Erroneous Diagnosis of Meningitis Due to False-Positive Gram Stains

During a two-month period, 6% of Gram stains of predominantly CSF specimens revealed gram-negative bacilli with no growth. The source of the false-positive Gram stain results was an alcohol storage bath from which slides were taken and flamed to remove residual alcohol. All 11 patients in the outbreak had further diagnostic tests or changes in therapy. In the laboratory, false-positive slides could be created by contaminating a slide bath with 10(5) bacilli/ml, flame drying, and staining. In addition, contaminated crystal violet caused a false-positive result when applied to a warm but not to a cool slide. To prevent false-positive Gram stain results, the storage of slides in alcohol baths should be abandoned, slides should not be flamed to remove alcohol, and specimens should be Gram-stained only when the heat-fixed slide has thoroughly cooled.