Comparison of acridine orange and Gram stains for detection of microorganisms in cerebrospinal fluid and other clinical specimens

Acridine orange, a fluorochrome strain, is potentially superior to the Gram stain in the direct microscopic examination of clinical specimens because it gives striking differential staining between bacteria and background cells and debris. Its value in clinical laboratories was evaluated by testing 209 cerebrospinal fluids and 288 other body fluids, tissues, and exudates by both techniques. Smears were made in duplicate, fixed with methanol, stained, and examined without knowledge of the result of the companion smear or culture. Overall, acridine orange was slightly more sensitive than the Gram stain (acridine orange, 59.9%; Gram stain, 55.8%) and equally specific in detecting microorganisms. One smear was falsely positive by the Gram stain; none was falsely positive by the acridine orange stain. We conclude that acridine orange staining is a sensitive method for screening clinical specimens and reviewing selected specimens that are purulent, but negative by the Gram stain. Bloody fluids, thick exudates, and other normally difficult-to-read specimens were easily and quickly examined. We recommend, however, that positive smears be reexamined with the Gram stain to confirm the result and determine the Gram reaction of the microorganisms.