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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S
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S
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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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R
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S
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S
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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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G
R
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G
More complicated..
Enterobacteriaceae
Result: tobramycin non-susceptible
amikacin & gentamicin susceptible
Infer: acquired -I enzyme
Interpret: report amikacin as Intermediate
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T
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B
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R
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G
Escherichia coli
A
N
T
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B
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O
G
R
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M
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E
A
D
I
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G
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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R
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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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X
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O
N
A
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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
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