Evaluation of Boronic Acid Disk Tests for Differentiating KPC-Possessing Klebsiella pneumoniaeIsolates in the Clinical Laboratory

ABSTRACTThe worldwide increase in the occurrence and dissemination of KPC ß-lactamases among gram-negative pathogens makes critical the early detection of these enzymes. Boronic acid disk tests using different antibiotic substrates were evaluated for detection of KPC-possessing Klebsiella pneumoniaeisolates. A total of 57 genotypically confirmed KPC-possessing K. pneumoniaeisolates with varying carbapenem MICs were examined. To measure the specificity of the tests, 106 non-KPC-possessing isolates (89 K. pneumoniaeand 17 Escherichia coliisolates) were randomly selected among those exhibiting reduced susceptibility to cefoxitin, expanded-spectrum cephalosporins, or carbapenems. As many as 56, 53, and 40 of the non-KPC-possessing isolates harbored extended-spectrum ß-lactamases, metallo-ß-lactamases, and plasmid-mediated AmpC ß-lactamases, respectively. By use of CLSI methodology and disks containing imipenem, meropenem, or cefepime, either alone or in combination with 400 µg of boronic acid, all 57 KPC producers gave positive results (sensitivity, 100%) whereas all 106 non-KPC producers were negative (specificity, 100%). The meropenem duplicate disk with or without boronic acid demonstrated the largest differences in inhibition zone diameters between KPC producers and non-KPC producers. By use of disks containing ertapenem, all isolates were correctly differentiated except for five AmpC producers that gave false-positive results (sensitivity, 100%; specificity, 95.3%). These practical and simple boronic acid disk tests promise to be very helpful for the accurate differentiation of KPC-possessing K. pneumoniaeisolates, even in regions where different broad-spectrum ß-lactamases are widespread.