Direct microscopic screening of blood cultures by Gram stain or methylene blue stain is time consuming and frequently insensitive. Therefore, we evaluated a fluorescent-staining procedure that uses acridine orange (AO) at pH 3.5 and compared it with the methylene blue and Gram stain procedures. All smears were prepared within 24 h of receiving the culture, fixed with methanol, and examined without the results of the companion smears being known. AO-stained smears were examined with incident-light fluorescence at 600x magnification and confirmed at 1,500x magnification. All bottles macroscopically positive within 24 h were excluded from the study. Of 2,946 cultures entered into the study, 204 (6.9%) were positive within 3 days. The sensitivity and specificity of AO based on these culture results were 52 and 98%, respectively, compared with 38% sensitivity and 99% specificity by methylene blue and Gram stains. The AO staining procedure is a simple, sensitive, screening technique for the early detection of positive blood cultures.

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