Clinical impact of cerebrospinal fluid Gram stain and culture testing: A retrospective cohort study

Stewardship of microbiological tests can improve laboratory efficiency. One indicator of appropriate test stewardship is test impact on patient management decisions. We sought to assess the impact of cerebrospinal fluid (CSF) Gram stain and culture results on treatment decisions. Our hypothesis was that CSF Gram stain and culture have low impact on patient management.

CSF specimens received at a tertiary microbiology laboratory between January 1, 2013, and December 31, 2013, were included. Clinical information and data on antibiotic treatment before CSF collection, antibiotic treatment after CSF Gram stain results, and antibiotic treatment after CSF culture results were collected. Ethics approval for secondary use of data was obtained.

We received 242 CSF specimens for Gram stain and culture during the study period; 120 were excluded (84 from children, 2 from indwelling ventricular drains, 12 collected at outside hospitals, 21 data missing, 1 duplicate). No Gram stains or cultures were positive among patients not already treated empirically. The number needed to test to influence treatment was 17 for Gram stain (11 for abnormal cytochemistry, 29 for normal cytochemistry) and 6 for culture (3 for abnormal cytochemistry, 6 for normal cytochemistry).

CSF Gram stain and culture are rarely positive and are being performed on inappropriate specimens. CSF results never prompt physicians to start treatment, so results are affecting not outcome but antibiotic stewardship. Negative CSF culture often leads to discontinuation of antibiotics. Labs could consider rejecting CSF Gram stain if cytochemistry is normal.