the online version of this article ( doi:10.1007/s40121 - 015 - 0094 - 6 ) contains supplementary material , which is available to authorized users . extended spectrum -lactamases ( esbl ) are enzymes capable of hydrolyzing penicillins , broad - spectrum cephalosporins , and monobactams . these infections increase cost of care if inappropriate therapy , defined as non - susceptibility of pathogen to therapy by laboratory criteria , is administered . at present , the clinical and laboratory standards institute ( clsi ) and european committee on antimicrobial susceptibility testing ( eucast ) do not recommend routine screening for esbls or subsequent phenotypic confirmation testing that had been advocated previously [ 3 , 4 ] . while this approach has been supported in the clinical practice setting by the introduction of reduced breakpoints for many beta - lactams , it may be of value , particularly when evaluating novel therapies , to understand the potency of various agents against esbl producers . however , if one utilizes esbl classification with aztreonam ( atm ) , ceftriaxone ( cro ) , or ceftazidime ( caz ) minimum inhibitory concentrations ( mic ) as a simple screen , without phenotypic confirmation via the clavulanate test , this approach may include other resistant genotypes ( i.e. , carbapenemase producers ) that may not accurately predict the clinical utility of therapeutic agents under consideration . phenotypic screening tests to detect resistance mechanisms such as carbapenemase production or ampc expression are available and can be used . while molecular testing would yield the most definite results , it is also cost prohibitive and unavailable to most clinical microbiological laboratories . in this study , we tested the recently us food and drug administration ( fda)-approved compound , ceftolozane / tazobactam ( c / t ) , which has demonstrated in vitro activity against some esbl organisms . the purpose of this study was to compare the impact of mic - based screening versus confirmatory testing of esbls on the susceptibility profile of c / t versus selected antimicrobials . in this study , 44 us hospitals collected non - duplicate , non - urine escherichia coli ( n  =  1306 ) and klebsiella pneumoniae ( n  =  1205 ) over the period of june 2013 and october 2014 . participating sites identified the organisms using their established method and then transferred each isolate to trypticase soy agar slants for shipping . isolates were transferred onto trypticase soy agar plates containing 5% sheep blood at the central processing laboratory ( center for anti - infective research and development , hartford hospital , hartford hospital , hartford , ct , usa ) for mic determination using broth microdilution as described by clsi . mics were determined for c / t , cefepime ( fep ) , cro , caz , ciprofloxacin ( cip ) , atm , ertapenem ( etp ) , piperacillin / tazobactam ( tzp ) , meropenem ( mem ) , and tobramycin ( tob ) against this collection of 2511 contemporary clinical enterobacteriaceae isolates . c / t was provided by cubist pharmaceuticals , while all others antibiotics were purchased from sigma - aldrich ( st . e. coli 25922 and pseudomonas aeruginosa 27853 were utilized as quality control strains as recommended by clsi . clsi states that the initial mic - based esbl screening for e. coli and k. pneumoniae should be performed by evaluating the mic profile of one of the following drugs : cefpodoxime 4  g / ml , caz 1  g / ml , atm 1  g / ml , or cefotaxime 1  g / ml . it is noted that the use of more than one of these agents will improve detection of esbls . we defined a positive esbl screen as having an mic of 1  g / ml to 2 of the following : atm , cro , or caz . subsequent clsi defined , phenotypic , esbl confirmation studies were undertaken using caz and cefotaxime with and without clavulanate . probable carbapenemase - producing organisms were excluded based on etp ( 1  g / ml ) or mem ( 2  g / ml ) susceptibility profiles . while molecular - based confirmation of these enzymes would have been definitive , methodologies readily available to the clinical microbiology laboratory were utilized . recently , c / t has been approved in the us and susceptibility interpretative criteria have now been provided by the fda of 2  g / ml for the enterobacteriaceae . this surveillance program was reviewed by the institutional review board ( irb ) at the coordinating center , hartford hospital . since all samples were collected as part of routine medical care and no interventions were undertaken as a result of this testing , informed consent was deemed unnecessary by the irb . four - hundred and forty - two ( 18% ) isolates [ e. coli ( n  =  231 ) , k. pneumoniae ( n  =  211 ) ] screened positive for esbl production based on mic - based criteria . of these , 274 ( 62% ) isolates [ e. coli ( n  =  146 ) , k. pneumoniae ( n  =  128 ) ] were phenotypically confirmed positive for esbl production . with further exclusion of the 28 ( 10% ) isolates [ e. coli ( n  =  13 ) , k. pneumoniae ( n  =  14 ) ] that demonstrated carbapenem non - susceptible , a left shift in the mic distribution ( i.e. , enhanced potency ) against phenotypically confirmed esbl was noted for c / t , etp , tzp , and mem ( table  1 ) . figure  1 highlights these screening- and confirmation - based observations for c / t and tzp . improvements in % s between 8% and 17% were noted for confirmed versus screened - positive esbl isolates for c / t , etp , tzp , and mem . ( table  1 ) overall , % s of the other -lactams , tob and cip , was poor against phenotypically confirmed esbls . comparing the mic90 values for all esbl mic - based screen - positive versus phenotypically confirmed positive , we observed a reduction of twofold for c / t , sixfold for etp , and eightfold for mem ( table  2).table  1percent susceptibility of all agents against mic - based screen - positive ( screen + ) and phenotypically confirmed ( confirmed + ) esbl escherichia 
 coli and klebsiella 
 pneumoniae isolatespercent susceptibility for antimicrobialsc / tfepcrocazcipatmetptzpmemtoball  screen + ( n  =  442)64211018231674537945  confirmed + ( n  =  274)7582914691619342  confirmed + , carbapenem ns removed ( n  =  246)829291371006710042 
 e. coli 
 screen + ( n  =  231)79291324272386699054  confirmed + ( n  =  146)889310101096759847  confirmed + , carbapenem ns removed ( n  =  139)91941110101007810047 
 k. pneumoniae 
 screen + ( n  =  211)471251118962366534  confirmed + ( n  =  128)6171718282468835  confirmed + , carbapenem ns removed ( n  =  107)717171621005210036 
 atm aztreonam , caz ceftazidime , cip ciprofloxacin , cro ceftriaxone , c / t ceftolozane / tazobactam , etp ertapenem , esbl extended spectrum -lactamase , fep cefepime , mem meropenem , mic minimum inhibitory concentration , ns non - susceptible , tob tobramycin , tzp piperacillin / tazobactamfig . 1the mic distribution of c / t and tzp for the mic - based screen - positive isolates ( screen + ) , phenotypically confirmed positive ( confirmed + ) and confirmed + with carbapenem ns isolates removed ( confirmed + , no carbapenem ns ) of all enterobacteriaceae . c / t ceftolozane / tazobactam , mic minimum inhibitory concentration , ns non - susceptible , tzp piperacillin / tazobactamtable  2mic90 ( g / ml ) of all agents against mic - based screen - positive ( screen + ) and phenotypically confirmed ( confirmed + ) escherichia 
 coli and klebsiella 
 pneumoniae isolatesmic90 ( g / ml ) for antimicrobialsc / tfepcrocazcipatmetptzpmemtoball  screen  +  ( n  =  442)12812812812832128325123264  confirmed  +  ( n  =  274)32128128128321280.55120.12564  confirmed + , carbapenem ns removed ( n  =  246)212812812832640.125640.06464 
 atm aztreonam , caz ceftazidime , cip ciprofloxacin , cro ceftriaxone , c / t ceftolozane / tazobactam , etp ertapenem , esbl extended spectrum -lactamase , fep cefepime , mem meropenem , mic minimum inhibitory concentration , ns non - susceptible , tob tobramycin , tzp piperacillin / tazobactam percent susceptibility of all agents against mic - based screen - positive ( screen + ) and phenotypically confirmed ( confirmed + ) esbl escherichia 
 coli and klebsiella 
 pneumoniae isolates 
 atm aztreonam , caz ceftazidime , cip ciprofloxacin , cro ceftriaxone , c / t ceftolozane / tazobactam , etp ertapenem , esbl extended spectrum -lactamase , fep cefepime , mem meropenem , mic minimum inhibitory concentration , ns non - susceptible , tob tobramycin , tzp piperacillin / tazobactam the mic distribution of c / t and tzp for the mic - based screen - positive isolates ( screen + ) , phenotypically confirmed positive ( confirmed + ) and confirmed + with carbapenem ns isolates removed ( confirmed + , no carbapenem ns ) of all enterobacteriaceae . c / t ceftolozane / tazobactam , mic minimum inhibitory concentration , ns non - susceptible , tzp piperacillin / tazobactam mic90 ( g / ml ) of all agents against mic - based screen - positive ( screen + ) and phenotypically confirmed ( confirmed + ) escherichia 
 coli and klebsiella 
 pneumoniae isolates 
 atm aztreonam , caz ceftazidime , cip ciprofloxacin , cro ceftriaxone , c / t ceftolozane / tazobactam , etp ertapenem , esbl extended spectrum -lactamase , fep cefepime , mem meropenem , mic minimum inhibitory concentration , ns non - susceptible , tob tobramycin , tzp piperacillin / tazobactam after confirmatory esbl testing using the clavulanate test , 38% of strains identified as potential esbl producers using a simple mic screen were excluded . for agents with potential activity against esbls such as c / t , tzp , etp , and mem , reduced susceptibility the reason for this discordance is that carbapenemase producers , ampc , and other types of resistance would not be differentiated by mic - based screen testing alone . as such , although mic screening selects for resistant organisms , inclusion of these other genotypes may falsely under - predict the potency of several antibiotics against esbl producers and if such a method is used during published surveillance studies , it may limit applicability of these data to geographic areas not plagued with carbapenemase producers . for this reason , in scenarios , such as the clinical laboratory , when molecular testing is not available , clavulanate - based phenotypic confirmatory testing is recommended to better evaluate empiric therapy options against esbl . david p. nicolau is on the speaker bureau for cubist and has received grants from cubist . this surveillance program was reviewed by the institutional review board ( irb ) at the coordinating center , hartford hospital . since all samples were collected as part of routine medical care and no interventions were undertaken as a result of this testing , informed consent was deemed unnecessary by the irb . this article is distributed under the terms of the creative commons attribution - noncommercial 4.0 international license ( http://creativecommons.org/licenses/by-nc/4.0/ ) , which permits any noncommercial use , distribution , and reproduction in any medium , provided you give appropriate credit to the original author(s ) and the source , provide a link to the creative commons license , and indicate if changes were made .
introductionwhile the clinical and laboratory standards institute ( clsi ) recommends against routine screening for extended spectrum -lactamases ( esbls ) , knowledge of these data can provide valuable insights regarding epidemiology and drug therapy decisions . 
 the purpose of this study was to compare the impact of minimum inhibitory concentration ( mic)-based screening versus phenotypic confirmatory testing of esbls on the susceptibility profile of selected antimicrobials.methodsbroth microdilution mics were determined for various antimicrobial agents against a collection contemporary clinical escherichia coli and klebsiella pneumoniae isolates . 
 isolates identified as esbl - positive by mic screening were then subjected to confirmatory phenotypic testing . 
 percent susceptibility was based on clsi or united states food and drug administration breakpoints.resultsfour-hundred and forty - two ( 18% ) isolates screened positive for esbl production . of these , 
 274 ( 62% ) were confirmed positive for esbl production ; 28 ( 10% ) were also carbapenem non - susceptible . 
 we found an under - prediction of activity for ceftolozane / tazobactam ( c / t ) , ertapenem ( etp ) , meropenem ( mem ) , and piperacillin / tazobactam ( tzp ) when considering only the screen - positive testing.conclusionfor agents with potential activity against esbls such as c / t , tzp , etp , and mem , reduced susceptibility was noted when only considering the mic screen - positive test . 
 although phenotypic screening selects for resistant organisms , inclusion of other genotypes besides esbl ( i.e. , ampc , carbapenemase ) may falsely under - predict the potency against some esbl producers and may limit applicability of surveillance data to geographic areas not plagued with carbapenemase producers.fundingcubist pharmaceuticals.electronic supplementary materialthe online version of this article ( doi:10.1007/s40121 - 015 - 0094 - 6 ) contains supplementary material , which is available to authorized users .