Where is sensitivity report on drug xyz?

Microbiology Laboratory frequently receives above and other similar questions from our clients or their representatives. It is the goal of this newsletter to provide answers to such questions as accurately as possible. One of the most common questions being asked is – “Your final report has organism and sensitivity results, but drug xyz is not listed. I have the patient on antibiotic xyz. How do I know if the antibiotic I am using will work?”

Antimicrobial susceptibility testing is far more complex than merely mixing the bacteria and antibiotic together, incubating the mixture, and determining if the organism grows or not in the presence of a specific antibiotic. Most clinical laboratories in the USA are licensed and regulated by U. S. Department of Health and Human Services (HHS) under Clinical Laboratories Improvement Act (CLIA). CLIA in turn uses national standards developed by joint efforts of authoritative organizations/agencies/persons, e.g. Federal Drug Administration (FDA), The Centers for Disease Control and Prevention (CDC), College of American Pathologists (CAP), pharmaceutical and instrument manufacturers, physicians, and laboratory scientists, to regulate clinical laboratories. These standards are compiled, updated and published by Clinical and Laboratory Standards Institute (CLSI). Due to rapid paced development of new technologies and tests, and equally rapid emergence of antimicrobial-resistant strains of microbes, some standards have to be revised annually. Thus, the standards for antimicrobial susceptibility testing are revised each year. The performance standards for antimicrobial susceptibility testing are based upon many factors, e.g., if the antimicrobial agent has been approved for use in the USA, if reliable testing methods are available to test its effectiveness, if the laboratory results can be directly applied for its use on human subjects, if increased resistance to specific antimicrobial has been observed in some or all types of bacteria, etc. These standards, in effect, are the “best practice” standards for antimicrobial susceptibility testing.

Doctors Laboratory follows the most current CLSI standards, and thus revises its testing protocol annually, not only to comply with CLIA regulations, but also to provide the most current antimicrobial treatment guidance to medical practitioners. At each revision, newer or more effective antimicrobials may be added for testing, and less effective or problematic antimicrobials may be removed from recommendation lists against some or all organisms. This may help explain why a certain antimicrobial may no longer be reported against certain bacterium. If an interpretative standard for a certain antimicrobial against certain microbe is not published in a CLSI manual, the laboratory has no way to test or report that antimicrobial against that microbe. One recent example of this is no testing of Enterococcus isolate against Trimethoprim/Sulfa. (Bactrim or Septra or SXT), where no interpretative guidelines are published in 2009 CLSI manual, because SXT is not clinically effective to treat infections caused Enterococci. Sometimes, laboratory testing for a specific drug/bug combination is not reliable enough to apply under actual patient treatment situation, or in vivo. Due to complexities of antimicrobials, microorganisms and their ever changing interactions, CLSI standards typically have many exceptions and fine prints that the laboratory has to apply before test results are released to clinicians. For example, even though over 20 antimicrobials may be tested and reported against Enteric Gram Negative Bacilli group, which includes E. coli, Salmonella and Shigella, CLSI standards recommend that, if the isolate is a Salmonella or Shigella from fecal source, only three antimicrobials (Ampicillin, Trimethoprim/Sulfamethoxazole, and a fluoroquinolone) be reported.

Due to similarities in their effectiveness, often a “class or group representative” drug is tested and reported, e.g. Tetracycline to represent Doxycycline. Since not all antimicrobials can be tested due to space limitations, all laboratories test and report limited antimicrobials, selected from “best practice” data, and prevalence in the communities they serve. Our attempt at Doctors Laboratory continues to be to provide its clients with as wide a selection of oral and IM/IV antimicrobials, and include representatives from as many drug groups as possible, to meet the needs of its highly diverse clientele, from hospitals to nursing homes to out-patient clinics. The attached table lists antimicrobials currently reported by Doctors Laboratory against most common groups of bacteria.

Micro. Matters August 2009

Prafull C. Shah