How often do cerebrospinal fluid (CSF) bacterial cultures or antigen assays yield the bacterial cause of meningitis?

Updated: Jul 16, 2019
  • Author: Rodrigo Hasbun, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

CSF bacterial cultures yield the bacterial cause in 70-85% of cases. The yield diminishes by 20% in patients who have received antimicrobial therapy. In these cases, some experts advocate the use of a CSF bacterial antigen assay. This is a latex agglutination technique that can detect the antigens of H influenzae type B (Hib), S pneumoniae, N meningitidis, E coli K1, and S agalactiae (group B streptococcus [GBS]). Its theoretical advantage is the detection of the bacterial antigens even after microbial killing, as is observed after antibacterial therapy.

Another attractive alternative is using the Binax NOW for S pneumoniae in the CSF. This assay has a 99-100% sensitivity and specificity for ruling out the most common cause of bacterial meningitis. [25]

Others studies, however, have shown that the CSF bacterial antigen assay may not be better than the Gram stain. Although it is specific (a positive result indicates a diagnosis of bacterial meningitis), a negative finding on the bacterial antigen test does not rule out meningitis (50-95% sensitivity).


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