Role of combination antibiogram in empirical treatment of infection due to multidrug-resistant Acinetobacter baumannii.

To the Editor—We read with interest the report by Mizuta and colleagues on the role of a combination antibiogram for empirical treatment of Pseudomonas aeruginosa infection. The emergence of multidrug-resistant (MDR) gramnegative microorganisms, especially Acinetobacter baumannii, has created prescribing dilemmas for physicians trying to select empirical therapy. In Thailand, the national incidence of MDR-A. baumannii—which is defined as A. baumannii that is resistant to 3 or more classes of antimicrobial agents—peaked at 45% in 2006. Although dual therapy is commonly used when P. aeruginosa infection is suspected, most infectious diseases experts in Thailand also recommend dual therapy for suspected MDR-A. baumannii infections. Given reports that infections with MDR-A. baumannii were associated with higher mortality, one potential option is to use dual or triple empirical antimicrobial therapy. We, therefore, conducted a feasibility assessment to determine the optimal initial therapy for patients with MDR-A. baumannii infection. We identified all hospitalized adults who had A. baumannii isolates recovered at Thammasat University Hospital from January 1, 2007, through December 31, 2007. If multiple A. baumannii isolates were obtained from the same patient during the same hospitalization, only the first isolate was evaluated. We used criteria suggested by the Clinical and Laboratory Standards Institutes to identify A. baumannii and to establish antimicrobial susceptibility profiles. Antimicrobial susceptibility testing was performed with conventional susceptibility microbroth dilution trays, and we tested susceptibility for the following antimicrobials: gentamicin, amikacin, netilmicin, cefepime, ceftazidime, cefoperazone-sulbactam, ampicillin-sulbactam, ciprofloxacin, imipenem, meropenem, piperacillin-tazobactam, and colistin. We created a standard antibiogram for A. baumannii isolates, along with annual combination antibiograms created in a matrix fashion, which listed the antimicrobial agents tested both horizontally and vertically, as had been previously done by Mizuta and colleagues. In each matrix box of the combination antibiogram, we noted the percentage of isolates susceptible to at least 1 of the 2 agents (Table). The backbone of noncolistin-based, 2-antibiotic regimens for triple antimicrobial agents was selected from the 2 antibiotics to which MDR-A. baumannii had the highest percentage of susceptibility: cefoperazone-sulbactam (32%) and netilmicin (27%). There were 560 A. baumannii isolates identified during the study period, of which 381 (68%) were recovered from urine, 45 (8%) from blood, and 134 (24%) from other sites; 218 isolates (39%) were MDR-A. baumannii. The majority of isolates (308 [55%]) were recovered from intensive care unit patients. Antimicrobial susceptibility for A. baumannii as shown by the combination antibiogram (dual vs triple antibiotics) revealed that the combinations with the broadest coverage were consistently colistin-based regimens (100%), whereas cefoperazonesulbactam and netilmicin (54%) provided the broadest coverage among noncolistin-based regimens (Table). Triple combinations of imipenem, cefoperazone-sulbactam, and