In Defense of Routine Antimicrobial Susceptibility Testing of Operative Site Flora in Patients with Peritonitis

S254 In Defense of Routine Antimicrobial Susceptibility Testing of Operative Site Flora in Patients with Peritonitis Samuel Eric Wilson and Joseph Huh From the Department of Surgery, University of California, Irvine, Irvine, California The species and number of bacteria present at a surgical site correlate with postoperative wound infection. When organisms cultured from intraabdominal infections are resistant to the presumptive antimicrobial therapy, the incidence of postoperative wound and intraabdominal infections is significantly increased. Knowledge of operative site culture data allows identification of resistant organisms, leads to an early change in therapy, and guides selection of antimicrobials for treatment of postoperative complications. Anaerobic susceptibility data vary geographically, even differing within hospitals in the same city. Surveillance of resistance patterns of bacteria causing intraabdominal infections facilitates accurate initial therapy. Failure of treatment in the absence of bacteriologic results confirming appropriate antimicrobial therapy may be difficult to rationalize on a medicolegal basis. In summary, it is advisable for surgeons to perform cultures and susceptibility tests for both aerobic and anaerobic organisms present in intraabdominal infections. Clinicians and microbiologists have engaged in a vigorous debate on whether testing the antibiotic susceptibilities of bacterial isolates obtained from purulent fluid within the peritoneal cavity or abdominal incision (operative sites) during an operation for peritonitis is indicated routinely [1]. Some infectious disease specialists have argued that since the susceptibilities of anaerobes are reasonably predictable, periodic reference testing should be adequate for selecting antibiotics for the treatment of anaerobic infections [2]. Even among surgeons, there are differing opinions regarding routine antibiotic susceptibility testing. One of us attended a scientific meeting in 1996 at which an informal survey of 60 surgeons experienced in the treatment of intraabdominal infections revealed that only 36% routinely performed cultures and susceptibility testing for patients with peritonitis. Up to 18% indicated that they never obtained specimens for culture. A retrospective survey of 480 patients with secondary bacterial peritonitis treated in Albuquerque, New Mexico, showed that surgeons typically ignored culture data [3]. In a 1989 study on complicated appendicitis, Dougherty et al. [4] discovered that culture reports influenced antimicrobial therapy for only 7% of patients. Even if the surgeon decides to obtain a speciman for culture, anaerobic microbiology may not be available to the clinician in many medical centers. According to a 1995 survey of United States hospital laboratories, 77% of these laboratories did not routinely test anaerobic susceptibilities, and 59% would not offer the susceptibility testing even if an individual physician re- This work was presented at the annual meeting of the Anaerobe Society of the Americas held on 19 July 1996 in Chicago. Reprints or correspondence: Dr. Samuel E. Wilson, Department of Surgery, UCI Medical Center, Building 53, Room 208, 101 The City Drive South, Orange, California 92868. Clinical Infectious Diseases 1997;25(Suppl 2):S254–7 q 1997 by The University of Chicago. All rights reserved. 1058–4838/97/2503–0053$03.00 / 9c36$$se69 quested it [5]. These figures show a decline from those in a 1993 survey showing that only 30% of hospital laboratories did not perform anaerobic susceptibility testing [6]. Internists as much as surgeons have come to rely on ‘‘the primacy of drainage procedures’’ or debridement in determining the outcome of anaerobic infections, which deemphasizes the importance of knowledge of individual pathogens [2]. Antimicrobial therapy is considered adjunctive to the intervention and thus is not directed at specific virulent organisms. In this article, we will address the causes of this drift toward incomplete culture and susceptibility testing for mixed infections, and we will present the case for more general use of these procedures. Methods of Susceptibility Testing Lack of physician confidence in the ability of the clinical microbiological laboratory to accurately identify pathogens in surgical specimens and variations in antimicrobial susceptibility data have been cited as reasons why surgeons ignore culture data [3]. Indeed, before 1980, 15% – 20% of cultures of surgical infections yielded no microbial growth, but with improvements in methods of collection, transport, and culture, anaerobic organisms have been recovered in circumstances where routine cultures previously did not yield any identifiable bacteria [7]. The reasons for selecting certain techniques, as well as the differing results in susceptibility testing according to the laboratory method that was chosen, are poorly understood by the surgical community and deserve explanation. For example, of the three generally accepted methods of testing bacterial susceptibilities, the agar dilution method preferred by many microbiologists is labor intensive. The broth microdilution method reduces the workload, although it limits susceptibility testing to the antibiotics and the concentrations available in commercially prepared tray panels [8]. The disk diffusion method, with the longer incubation period for anaerobes and the unsteady levels of antibiotic gradients over time, has not been endorsed by the 08-11-97 22:52:09 cidal UC: CID CID 1997;25 (Suppl 2) Routine Antimicrobial Testing S255 National Committee for Clinical Laboratory Standards (NCCLS). Several authors, however, have shown reproducible results [8], and many smaller clinical laboratories use the disk diffusion method because it is simple and inexpensive [5, 6]. The Etest (Epsilometer test, AB BIODISK, Solna, Sweden), a modification of the disk method that establishes stable antibiotic levels surrounding an antibiotic-coated plastic strip, is largely unknown to surgeons [9]. The strip is placed on an agar plate streaked with an organism and is incubated for 24 – 48 hours. An inhibition zone is established around the strip, and the MICs can be read directly from the strip. Correlation of the results with those of the agar dilution technique has generally been good; however, before clinical use of the Etest in anaerobic susceptibility testing is accepted, more experience with the optimal inoculum size, medium, and duration of incubation needs to be determined [10, 11]. Surgeons should know that method-dependent factors also affect susceptibility data. The type of medium used may not support all organisms present in a clinical specimen. WilkinsChalgren agar has been recommended as a medium for anaerobes by the NCCLS; however, not all anaerobic organisms are recovered from Wilkins-Chalgren agar [12]. MICs may vary depending on the test method. Aldridge and Schiro [13] have shown that MICs of c (...truncated)