's stain, and culture 2
- CSF lactic acid (LA) levels, which can provide a rapid and reliable diagnosis of acute bacterial meningitis (ABM) and help differentiate it from viral meningitis 3
- Limulus lysate assay and tests to identify bacterial antigens, which can allow for rapid diagnosis of bacterial meningitis 2, 4
- Latex agglutination test, which can detect soluble bacterial antigens in CSF 5
- Biomarkers such as CRP, procalcitonin, or sTREM-1, which can help differentiate between viral and bacterial meningitis 5, 6
- Microbiological pathogen-focussed methods, such as highly multiplexed nucleic acid amplification tests, which can rapidly detect bacterial pathogens 6
- Proteomic and transcriptomic signatures, which can provide newer biomarkers for differentiating between bacterial and viral meningitis 6
CSF Analysis
CSF analysis is the gold-standard test for diagnosis of meningitis, and it includes:
- Gram staining of CSF, which can reveal bacteria in about 50 to 80 % of cases 5
- CSF culture, which can be positive in at best 80 % of cases 5
- CSF leukocyte count and concentration of protein and glucose, which can lack specificity and sensitivity for the diagnosis of meningitis 5
Limitations of Laboratory Tests
It's worth noting that the sensitivity of laboratory tests for meningitis can be limited, especially in patients who are already on antibiotic treatment 5. Additionally, the early phases of acute bacterial and viral meningitis can have non-specific signs and symptoms, making it challenging to make a differential diagnosis 5.