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7/28/2019 Principles and Practice of AST
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Principles and Practice of
Antimicrobial Susceptibility Testing
Microbiology Technical Workshop
25
th
September 2013
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Scope History
Why Perform Antimicrobial Susceptibility Testing?
How to Perform an Antimicrobial Susceptibility Test Isolate
Method (phenotypic, special tests, genotypic) Drug Standards (how breakpoints are derived; CLSI, EUCAST, BSAC, CDS)
Understanding An Antimicrobial Susceptibility Test
Categorical interpretation 90-60 rule
Summary
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History
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Source: http://www.biography.com/
Source: http://www.s1darvel.com/
Sir Alexander Fleming
(1881-1955)
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When I woke up just after dawn onSeptember 28, 1928, I certainly didn't plan
to revolutionize all medicine by discovering
the world's first antibiotic, or bacteria killer.But I suppose that was exactly what I did.
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Why Perform Antimicrobial
Susceptibility Testing?
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EmpiricTherapy
InvestigationsDefinitiveTherapy
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Patient LAM 89 year old Chinese female
Previously hospitalized 2 months ago for cystitis Urine culture ESBL positive E. coli
Blood cultures negative
Now admitted from the A&E with fever, dysuria andincreased urinary frequency for 3 days Urine microscopy RBC 3, WBC > 2000, EC 0
Urine culture no bacterial growth
Started empirically on IV Piperacillin/tazobactam
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Predict treatment outcome
Guide selection of most appropriate agent
Provide alternatives drug allergy, oral option
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How to Perform an Antimicrobial
Susceptibility Test
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Results
Standards
Method Drug
Isolate
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Isolate Identity of bacterial isolate
Normal flora
Clinically significant number of colonies
Predictable antibiotic susceptibility profile
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Method Phenotypic
Zone diameter
Minimum inhibitory concentration (MIC)
Special tests
Genotypic
Source: CLSI M100-S23E Performance Standards for Antimicrobial Susceptibility Testing - 23rd Informational Supplement
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Source: htt p://www.cdc.gov/
Source: htt p://www.cdc.gov/
Disk Diffusion
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Source: http://bacterioweb.univ-fcomte.fr/
Etest (Epsilometer test)
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Source: http://www.cdc.gov/
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Broth Macrodilution
Source: http://web.carteret.edu/ Source: http://web.carteret.edu/
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Automated (Vitek2)
Source: www.biomerieux.com
Source: www.biomerieux.com
Source: www.biomerieux.com
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7/28/2019 Principles and Practice of AST
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Source: http://microblog.me.uk/
D-Test for inducible clindamycin resistance
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Clavulanic acid Cephalosporin
ESBL Detection (double-disk approximation)
Cephalosporin
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Source: http://www.cepheid.com/
Source: http://www.pih.org/ Source: http://www.pih.org/
Source: http://www.ahsoman.com/
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Drug Species to be tested
Institution formulary
Commonly used antimicrobials
Availability of antimicrobial agent for testing
Tailored to specific needs of institution Infectious disease physicians
Clinical microbiologists Pharmacists
Committees concerned with institutional formulary
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Source: http://www.eucast.org/
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Standards MIC distribution
Wild-typeEpidemiological cut-off (ECOFF)
Pharmacokinetic
Pharmacodynamic
Clinical data
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0
10
20
30
40
50
60
70
80
0.125 0.25 0.5 1 2 4 8 16 32 64 128 25 6
increasing concentration of antibiotic
susceptible resistant
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0
10
20
30
40
50
60
70
80
0.125 0.25 0.5 1 2 4 8 16 32 64 128 25 6
increasing concentration of antibiotic
susceptible resistant
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Source: http://mic.eucast.org/
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Source: http://mic.eucast.org/
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Source: http://mic.eucast.org/
???
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Standard Location Media Inoculum
CLSI America, several
areas of Europe,
Asia, Australia
Mueller-Hinton agar
Mueller-Hinton agar with 5% sheep blood
0.5 McFarland
EUCAST Europe Mueller-Hinton agar
Mueller-Hinton agar with 5% defibrinated horse
blood + 20 mg/L -NAD
0.5 McFarland
BSAC United Kingdom Iso-Sensitest agar
Iso-Sensitest agar with 5% defibrinated horse
blood + 20 mg/L NAD
0.5 McFarland
then dilute
(refer to Table)
CDS Australia Sensitest agarSensitest agar with 5% horse blood
Refer to Figure
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Source: http://bsac.org.uk/ (Version 12 May 2013)
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Source: http://bsac.org.uk/ (Version 12 May 2013)
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Source: http://bsac.org.uk/ (Version 12 May 2013)
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Source: http://web.med.unsw.edu.au/ (Sixth Edit ion)
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Source: http://web.med.unsw.edu.au/ (Sixth Edit ion)
CLSI
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Source: CLSI M100-S23E Performance Standards for Antimicrobial Susceptibility Testing - 23rd Informational Supplement
CLSI
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Source: CLSI M100-S23E Performance Standards for Antimicrobial Susceptibility Testing - 23rd Informational Supplement
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Source: http://www.eucast.org/
EUCAST
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Source: http://www.eucast.org/
BSAC
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Source: http://bsac.org.uk/ (Version 12 May 2013)
BSAC
CDS
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CDS
Source: http://web.med.unsw.edu.au/ (Sixth Edit ion)
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Source: http://web.med.unsw.edu.au/ (Sixth Edition)
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Source: http://web.med.unsw.edu.au/ (Sixth Edit ion)Source: http://web.med.unsw.edu.au/ (Sixth Edit ion)
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Understanding An AntimicrobialSusceptibility Test
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Identify
Test
Report
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Categorical InterpretationCategory Interpretation
Susceptible High likelihood of therapeutic success
Resistant High likelihood of therapeutic failure
Intermediate Uncertain therapeutic effect
Drug concentration at body sites
Buffer zoneSusceptible-dose dependent (S-DD) Antifungal susceptibility testing
Susceptibility dependent on achieving
maximal possible blood level
Nonsusceptible Often seen with new antimicrobialagents
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Source: Rex and Pfaller CID 2002 -Has Antifungal Susceptibility Testing Come of Age?
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Summary
Antimicrobial susceptibility tests allows us to Predict treatment outcome
Guide selection of most appropriate agent Provide alternatives
Factors to consider
Isolate Method
Drug
Standards
Understanding the results Categorical interpretation
90-60 rule
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Thank you for your attention!Dr Jamie Tan ([email protected])
Registrar
Department of Pathology
Singapore General Hospital
Source: http://www.channelnewsasia.com/