Acute Bacterial Meningitis CSF Diagnosis
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Direct answer
Acute bacterial meningitis is diagnosed by examining cerebrospinal fluid, where a neutrophil-predominant pleocytosis, low glucose and raised protein point to bacterial infection. Gram stain and culture identify the organism, while latex agglutination detects capsular antigen when antibiotics have already been given. Empiric antibiotic choice depends on age, because neonates, adults and older patients harbour different common organisms.
What you must remember
- Cell count: bacterial meningitis gives a neutrophil-predominant pleocytosis, often in the hundreds to thousands of cells per microlitre.
- Glucose: CSF glucose below 40 mg/dL, or a CSF-to-blood glucose ratio below 0.4, supports a bacterial cause.
- Protein: raised protein, commonly above 100 mg/dL, is typical of bacterial infection.
- Gram stain: positive in a majority of untreated cases, but its yield falls sharply once antibiotics are started.
- Latex agglutination: detects capsular antigens of Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b and group B streptococcus.
- Empiric therapy in adults: a third-generation cephalosporin with vancomycin, plus ampicillin for Listeria in those over 50 years, with dexamethasone given before or with the first antibiotic dose.
A stepwise reading of the CSF report
A CSF report only makes sense when read in order, from the opening pressure to the antigen test. The steps below set out the sequence a microbiologist and clinician follow together.
- Record pressure and appearance: A raised opening pressure and a turbid fluid suggest a bacterial process, and a clear fluid argues against it.
- Read the cell count and type: Neutrophil predominance favours bacteria, while lymphocyte predominance points to viral, tuberculous or fungal causes.
- Compare glucose with blood: Low CSF glucose with a low ratio points to bacteria, tuberculosis or fungi, while normal glucose favours viral infection.
- Stain and culture: Perform Gram stain on the centrifuged deposit, and culture on blood agar and chocolate agar incubated in CO2 to recover fastidious organisms.
- Use antigen tests after pretreatment: Latex agglutination helps when culture is sterile after antibiotics, although its use has declined as molecular tests have improved.
- Start empiric therapy by age: Neonates need ampicillin with cefotaxime, covering Listeria and enteric Gram-negative bacilli, while older children and adults need a third-generation cephalosporin with vancomycin.
Where the exam tries to catch you
The exam often presents a CSF table and expects you to recognise the pattern before the organism.
- Trap: Calling a lymphocyte-predominant, low-glucose CSF bacterial Correct: lymphocyte predominance with low glucose suggests tuberculous or fungal meningitis, whereas pyogenic meningitis is neutrophil predominant.
- Trap: Delaying the first antibiotic dose for lumbar puncture Correct: give the first dose without waiting for CSF results when the lumbar puncture is delayed, after blood cultures are drawn.
- Trap: Using ceftriaxone alone for a neonate Correct: neonates need ampicillin as well, because Listeria and group B streptococcus are common neonatal pathogens.
Frequently asked questions
Which CSF pattern indicates acute bacterial meningitis?
A CSF with neutrophil-predominant pleocytosis, low glucose and high protein indicates acute bacterial meningitis. A turbid appearance and raised opening pressure often accompany this pattern.
Which meningitis organism is covered by ampicillin in empiric therapy?
Ampicillin covers Listeria monocytogenes, which matters in neonates and in adults over 50 years of age. Listeria also causes meningitis in immunocompromised patients, so it is not excluded by age alone.
Why does latex agglutination help after antibiotics have been given?
Latex agglutination detects bacterial capsular antigens in CSF, which can remain detectable even when culture is sterile after antibiotics. It is valuable for Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b and group B streptococcus.
Which CSF findings favour tuberculous meningitis over pyogenic meningitis?
Tuberculous meningitis shows lymphocyte predominance, low glucose and often a fibrin web on standing the fluid. Acid-fast bacilli on Ziehl-Neelsen staining are seen only in a minority of patients.
Which culture medium helps isolate Haemophilus influenzae from CSF?
Chocolate agar incubated in CO2 supports the growth of Haemophilus influenzae and Neisseria meningitidis from CSF. Blood agar alone is insufficient for these fastidious organisms.