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Quality Assurance of Antimicrobial Susceptibility Testing

Quality Assurance of Antimicrobial Susceptibility Testing. Gunnar Kahlmeter Dept of Clinical microbiology, Växjö, Sweden The National Institute for Infectious Disease Control SRGA & SRGA-M, BSAC and EUCAST. Antimicrobial susceptibility testing is about short-term and

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Quality Assurance of Antimicrobial Susceptibility Testing

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  1. Quality Assurance of Antimicrobial Susceptibility Testing Gunnar Kahlmeter Dept of Clinical microbiology, Växjö, Sweden The National Institute for Infectious Disease Control SRGA & SRGA-M, BSAC and EUCAST

  2. Antimicrobial susceptibility testing is about short-term and long-term credibility!Surprising findings or trends will be questioned!

  3. Standardisation/Calibration of method Routine quality control Education Quality Assurance Expert rules Internal Quality Assessment (specimen reprocessing) External Quality Assessment Adapted from Derek Brown

  4. Standardisation and calibrationof method • Standardised methodology (BSAC, CA-SFM, DIN, NCCLS, SRGA) • QC-strains with target values • Reference MIC-distributions of wild type bacteria (on www.eucast.org) • Reference zone diameter distributions of wild type bacteria (on SRGA website and within the BSAC system)

  5. E.coli ATCC 25922 Uppdaterad 2000-04-18

  6. Reference MIC-distributions onwww.eucast.org

  7. Reference MIC-distributions onwww.eucast.org

  8. Reference zone diameter distributions throughSRGA and BSAC Data by Trevor Winstanley

  9. Reference zone diameter distributions through SRGA and BSAC

  10. E.coli vs. gentamicin 30 µgn=2478 from 17 countries S/R 21/17

  11. Discrepancies discovered in the validation procedure - analysis and suggested measures • - one or more of the distributions deviate from what is expected (the median is off, the distribution is wider than expected, the part of the distribution that ought to be unimodal is bimodal, etc) the pattern of the deviation can be used to diagnose the problem: • are all basic parameters alike for the two histograms (disk strengths, additives, atmosphere, calibration of the CO2-incubator, temperature etc). • are species-identification procedures equivalent? • do the aberrations occur with media containing supplements? or with media or disks sensitive to storage? or with antibiotics sensitive to changes in pH (aminoglycoside, erythromycin)? or with antibiotics sensitive to the autoclaving of the medium (nitrofurantoin).

  12. Zone diameter distributions from 4 Swedish laboratories illustrating different methodological ”outputs”. Reference SRGA distribution Median

  13. Discrepancies discovered in the validation procedure - analysis and suggested measures • If the majority of the distributions deviate from the expected there should be a pattern: • inhibition zones smaller than the reference histograms: inoculum too dense or incorrect agar volume in the plates (depth >4 mm). • inhibition zones larger than the reference histograms: incorrect agar volume in the plates (depth <4 mm).

  14. Standardisation/Calibration of method Routine quality control Education Quality Assurance Expert rules Internal Quality Assessment (specimen reprocessing) External Quality Assessment Adapted from Derek Brown

  15. Routine quality control • QC-strains with target values (target values, permitted range, trend analysis in Shewart diagrams)- general: overall proficiency check- specific: rare organisms N.gonorrhoeae, H.pylorii, fungi etc • Serial analysis of zone diameter distributions (or MIC-distributions)

  16. E.coli ATCC 25922 Uppdaterad 2000-04-18

  17. Period of calibration

  18. Variation attributable to the lab.technician (the inoculum) One of my oldest friends – but her inocula were always a bit too dense!

  19. G Kahlmeter, SRGA & SRGA-M

  20. Discrepancies in results with quality control strains - analysis and remedies Random errors cannot be avoided but shall be controlled. Reference strains have been assigned target values and acceptable intervals for random variation. Occasional deviation can be accepted but trends and recurring patterns should be investigated, explained and corrected.

  21. Discrepancies in results with quality control strains - analysis and remedies Systematic errors (systematic high or low values, or a trend) should be investigated and corrected. - wrong strain- disc content- volume of medium- pH of medium - atmosphere and incubation temperature)- aging of plates If this does not explain and solve the problem the most probable cause is the density of the inoculum.

  22. A change in the ion-content of ISA (Oxoid)

  23. Routine quality control • QC-strains with target values (target values, permitted range, trend analysis in Shewart diagrams)- general: overall proficiency check- specific: rare organisms N.gonorrhoeae, H.pylorii, fungi etc • Serial analysis of zone diameter distributions (or MIC-distributions)

  24. Internal QA Serial analysis of zone diameter distributions The regular analysis of zone diameter distributions of consecutive clinical isolates provide good internal quality asessment: median(accuracy) width (reproducibility) shape (tells you all sorts of things about the drug, species, lab)

  25. A high degree of reproducibility! A high degree of credibility!

  26. Epidemic in the Lagan area

  27. Standardisation/Calibration of method Routine quality control Education Quality Assurance Expert rules Internal Quality Assessment (specimen reprocessing) External Quality Assessment Adapted from Derek Brown

  28. Expert rules are an important part of QA Penicillin resistance does not exist in S.pyogenes- ”faulty disk” Ampicillin resistance in E.faecalis is very rare- ”erroneous species identification” Methicillinresistance in Staphylococci should not permit an ”S” for other betalactam antibiotics- ”don´t do them – you´ll get it wrong!” Mistrust clarithromycin ”S” if erythromycin ”R”- ”someone got the breakpoint wrong”

  29. Standardisation/Calibration of method Routine quality control Education Quality Assurance Expert rules Internal Quality Assessment (specimen reprocessing) External Quality Assessment

  30. External Quality Assessment • The distribution of strains with known antimicrobial resistancies or MIC-values - NEQAS, EQUALIS, RING, LABKA etc - panel of experts to chose strains and to make the analysis of results and prepare comments • The analysis of yearly distributions of the quantitative results of consecutive clinical isolates – combined EQA and resistance surveillance.

  31. EQA in NEQAS and EARSS- please refer to NEQAS and EARSS reports

  32. Sweden 100 consecutive patient isolates per bacterium and antibiotic and year and lab. Each lab enter zone diameters over internet. QA - the distributions are compared (median, width, scew) with reference. Resistance surveillance - using SRGA breakpoints the resistance frequencies calculated and tabulated

  33. S/R 18/17 • Each laboratory is asked to test 100 consecutive patient strains of defined species (3 - 5) against definedantibiotics (4 - 6). • The zones are entered over the internet (see www.srga.org) • 30 labs x 100 strains = 3000 strains per species & antibiotic & year with good geographical distribution.

  34. ResNet

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