Substantial hemorrhage or substantial numbers of red cells in the CSF are uncommon in meningitis, the exceptions being anthrax meningitis (see Lanska) as well as certain viral infections (hantavirus, dengue fever, Ebola virus, etc.) and some cases of amebic meningoencephalitis.
The protein content is higher than 45 mg/dL in more than 90 percent of the cases; in most cases, it falls in the range of 100 to 500 mg/dL. The glucose content is diminished (hypoglycorrhachia), usually to a concentration less than 40 mg/dL, or less than 40 percent of the blood glucose concentration (measured concomitantly or within the previous hour), provided that the latter is less than 250 mg/dL. However, in atypical or culture-negative cases, other conditions associated with a reduced CSF glucose should be considered. These include hypoglycemia from any cause; sarcoidosis of the CNS; fungal or tuberculous meningitis; and some cases of subarachnoid hemorrhage, meningeal carcinomatosis, chemically induced inflammation from craniopharyngioma or teratoma, and meningeal gliomatosis. The factors that alter CSF glucose concentration, especially at the extremes of blood glucose, are discussed in Chap. 2.
A special problem pertains to identifying patients with a meningitic syndrome and CSF pleocytosis who do not, in fact, have bacterial meningitis but likely have a viral or other cause for their syndrome. This is driven by a desire to avoid exposure to high-potency intravenous antibiotics that are potentially dangerous may have side effects. To address this problem, Nigrovic and colleagues have developed a clinical prediction rule that classifies children at very low risk for bacterial meningitis if they have all of the following: negative CSF Gram stain, CSF absolute neutrophil count under 1,000 cells/mL, CSF protein under 80 mg/dL, peripheral absolute neutrophil count under 10,000 cells/mL, and no history of a seizure at or after the time of presentation. This rule was validated in a multicenter retrospective cohort study that encompassed 3,295 patients. Of those who were categorized at very low risk, only two had bacterial meningitis. Whether this low rate justifies withholding antibiotics is, of course, a clinical judgement made at the bedside.
The Gram stain of the spinal fluid sediment permits identification of the causative agent in most cases of bacterial meningitis; pneumococci and H. influenzae are identified more readily than meningococci. Small numbers of gram-negative diplococci in leukocytes may be indistinguishable from fragmented nuclear material, which may also be gram-negative and of the same shape as bacteria. In such cases, a thin film of uncentrifuged CSF may lend itself more readily to morphologic interpretation than a smear of the sediment. The most common error in reading Gram-stained smears of CSF is the misinterpretation of precipitated dye or debris as gram-positive cocci or the confusion of pneumococci with H. influenzae. The latter organism may stain heavily at the poles, so that they resemble gram-positive diplococci, and older or rapidly growing pneumococci often lose their capacity to take a gram-positive stain.
Cultures of the spinal fluid, which prove to be positive in 70 to 90 percent of cases of bacterial meningitis, are best obtained by collecting the fluid in a sterile tube and immediately inoculating agar plates; tubes of thioglycolate (for anaerobes); and other media. The advantage of using broth media is that large amounts of CSF can be cultured. The importance of obtaining blood cultures is mentioned in the following text.