In cases of suspected infection of the CNS, chemical analyses and differential cell counts of the CSF are helpful but not always decisive in differentiating between a bacterial and a viral etiology. Gram's stains and cultures are negative in 10% to 30% of cases, especially when antibiotic therapy has been started before lumbar puncture is done.
Recent reports, however, indicate that a simple rapid test, determination of the level of lactic acid in the CSF, can accurately discriminate between the two causes of meningitis. The test can also aid in differentiating between two forms of monoarticular arthritis.
In one investigation, physicians at the Wadsworth Veterans Administration Hospital in Los Angeles studied 97 patients with a tentative diagnosis of infection of the CNS, based on the presence of fever and neurological signs (J Infect Diseases 137:384-390, 1978). For all patients, the CSF was analyzed for number of WBCs and polymorphonuclear leukocytes