Reproducibility of the analytab (API 20E) system.

The reproducibility of the Analytab (API 20E) system for identification of Enterobacteriaceae was evaluated with 110 clinical isolates. Each isolate was identified by two technologists at different times. Genus-species identification was 97.3% reproducible; however, only 55.5% of the strains gave identical reactions in all 20 of the API 20E biochemical tests on repeat testing. Of those strains which varied, 56% possessed only one variable biochemical test. The reproducibility for each biochemical test was calculated and ranged from 89 to 100%. A subset of 20 of the most variable strains was tested further under conditions of varying incubation time (15 and 22 h) and inoculum concentration (10(7), 10(5), and 10(3) colony-forming units per ml), and by having four technologists interpret the test results. The reproducibility for each biochemical test for these 20 variable strains ranged from 86 to 99%. Less variation in interpretation by technologists was seen at an incubation time of 22 h and an inoculum concentration of 10(7) colony-forming units per ml. Consideration of the reproducibility for each biochemical test can aid in determining the probability that two isolates suspected of being the same strain, but with API profiles which differ by one or more biochemical test results, are in fact the same strain. Variables such as inoculum size, incubation time, technologist interpretation, and strip variability affect the API test results and should be standardized to minimize their effects.