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INTERPRETATIVE
READING
Tan Thean Yen
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To
interpret
explain the meaning of (information, words, or actions)
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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!?
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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.
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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
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Description of action to be taken, based on
current evidence, in response to specific
antimicrobial susceptibility test results
E
X
P
E
R
T
R
U
L
E
Expert rule
- Katharine Hepburn
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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
E
X
P
E
R
T
R
U
L
E
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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
E
X
P
E
R
T
R
U
L
E
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Three areas to cover
1. Intrinsic resistance
2. antibiogram
3. Exceptional phenotypes
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INTRINSIC RESISTANCE
Part One
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Chromosomal
(born with it)
I
N
T
R
I
N
S
I
C
R
E
S
I
S
T
A
N
C
E
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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I
N
T
R
I
N
S
I
C
R
E
S
I
S
T
A
N
C
E
Examples Organism Antibiotic resistance
Enterobacteriaceae Vancomycin
Gram-positive organism Aztreonam
Klebsiella species Ampicillin
Proteus mirabilis Nitrofurantoin Polymyxins
Serratia marcescens Polymyxins
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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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N
T
R
I
N
S
I
C
R
E
S
I
S
T
A
N
C
E
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I
N
T
R
I
N
S
I
C
R
E
S
I
S
T
A
N
C
E
http://www.eucast.org/expert_rules/
http://www.eucast.org/expert_rules/
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ANTIBIOGRAM READING
Part Two
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Test susceptibility
Infer resistance mechanism
Interpret clinical susceptibility on the basis
of the resistance mechanism
A
N
T
I
B
I
O
G
R
A
M
R
E
A
D
I
N
G
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E
X
P
E
R
T
R
U
L
E
Species identification
Antibiotic susceptibility
REPORT
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E
X
P
E
R
T
R
U
L
E
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
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Simple..
Staphylococcus aureus
Result: resistant to cefoxitin
Infer: resistance mediated by mecA
Interpret: resistant to all beta-lactams
A
N
T
I
B
I
O
G
R
A
M
R
E
A
D
I
N
G
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More complicated..
Enterobacteriaceae
Result: tobramycin non-susceptible
amikacin & gentamicin susceptible
Infer: acquired -I enzyme
Interpret: report amikacin as Intermediate
A
N
T
I
B
I
O
G
R
A
M
R
E
A
D
I
N
G
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Escherichia coli
A
N
T
I
B
I
O
G
R
A
M
R
E
A
D
I
N
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
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Klebsiella pneumoniae
A
N
T
I
B
I
O
G
R
A
M
R
E
A
D
I
N
G
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
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Limitations
High complexity of resistance mechanisms
Limited information about some mechanisms of resistance
Multifactorial multiresistance
Oversimplification interpretative
Mistakes when deducing mechanisms of resistance
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EXCEPTIONAL PHENOTYPE
Part Three
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Resistance in a species where resistance has not been seen or is
rare
may change with time
may also be regional or national differences
E
X
C
E
P
T
I
O
N
A
L
P
H
E
N
O
T
Y
P
E
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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
E
X
C
E
P
T
I
O
N
A
L
P
H
E
N
O
T
Y
P
E
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Streptococcus pyogenes resistant to penicillin
Staphylococcus aureus resistant to vancomycin
resistant to any third-generation cephalosporin, carbapenems,
and
E
X
C
E
P
T
I
O
N
A
L
P
H
E
N
O
T
Y
P
E
Examples
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Anaerobes resistant to metronidazole
Neisseria gonorrhoeae resistant to third-generation
cephalosporin
Enterobacteriaceae resistant to carbapenems
E
X
C
E
P
T
I
O
N
A
L
P
H
E
N
O
T
Y
P
E
Examples
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HOW TO BE AN EXPERT
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ICU patient 28 days in hospital oxidase negative
non-fermentative Gram-
negative bacillus
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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
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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
