CSF bacterial antigen detection tests offer no advantage over Gram's stain in the diagnosis of bacterial meningitis

While bacterial antigen detection (BAD) tests have been used on cerebrospinal fluid (CSF) with success in the diagnosis of bacterial infection in developing countries, their value in the developed world has been recently questioned. In Darwin, Northern Territory (NT), there are good diagnostic resources but high rates of infectious disease, so it was unclear which findings were applicable to our own population. This study aimed to determine the utility of the BAD tests in detection of bacterial meningitis from CSF in patients studied at Darwin, using a retrospective review of hospital case records and microbiology laboratory reports, over a 19 month period, and utilising a clinical component in the case definition of bacterial meningitis. The sensitivity of the BAD test in the diagnosis of acute bacterial meningitis was 28.6%, with a specificity of 98.7% and a positive predictive value of 85.7%. The cost per positive test was computed at $240. No cases of bacterial meningitis which were positive on the BAD test were missed on Gram's stain of CSF. We conclude that in our setting BAD tests alone are not sensitive enough to confidently diagnose bacterial meningitis. BAD tests are more costly and offer no advantage in speed of diagnosis or in antibiotic pre-treated patients, compared to routine Gram's stain.