During the 55 months studied, 2,635 CSF specimens were sent for Gram stain and routine culture (Table ). Based on the criteria presented above, there were 64 cases of infection, including 56 cases of bacterial or fungal meningitis, 4 cases of infections adjacent to the subarachnoid space, and 4 cases of sepsis without meningitis. Bacterial or fungal meningitis was further stratified into that associated or not associated with a CNS shunt. As shown in Table , Cryptococcus neoformans was the most common cause of meningitis in the absence of a shunt, followed by Streptococcus pneumoniae and Neisseria meningitidis. Coagulase-negative staphylococci and Enterobacter cloacae were the most common causes of meningitis associated with a CNS shunt.
TABLE 1 Overall results of 2,635 CSF specimens submitted for routineculture
TABLE 2 Results of 284 CSF specimens with positive routinecultures
Organisms were detected by microscopic examination of the Gram-stained CSF in 35 of 40 (88%) instances of bacterial or fungal meningitis and in 13 of 16 (81%) instances of shunt-associated meningitis (Table ). The Gram stain morphology was correctly interpreted in all cases. If patients who were receiving effective antimicrobial therapy (as determined by retrospective review of susceptibility data) at the time the CSF specimen was obtained are excluded, microscopic examination was 92% sensitive (35 of 38 without and 13 of 14 with CNS shunts). Of the three patients without CNS shunts whose CSF Gram stain failed to reveal organisms, two had cryptococcal meningitis which was diagnosed by simultaneous cryptococcal antigen testing and thus would not have gone undetected. The remaining patient had presumptive meningitis due to Streptococcus sanguis II; however, additional studies (such as computed tomagraphy or magnetic resonance imaging) were not done to rule out an abscess, and all other cultures were negative. Microscopic examination showed at least moderate WBC in 12 of 13 (92%) cases of bacterial meningitis without a CNS shunt, 15 of 27 (56%) cases with cryptococcal disease, and 8 of 16 (50%) cases with shunt infection.
TABLE 3 Summary of Gram stain results for 284 CSF specimens with positive routinecultures
Microscopic examination revealed no organisms in 218 of 220 CSF specimens in which the organisms isolated were judged to be contaminants (Table ). In 28 cases, WBC were quantitated as at least moderate. Review of the medical records for these patients indicated a corresponding elevated WBC count in the CSF of 10 who were subsequently diagnosed with viral meningitis (3 patients), tuberculous meningitis (2 patients), and neurosyphilis, toxoplasmosis, lymphocytic meningitis, systemic lupus erythematosus, and CNS lymphoma (1 patient each). For the remaining 18, CSF cell counts by hemocytometer were normal (≤4 WBC/mm3) and no associated disease was present. Generally, estimation of WBC as no more than few on Gram stain correlated with normal cell counts by hemocytometer, but less agreement was seen when WBC were quantified as at least moderate. Errors tended to be caused by uneven distribution of cells by cytocentrifugation and misidentification of other cell types and/or debris as WBC.
Microscopic examination was equivocal for two CSF specimens in which Gram stain and culture results were eventually found to be discrepant. In the first case, the Gram stain was initially interpreted as many WBC and no organisms seen. The senior reviewer identified possible rare gram-positive forms (suggestive of Listeria
organisms, which are often found in low numbers). With uncertainty conveyed to the clinician, the results were reported as rare gram-positive rods and/or cocci and many WBC. All cultures were negative, and the patient was diagnosed with a cerebrovascular accident. In the second case, microscopic examination revealed two clumps of gram-negative rods with moderate WBC in CSF from a patient thought to have recurrence of a brain abscess. Doubt that these organisms represented infection was expressed because they were not evenly dispersed or associated with the WBC. Broth culture grew Flavimonas oryzihabitans
, which infrequently causes infection but more often occurs as a contaminant of laboratory materials and Gram-staining equipment (3
). The positive culture in this case was interpreted as contamination. Aside from these two cases, microscopic examination was thought to yield a misleading false-positive result in only one additional CSF specimen. In this case, the Gram stain showed rare gram-positive cocci with no more than few WBC, but all other CSF values were normal, and culture grew only one coagulase-negative Staphylococcus
The majority (28 of 34 [82%]) of the bacterial isolates causing true infection were recovered both in broth and on solid media and were too numerous for accurate quantitation (Table ). Of the four cases of shunt-associated meningitis in which organisms were isolated in broth only, only one (Candida albicans) was detected in additional specimens. The observation that only one-third (10 of 30) of the fungal isolates causing infection were recovered on both media may be artifact, as it is laboratory policy to refer fungal isolates to the mycology section as soon as the first culture is positive and to discontinue incubation and examination of the remaining media.
In the remaining 220 CSF specimens with positive cultures, bacterial or fungal growth was judged to reflect contamination (Table ). In 80% (177 of 220), the isolates grew in broth only or on solid medium only with a colony count of ≤4. Coagulase-negative staphylococci were isolated in 154 cultures (as the only organism in 141, with other common contaminants in 11, and with potential pathogens in 2). Other common contaminants were coryneforms, alpha-hemolytic streptococci, and Staphylococcus aureus. Although bacterial growth was obtained from both broth and solid media in 43 cases, review of medical records indicated that these were the result of contamination.