by Samuel Cohen, MS (ASCP) and David Gibbs, Ph.D.
Microbiology testing based on NCCLS standards with the Biomic Vision Antibiotic and ID Tree
Reliability and rapidity of identification and susceptibility testing are of growing, and sometimes competing concern, as the threat of drug-resistant bacteria worsens. The National Committee on Clinical Laboratory Standards (NCCLS) in Wayne, Penn., has published numerous performance standards and guidelines for clinical microbiology laboratories, including M2-A7 and M100-S12 Antimicrobial Disk Susceptibility Tests (2002), M31 Antimicrobial Disk and Dilution Susceptibility Tests for Bacteria Isolated from Animals (1999), M39 Analysis and Presentation of Cumulative Antimicrobial Susceptibility Test Data (2002), M35 Abbreviated Identification of Bacteria and Yeast (2000), and the upcoming NCCLS Disk Susceptibility Test for Yeast (in 2003). The focus of these documents is to clearly present practical, relevant, and low-cost, non-proprietary methods, materials and procedures. NCCLS recommendations carry significant weight in the global microbiology community because of their objectivity.
the Biomic Vision microbiology system
Rapid simple test identification (ID) schemes, based on NCCLS M35 recommendations to identify species of bacteria and yeast in minutes for a few cents, are in standard use in many laboratories. Many hospitals identify over 80 percent of their isolates by these methods, which have been in common use for many years. NCCLS endorsement has given them greater credibility. Many susceptibility tests are performed on E. coli, Klebsiella and Citrobacter/Enterobacter species with predictable susceptibility patterns from urine, respiratory tract and wounds. These rapid final IDs can be reported earlier than card/panel ID systems, and knowing the species and cumulative antibiogram in an institution has more impact on initial therapy than rapid susceptibility results.
Rapid antibiotic susceptibility test (AST) results may be reported from some card/panel systems about 4 hours after isolation, or the afternoon of the day after the clinical specimen(s) was taken (24 + 4 = 28 hours total). Rapid AST results are under scrutiny, however, as to whether they can consistently and accurately detect resistance during this limited incubation period; a longer incubation may be required to express some mechanisms (e.g., ESßL). Though rapid results are desirable, accuracy and reliability in test results for susceptibility are of critical importance. False “S” may be reported too frequently. Users of automated panel /card systems often accept published discrepancy rates of five to 10 percent, whereas microbiologists using NCCLS M2-A7 disk or NCCLS broth tests have flexibility to see and adjust recorded endpoints. It is common when questioning a ‘walkaway’ system test result to determine the accurate result using the disk test or Etest. NCCLS disk tests also enable full flexibility to select the drugs tested, which can be important to accommodate changing formularies. Precise MICs, rather than breakpoint MICs from most cards/panels, have a higher reliability for dosing critically ill patients with toxic agents.
Antimicrobial therapy is usually initiated empirically before sending specimens to the laboratory, often guided by cumulative AST results. The ability to report rapid AST results, usually about 28 hours after starting therapy and 12 hours earlier than standard AST, is cited as allowing physicians to check and modify therapy based on individual isolate AST results. In practice, antibiotic changes based on AST results are infrequent. Recent studies that look at total patient costs in two groups receiving rapid versus standard AST results, rather than considering the actual number of antibiotics “changed after receiving” results, may erroneously assume that cost differences in the two groups are due to “wrong therapy corrected” after receiving rapid AST results 12 hours earlier than the standard. Further, if rapid card/panel AST results are unable to reliably detect resistance in 4+ hours, laboratories may still need to wait for standard overnight individual AST results as a final check on appropriate therapy, based on knowledge of the difficulties involved in expressing some resistance mechanisms. Initial empiric therapy may also be checked sooner based on species identification. If 80 percent of IDs can be reported minutes after isolation by rapid NCCLS methods versus 4 hours later by automated card/panel systems, NCCLS methods are in practice more rapid.
With NCCLS M35 guidelines, IDs can be followed by NCCLS antibiotic disk tests on bacteria and yeast, and result in an accurate AST and ID averaging just over $1 total per isolate. NCCLS recommended AST and ID methods also provide maximum flexibility to customize antibiotics to changing formularies and customize species identification to physician and laboratory needs.
Setup-labor time for disk test plates and automated AST-ID panel-card systems is comparable; but disk test zone reading and interpretation has been tedious and time consuming. With Biomic instantaneous plate reading, Expert-checking, and laboratory-information-system (LIS) interfacing, NCCLS disk tests may be accomplished with labor comparable to walk-away systems, and at lower the cost.
The Biomic Vision
The Biomic Vision from Giles Scientific, Inc. in Santa Barbara, Calif., is a complete microbiology laboratory system for antimicrobial susceptibility testing and species identification based on the above six NCCLS clinical microbiology published guidelines. The Biomic utilizes color-digital camera and PC technology to enable more efficient use of established NCCLS methods. It instantly reads and interprets bacteria and yeast test plates, including fastidious organisms on blood agar. The Epidemiology-Infection Control reports are consistent with the NCCLS M39.
The Biomic System software is flexible and user friendly. With a built-in two-way LIS interface, it provides test results with labor more comparable to “walk-away” card/panel systems than conventional methods.
All Biomic test results are automatically and instantaneously checked against an extensive NCCLS based Expert System. Predefined rules include improbable and unusual results, tests for ESßL screening and confirmation, other resistance mechanisms, uncontrolled results, QC rules, tips, and precautions. On-screen messages improve accuracy, reduce supervisory time, and provide ongoing training. Laboratories can select and define rules, add comments, and automatically suppress specified test results. Plate images are saved for the working day to enable review of plate images, test results and Expert messages at the supervisor’s convenience.
Biomic image analysis of zones provides precise and accurate MICs from the continuous agar gradient around each disk. Biomic MICs are much like Etest results in that both are calibrated against NCCLS reference dilution method MICs for accuracy. The NCCLS M2A7 states, “Disk diffusion zone diameters correlate inversely with MICs from standard dilution tests.” Biomic MICs have been determined by the FDA to be substantially equivalent in accuracy and reproducibility to the reference NCCLS method MICs.
The IDTree program included with Biomic is based on NCCLS M35 Abbreviated Identification of Bacteria and Yeast Guidelines. IDTree flowcharts outline the minimal characteristics required to identify most common bacteria and yeast to genus and or species with >= 95% accuracy using non-proprietary methods and materials.
Cost savings for the Biomic system can be computed on worksheets on the Biomic website, www.biomic.com. Biomic-read disk test results can be reviewed anytime that day by a technologist or supervisor with the actual plate image, interpretation and Expert messages on-screen, which may lower the skill level required for users.
Many automated broth system users find that testing urine isolates alone, representing about 50 percent of isolates, by NCCLS rapid-ID and disk methods and Biomic, reduces costs dramatically. Reimbursement costs may be an issue for outpatient tests, but inpatient costs are based on Diagnosis Related Grouping (DRG) and cost savings are immediately accrued as hospital savings.
A significant advantage of following NCCLS recommended guidelines and methods is their accuracy, flexibility and use of non-proprietary materials. The Biomic Vision with IDTree enhances these advantages with instant plate reading.
Sam Cohen is microbiology supervisor at ARUP Laboratories, and David Gibbs is president of Giles Scientific in Santa Barbara, Calif.