Reliability of Transport Medium in the Laboratory Evaluation of Corneal Ulcers

Abstract

To compare the microbiological yield of corneal ulcer cultures established by direct inoculation of culture media vs indirect inoculation by means of transport medium (Amies without charcoal). Single masked, prospective clinical trial. Scrapings were obtained for Gram and potassium hydroxide (KOH) stains from eyes with presumed infectious keratitis and cultured by direct plating onto standard media. Samples were also held in transport media (Amies without charcoal) at room temperature and then plated after 4 and 24 hours. Yields from direct plating vs cultures by means of transport media were compared. Of 100 consecutive eyes examined with presumed infectious keratitis, Gram or KOH stain revealed a bacterial or fungal agent in 69 cases (69%). Of these, 26 were bacterial and 43 fungal. Twenty-two bacterial infections produced positive cultures by direct plating, and all produced the same organism with Amies medium after 4 and 24 hours, respectively. For 43 fungal infections identified by KOH stain, 29 (67%) yielded a positive result after 4 hours in Amies transport medium and 27 (63%) after 24 hours in Amies medium. A total of three cases (7%) that showed fungal infection on KOH stain but did not yield organisms by direct plating did so after inoculation with Amies transport medium. For all comparisons, there was no difference in recovery rates by means of transport medium compared with direct plating (McNemar exact P > .05). In the clinical setting, Amies transport medium may be a useful alternative to direct inoculation onto blood agar for the laboratory evaluation of infectious keratitis.