Rapid Determination of Antimicrobial Susceptibility for Urgent Clinical Situations

Abstract

With conventional methods, results of antimicrobic susceptibility tests are not available until 2 to 3 days after the specimen is collected. Many clinical situations would benefit if results of susceptibility tests were available sooner. Information can be obtained more quickly by inoculating the test plates directly with clinical material such as exudate or urine. Data to show that this approach is too unreliable for clinical use, especially with specimens containing mixtures of microorganisms, are presented. Isolated colonies may be tested by means of a standardized disk diffusion method, but this requires an additional 18 to 24 hr. Faster results can be obtained by observing the test plates after incubation for only 5 to 6 hr. Such early zone measurements were compared with those obtained with the more conventional 16 to 18 hr. period of incubation. With the majority of drugs and bacterial genera, zones measured after a few hours are not altered with further incubation, but with certain genera and with certain drugs, the early preliminary readings are predictably unreliable, e.g., Enterobacter with ampicillin or cephalothin and Serratia with polymyxin B, both of which appear susceptible after incubation for 5 to 6 hr. but resistant after incubation overnight.