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




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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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???


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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://web.med.unsw.edu.au/ (Sixth Edit ion)


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CLSI


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CLSI


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EUCAST


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BSAC


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BSAC

CDS


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CDS



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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/

Principles and Practice of AST

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