Meningitis: positive Gram stain and initial observations may help differential diagnosis.

Clinical bottom line (level 2c)

In patients with meningitis, the differentiation between bacterial and viral may be made with a positive Gram stain.
Differentiation between bacterial and viral meningitis may be helped with the use of age of onset, time of year of onset, CSF-blood glucose ratio, CSF leukocyte count and CSF PMN count.

Expires November 2003

The study

Setting: University medical centre, USA

422 patients (aged ?, ?% male) diagnosis of acute meningitis

Excluded if

presentation within the first month of life
associated with neurosurgical procedures, including CSF shunt infections
immunosuppressed patients
aseptic meningitis (with negative viral cultures) in patients with noninfectious diseases that themselves can cause aseptic meningitis
CSF and blood test data missing from chart
no etiology was confirmed but antibiotics were given for 2 or more days after admission
missing any independently important data
CSF Gram stained smear was thought to show bacteria, but cultures and counterimmunoelectrophoresis were negative

The evidence

differential diagnosis	number of patients	prevalence (95% CI)
positive Gram stain of CSF in bacterial meningitis	154	71.0% (64.9% to 77.0%)
positive Gram stain of CSF in viral meningitis	2	1.00% (0% to 2.30%)

Bacterial meningitis was found to be more prevalent in winter months, whereas viral meningitis was more prevalent in the summer months.
Bacterial meningitis is more prevalent in age groups of 1 month to 1 year, and then gradually declining and increasing again after about 22 years, into old age.
CSF-blood glucose ratio in bacterial meningitis was 0.29, and in viral meningitis was 0.61.
CSF leukocyte count in bacterial meningitis was 1195x10^6/L, and in viral meningitis was 100x10^6/L.
Protein level in bacterial meningitis was 1.72 g/L, and in viral meningitis was 0.45 g/L.
CSF PMNs in bacterial meningitis were 86%, and in viral meningitis were 33%.

Comments

Exclusion of some gram-stain positive CSF samples may falsely improve the test characteristics
The data has to be validated for temporal as well as geographic applicability as it incorporates the seasonal patterns of meningitis.
In situations where TBM is the third differential diagnosis, with the exception of gram smear examination, the utility of other tests has to be established.

Citation

Spanos A, Harrell FE, Durack DT: Differential diagnosis of acute meningitis: An analysis of the predictive value of initial observations. Journal of the American Medical Association 1989; 262: 2700-2707

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