NEW MICROBIOLOGICA, 34, 97-99, 2011 Accuracy of automated and manual systems for susceptibility testing of Pseudomonas aeruginosa to piperacillin and piperacillin-tazobactam Carlo Gagliotti 1 , Mario Sarti 2 , Carla Sabia 3 , Raffaele Gargiulo 2 , Gian Maria Rossolini 4 , Carmelina Carillo 5 , Carla Cassani 6 , Antonio Paolo Cipolloni 7 , Federica Pedna 8 , Maria Rita Rossi 9 , Silvia Storchi Incerti 10 , Giovanna Testa 11 , Claudia Venturelli 12 , Maria Luisa Moro 1 1 Agenzia Sanitaria e Sociale Regionale Emilia-Romagna, Bologna, Italy; 2 AUSL Modena. Nuovo Ospedale di Modena S. Agostino-Estense, Baggiovara (MO), Italy; 3 Università degli Studi di Modena e Reggio-Emilia, Dipartimento di Scienze Biomediche, Modena, Italy; 4 Università degli Studi di Siena, Dipartimento di Biologia Molecolare Laboratorio di Fisiologia e Biotecnologia dei Microrganismi. Policlinico Le Scotte, Siena, Italy; 5 AUSL Ferrara. Ospedale Mazzolani Landini, Argenta (FE), Italy; 6 AUSL Imola. Ospedale S. Maria della Scaletta, Imola (BO), Italy; 7 AUSL Cesena. Ospedale Bufalini, Viale Ghiotti, 286; 47023 Cesena, Italy. 8 AUSL Ravenna. Ospedale S. Maria Delle Croci, Viale Randi, 5; 48100 Ravenna, Italy. 9 Azienda Ospedaliero-Universitaria di Ferrara. Arcispedale S. Anna, Ferrara, Italy; 10 AUSL Reggio-Emilia. Ospedale di Guastalla, Guastalla (RE), Italy; 11 AUSL Rimini. Ospedale Infermi, Rimini, Italy; 12 Azienda Ospedaliero-Universitaria di Modena, Modena, Italy Data on the accuracy of automated and manual systems for testing the antimicrobial susceptibil- ity of Pseudomonas aeruginosa have been report- ed (Burns et al., 2000; Juretschko et al., 2007; Sader et al., 2006; Steward et al., 2003). High rates of false susceptibility (very major errors: VMEs) of the automated systems have been observed, es- pecially for piperacillin and piperacillin-tazobac- tam, where the interpretative categories of the re- sults are only two: susceptible and resistant with- out the intermediate susceptibility (Juretschko et Corresponding author Maria Luisa Moro Agenzia Sanitaria e Sociale Regionale Emilia-Romagna Area di Programma Rischio Infettivo Viale Aldo Moro, 21 - 40127 Bologna, Italy E-mail: mlmoro@regione.emilia-romagna.it al., 2007; Sader et al., 2006). The present study aimed to compare the accuracy of one automat- ed system (Vitek2 (V2)), considering both the card AST-N022 and the recently released AST-N026, and two manual systems (Kirby-Bauer (KB) and E-test (ET)) for susceptibility testing of P. aerug- inosa to piperacillin and piperacillin-tazobactam. Nine laboratories in the Emilia-Romagna region (Italy) were recruited on a voluntary basis to par- ticipate to the study. These laboratories (7 hospi- tal laboratories and 2 teaching hospital laborato- ries) collected all the consecutive clinical isolates of P. aeruginosa; identified between January and February 2008, for which V2 (AST-N022) esti- mated a Minimum inhibitory concentration (MIC) ≥8 mg/ml for piperacillin and/or piperacillin-tazobactam. A further sample of 40 consecutive isolates with MICs≤4 mg/ml for both This study aimed to evaluate the accuracy of routine systems (Vitek2 cards AST-N022 and AST-N026; Kirby Bauer; E- test) for susceptibility testing of Pseudomonas aeruginosa to piperacillin and piperacillin-tazobactam. Vitek2 (card AST-N022) showed the worst performance; the other three methods (Vitek2 card AST-N026, Kirby-Bauer and E-test) performed comparably but never fulfilled the minimal standard proposed by FDA. KEY WORDS: Diagnostic tests; antimicrobial resistance; Pseudomonas aeruginosa; very major error SUMMARY Received March 25, 2010 Accepted August 08, 2010