Multilaboratory validation of rapid spot tests for identification of Escherichia coli

To validate the accuracy of rapid tests for identification of Escherichia c oli, five laboratories sequentially collected 1,064 fresh, clinically signi ficant strains with core criteria of indole-positive, oxidase-negative, non spreading organisms on sheep blood agar plates (BAP), having typical gram-n egative rod plate morphology, defined as good growth on gram-negative rod-s elective media. An algorithm using beta-hemolysis on BAP, lactose reaction on eosin-methylene blue or MacConkey agar, L-pyrrolidonyl-beta-naphthylamid e (PYR), and 4-methylumbelliferyl-beta-D-glucuronide (MUG) was evaluated. I dentifications using the algorithm were compared to those obtained using co mmercial kit system identifications. One thousand strains were E. coli and 64 were not E. coli by kit identifications, which were supplemented with co nventional biochemical testing of low probability profiles. Of the 1,064 is olates meeting the core criteria, 294 were beta-hemolytic and did not requi re further testing to be identified as E. coli, None of the 64 non-E. coli strains were hemolytic, although other indole-positive, lactose-negative sp ecies were found to be hemolytic when further strains were examined in a fo llow-up study. Of the remaining strains, 628 were identified as E. coil by a lactose-positive and PYR-negative reaction. For nonhemolytic, lactose-neg ative E. call, PYR was not helpful, but a positive MUG reaction identified 65 of 78 isolates as E. coli, The remaining 13 E, coli strains required kit identifications. This scheme for E. coli identification misidentified thre e non-E. coil strains as E. coli, for an error rate of 0.3%, A total of 13 kit identifications, 657 PYR tests, and 113 MUG tests were needed to identi fy 1,000 E. coli strains with the algorithm. The use of this rapid system s aves laboratory resources, provides timely identifications, and yields rare misidentifications.