INTERPRETATIVE

READING

Tan Thean Yen

To

interpret

explain the meaning of (information, words, or actions)

What Women Say What It Means

Yes No

No Yes

You have to learn to communicate...

Just agree with me.

Are you listening to me!?

To

analyse susceptibility results

based: organism identification

all antibiotic results (pattern)

interpret susceptibility based on above

Courvalin P. Interpretive reading of in vitro antibiotic susceptibility tests (the antibiogramme).

Clinical Microbiology and Infection. 1996;2:S26S34.

Why interpret?

To detect unusual results that need further testing or confirmation

To report appropriate antibiotic results for the organism-infection combination

To detect emerging resistance

Description of action to be taken, based on

current evidence, in response to specific

antimicrobial susceptibility test results

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Expert rule

- Katharine Hepburn

Actions

Recommendations on reporting

Inference of susceptibility

Edit results from S to I or R, from I to R, but NEVER I or R to S

Suppression of results

Addition of comments

Advice on further tests

Advice on referral of isolates

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Requirements

Identification of the organism (fully)

May need to test an extended range of appropriate antibiotics

Access to a set of expert rules

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Three areas to cover

1. Intrinsic resistance

2. antibiogram

3. Exceptional phenotypes

INTRINSIC RESISTANCE

Part One

Chromosomal

(born with it)

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resistance which is a characteristic of the species

results?

errors in identification or susceptibility testing

drug should be used with caution

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Examples Organism Antibiotic resistance

Enterobacteriaceae Vancomycin

Gram-positive organism Aztreonam

Klebsiella species Ampicillin

Proteus mirabilis Nitrofurantoin Polymyxins

Serratia marcescens Polymyxins

Livermore DM, et al. Interpretative reading: recognizing the unusual and inferring resistance mechanisms from resistance phenotypes.

J Antimicrob Chemother. 2001 Jul 1;48(suppl 1):87 102

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http://www.eucast.org/expert_rules/

http://www.eucast.org/expert_rules/

ANTIBIOGRAM READING

Part Two

Test susceptibility

Infer resistance mechanism

Interpret clinical susceptibility on the basis

of the resistance mechanism

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Species identification

Antibiotic susceptibility

REPORT

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Species identification

Antibiotic susceptibility

Antibiogram interpretation

REPORT

deduce phenotype

deduce biochemical resistance

clinical relevance

additional tests

1. re-define susceptibility (categorical)

2. deduce susceptibility to non-tested antibiotics

3. clinical comments 4. clinical advice

Simple..

Staphylococcus aureus

Result: resistant to cefoxitin

Infer: resistance mediated by mecA

Interpret: resistant to all beta-lactams

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More complicated..

Enterobacteriaceae

Result: tobramycin non-susceptible

amikacin & gentamicin susceptible

Infer: acquired -I enzyme

Interpret: report amikacin as Intermediate

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Escherichia coli

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Antibiotic Category

Ampicillin R

Amoxicillin-clavulanate S

Piperacillin-tazobactam S

Cephalothin R

Ceftriaxone R

Ceftazidime S

Cefepime R

Cefoxitin S

Ertapenem S

Gentamicin S

Amikacin S

Ciprofloxacin S

Trimethoprim-sulfamethoxazole S

Infer: Extended spectrum beta-lactamase (CTX-M) Interpret: Consider reporting ceftazidime as resistant OR Reporting presence of ESBL with comment

Klebsiella pneumoniae

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Infer: ampC enzyme (plasmid-acquired) Interpret: Consider reporting ceftriaxone as resistant

Antibiotic Category

Ampicillin R

Amoxicillin-clavulanate R

Piperacillin-tazobactam S

Cephalothin R

Ceftriaxone I

Ceftazidime R

Cefepime S

Cefoxitin R

Ertapenem S

Gentamicin S

Amikacin S

Ciprofloxacin S

Trimethoprim-sulfamethoxazole S

Limitations

High complexity of resistance mechanisms

Limited information about some mechanisms of resistance

Multifactorial multiresistance

Oversimplification interpretative

Mistakes when deducing mechanisms of resistance

EXCEPTIONAL PHENOTYPE

Part Three

Resistance in a species where resistance has not been seen or is rare

may change with time

may also be regional or national differences

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Resistance in a species where resistance has not been seen or is rare

errors in identification or susceptibility testing

send the isolate to a reference laboratory for independent confirmation

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Streptococcus pyogenes resistant to penicillin

Staphylococcus aureus resistant to vancomycin

resistant to any third-generation cephalosporin, carbapenems, and

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Examples

Anaerobes resistant to metronidazole

Neisseria gonorrhoeae resistant to third-generation cephalosporin

Enterobacteriaceae resistant to carbapenems

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Examples

HOW TO BE AN EXPERT

ICU patient 28 days in hospital oxidase negative non-fermentative Gram-

negative bacillus

ICU patient 28 days in hospital oxidase negative non-fermentative Gram-

negative bacillus

Antibiotic Zone

(mm)

Result

Amoxicillin-clavulanate 6 R

Piperacillin-tazobactam 6 R

Ceftriaxone 6 R

Ceftazidime 26

Cefepime 18

Imipenem 6 R

Ertapenem 6 R

Meropenem 6 R

Amikacin 6 R

Gentamicin 6 R

Ciprofloxacin 30

Trimethoprim-

sulfamethoxazole

35

What is the likely organism? How would you report

susceptibility to: ceftazidime imipenem ciprofloxacin

Antibiotic Zone

(mm)

Result

Amoxicillin-

clavulanate

6 R

Cefoxitin 6 R

Ceftriaxone 27 S

Ceftazidime 26 S

Cefepime 24 S

Imipenem 24 S

Ertapenem 23 S

Meropenem 24 S

Ciprofloxacin 25 S

Trimethoprim-

sulfamethoxazole

20 S

Enterobacter cloacae

Isolate from blood culture Patient with suspected ventilator-

associated pneumonia