A laboratory that follows EUCAST tests a Pseudomonas aeruginosa blood isolate from a ventilated patient against piperacillin-tazobactam with a gradient strip. The inhibition ellipse meets the strip at 12 mg/L. Quality control with P. aeruginosa ATCC 27853 is within range. The laboratory's current EUCAST breakpoint table lists, for P. aeruginosa and piperacillin-tazobactam: S ≤0.001 mg/L, R >16 mg/L, with a note that the breakpoints apply to high-dose therapy. How should the result be reported?

MIC 16 mg/L: susceptible, increased exposure

MIC 16 mg/L: intermediate, so avoid the drug

MIC 12 mg/L: susceptible, standard dosing

An Enterococcus faecium grows in two blood culture sets from a patient on a hematology ward. Vancomycin is tested by disk diffusion with a 30 µg disk on Mueller-Hinton agar. Read at 18 hours with reflected light, the zone is 19 mm. CLSI zone breakpoints: susceptible ≥17 mm, intermediate 15-16 mm, resistant ≤14 mm. A senior technologist holds the plate for the full 24 hours and examines it with transmitted light: a fine haze of tiny colonies runs from the disk edge to the zone margin. A purity plate grows only E. faecium. How should vancomycin be reported?

Resistant; confirm by an MIC method

Susceptible; ignore the faint haze

Intermediate; repeat the disk test

Susceptible, from the 19 mm zone

How should the gentamicin result in the broth microdilution panel shown be reported for this Klebsiella pneumoniae?

Report the highest MIC, 8 µg/mL

Susceptibility testing methods and interpretation

24 DOH Abu Dhabi (HAAD) lab licensing practice questions on Susceptibility testing methods and interpretation, part of Microbiology. The first 24 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A laboratory that follows EUCAST tests a Pseudomonas aeruginosa blood isolate from a ventilated patient against piperacillin-tazobactam with a gradient strip. The inhibition ellipse meets the strip at 12 mg/L. Quality control with P. aeruginosa ATCC 27853 is within range. The laboratory's current EUCAST breakpoint table lists, for P. aeruginosa and piperacillin-tazobactam: S ≤0.001 mg/L, R >16 mg/L, with a note that the breakpoints apply to high-dose therapy. How should the result be reported?
1. AMIC 16 mg/L: resistant
2. BMIC 16 mg/L: intermediate, so avoid the drug
3. CMIC 12 mg/L: susceptible, standard dosing
4. DMIC 16 mg/L: susceptible, increased exposure
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Answer: D · MIC 16 mg/L: susceptible, increased exposure
A gradient strip reading that falls between twofold dilutions is rounded up before interpretation, so 12 mg/L is reported as 16 mg/L. EUCAST sets the susceptible breakpoint for P. aeruginosa at an off-scale 0.001 mg/L, so wild-type isolates are never categorised 'S' but 'I'. An MIC of 16 mg/L is not above the resistant breakpoint of >16 mg/L, so the category is I: susceptible, increased exposure. This means the drug is expected to work if given at the high dose or by extended infusion in the EUCAST dosing table, and the report should make that clear. CLSI uses different breakpoints, so a laboratory must apply one system consistently.
- A: An MIC of 16 is not above the resistant breakpoint of >16 mg/L.
- B: EUCAST 'I' means susceptible at increased exposure, not a drug to avoid.
- C: Gradient readings are rounded up, and under these breakpoints no MIC above 0.001 mg/L is S.
Question 2 · Hard
An Enterococcus faecium grows in two blood culture sets from a patient on a hematology ward. Vancomycin is tested by disk diffusion with a 30 µg disk on Mueller-Hinton agar. Read at 18 hours with reflected light, the zone is 19 mm. CLSI zone breakpoints: susceptible ≥17 mm, intermediate 15-16 mm, resistant ≤14 mm. A senior technologist holds the plate for the full 24 hours and examines it with transmitted light: a fine haze of tiny colonies runs from the disk edge to the zone margin. A purity plate grows only E. faecium. How should vancomycin be reported?
1. ASusceptible; ignore the faint haze
2. BResistant; confirm by an MIC method
3. CIntermediate; repeat the disk test
4. DSusceptible, from the 19 mm zone
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Answer: B · Resistant; confirm by an MIC method
For enterococci, CLSI requires vancomycin disk tests to be incubated a full 24 hours and examined with transmitted light, because some resistant strains show only a faint haze inside the zone. Any haze or growth within the zone indicates resistance, whatever the measured diameter. Reading at 18 hours with reflected light gave a falsely susceptible 19 mm zone. The result should be confirmed by an MIC method such as broth microdilution, and vanA/vanB testing helps infection control. vanB strains in particular can have MICs close to the breakpoint and give subtle disk results.
- A: The purity plate shows the haze is E. faecium itself, so it cannot be ignored.
- C: Haze within the zone indicates resistance; intermediate refers to a 15-16 mm zone.
- D: Enterococcal vancomycin zones are read at 24 hours with transmitted light, and haze means resistance.
Question 3 · Easy
A meropenem gradient strip was tested against an Acinetobacter baumannii complex isolate, as shown. What MIC is read?
Figure: MedExams
1. A8 µg/mL
2. B2 µg/mL
3. C0.5 µg/mL
4. D32 µg/mL
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Answer: B · 2 µg/mL
Along a gradient strip the drug concentration rises steadily, and growth is inhibited wherever it exceeds the MIC. The MIC is read where the edge of the elliptical zone meets the strip, here at the 2 µg/mL mark. The widest part of the ellipse and the top of the strip show higher concentrations, not the MIC. If the edge falls between two marks, the higher mark is read.
- A: 8 µg/mL is near the widest part of the ellipse, not where its edge meets the strip.
- C: The 0.5 µg/mL mark lies below the zone, in confluent growth.
- D: 32 µg/mL is the top of the strip's range, not the MIC.
Question 4 · Medium
The broth microdilution panel shown is for a Pseudomonas aeruginosa isolate from sputum. CLSI breakpoints (µg/mL, S / I / R): piperacillin-tazobactam ≤16 / 32 / ≥64; ceftazidime ≤8 / 16 / ≥32; meropenem ≤2 / 4 / ≥8; ciprofloxacin ≤0.5 / 1 / ≥2. The drug reported as intermediate is:
Figure: MedExams
1. ACiprofloxacin
2. BPiperacillin-tazobactam
3. CCeftazidime
4. DMeropenem
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Answer: D · Meropenem
The MIC is the lowest concentration with no visible growth: piperacillin-tazobactam 64, ceftazidime 32, meropenem 4 and ciprofloxacin 0.5 µg/mL. Against the stated breakpoints, piperacillin-tazobactam and ceftazidime are resistant, ciprofloxacin is susceptible and meropenem, at 4 µg/mL, is intermediate. Taking the last turbid well instead of the first clear one would wrongly make piperacillin-tazobactam and ceftazidime intermediate. The growth control must show good growth before any well is read.
- A: The ciprofloxacin MIC is 0.5 µg/mL, which is susceptible.
- B: The piperacillin-tazobactam MIC is 64 µg/mL, the first clear well, which is resistant.
- C: The ceftazidime MIC is 32 µg/mL, which is resistant.
Question 5 · Medium
How should the gentamicin result in the broth microdilution panel shown be reported for this Klebsiella pneumoniae?
Figure: MedExams
1. ADo not report; repeat the test
2. BReport an MIC of 2 µg/mL
3. CReport an MIC of 4 µg/mL
4. DReport the highest MIC, 8 µg/mL
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Answer: D · Report the highest MIC, 8 µg/mL
Gentamicin grows at 0.25, 0.5 and 1 µg/mL, is clear at 2, grows again at 4 and is clear from 8 µg/mL. A clear well followed by growth at a higher concentration is a skipped well, usually from an inoculation or pipetting error. By CLSI, a single skipped well is disregarded and the highest MIC is read, here 8 µg/mL. If more than one well is skipped, the result is not reported and the test is repeated.
- A: A repeat is needed only when more than one well is skipped.
- B: Reading the first clear well ignores the growth at 4 µg/mL and understates the MIC.
- C: There is still growth at 4 µg/mL, so 4 µg/mL cannot be the MIC.
Question 6 · Hard
Blood cultures from a 66-year-old woman with pyelonephritis grow Escherichia coli, and the cefepime gradient strip is shown. CLSI cefepime breakpoints for Enterobacterales (µg/mL): S ≤2, SDD 4-8, R ≥16. How should cefepime be reported?
Figure: MedExams
1. ASusceptible at standard dosing
2. BResistant; avoid cefepime
3. CSusceptible-dose dependent (SDD)
4. DIntermediate; efficacy uncertain
Answer and explanation in LearnFree account
Question 7 · Medium
A Streptococcus pneumoniae from the blood culture of an adult with pneumonia was screened with the oxacillin disk shown, on Mueller-Hinton agar with 5% sheep blood. By CLSI, a zone of 20 mm or more predicts penicillin susceptibility. What should the laboratory do next for penicillin?
Figure: MedExams
1. ADetermine a penicillin MIC
2. BReport penicillin as resistant
3. CReport penicillin as intermediate
4. DReport penicillin as susceptible
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Question 8 · Medium
A disk diffusion inoculum is prepared at about a 2 McFarland density instead of 0.5 McFarland. The zones of inhibition will be:
1. ALarger, with falsely susceptible results
2. BSmaller, with falsely resistant results
3. CHazy but the same size as usual
4. DUnchanged, because the disk content is fixed
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Question 9 · Medium
Why is Escherichia coli ATCC 35218 included in QC when amoxicillin-clavulanate or piperacillin-tazobactam is tested?
1. AIt checks the thymidine content of the agar
2. BIt makes a beta-lactamase that checks the inhibitor works
3. CIt carries mecA and checks cefoxitin potency
4. DIt is resistant to all aminoglycosides
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Question 10 · Hard
Weekly disk QC with Escherichia coli ATCC 25922 shows imipenem and amoxicillin-clavulanate zones below range, while all other drugs are in range. The disk cartridges had been kept on the bench between runs. The most likely cause is:
1. AAgar poured deeper than 4 mm
2. BExcess thymidine in the agar
3. CLoss of potency of the labile disks
4. DAn inoculum lighter than 0.5 McFarland
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Question 11 · Easy
Which quality control strain is used for disk diffusion testing of Enterobacterales with most antimicrobials?
1. AStaphylococcus aureus ATCC 25923
2. BEscherichia coli ATCC 25922
3. CPseudomonas aeruginosa ATCC 27853
4. DEnterococcus faecalis ATCC 29212
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Question 12 · Medium
Under EUCAST, an isolate reported as 'I' for an antibiotic is described as:
1. AIntermediate, with uncertain therapeutic effect
2. BResistant at standard and high doses
3. CSusceptible, increased exposure
4. DNot tested, because no breakpoint exists
Answer and explanation in LearnFree account
Question 13 · Hard
A laboratory uses EUCAST rapid AST (RAST) directly from positive blood culture bottles and reads the plates at 6 hours. How must the zones be interpreted?
1. AWith CLSI zone breakpoints for direct testing
2. BWith RAST breakpoints set for the 6-hour reading
3. COnly after converting the zones to MIC values
4. DWith the standard 18-hour EUCAST zone breakpoints
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Question 14 · Medium
Disk diffusion plates for Pseudomonas aeruginosa are mistakenly incubated in 5% CO2. Which drug class is most likely to show falsely small zones?
1. ATetracyclines
2. BCarbapenems
3. CAminoglycosides
4. DCephalosporins
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Question 15 · Hard
On a disk diffusion plate of a Klebsiella pneumoniae blood isolate from an ICU patient, several discrete colonies grow inside the clear meropenem zone. What should be done?
1. AIgnore the colonies and measure the clear zone
2. BReport meropenem susceptible with a comment
3. CMeasure the zone again after another 48 hours
4. DSubculture the colonies, confirm identity and retest
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Question 16 · Hard
Daily disk QC with Escherichia coli ATCC 25922 gives zones 3-5 mm below range for every drug. S. aureus ATCC 25923, tested the same day with the same plate lot and disk cartridges, is in range. The most likely cause is:
1. ADisks that have lost potency from poor storage
2. BAn E. coli suspension denser than 0.5 McFarland
3. CIncubation at 35 °C instead of 37 °C
4. DMueller-Hinton agar with excess calcium and magnesium
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Question 17 · Medium
A gradient diffusion strip gives an MIC reading of 0.19 µg/mL where the ellipse meets the strip. Which value is used for interpretation?
1. A0.25 µg/mL
2. B0.125 µg/mL
3. C0.19 µg/mL
4. D0.38 µg/mL
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Question 18 · Hard
A laboratory changing from CLSI to EUCAST disk diffusion keeps using its 30-µg cefotaxime disks. Why would EUCAST zone breakpoints applied to these zones be invalid?
1. AEUCAST breakpoints apply only to MIC results
2. BEUCAST zones must be read after 48 hours
3. CEUCAST requires blood agar for Enterobacterales
4. DEUCAST cefotaxime breakpoints are set for a 5-µg disk
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Question 19 · Hard
A verification study tests 100 isolates; the reference method classifies 25 as resistant. The new method calls 2 of these 25 susceptible. What is the very major error rate?
1. A2%
2. B2.7%
3. C8%
4. D92%
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Question 20 · Hard
A new automated AST system is verified against broth microdilution with 80 isolates for one drug. Results: 74 in the same category, 4 minor errors, 1 major error and 1 very major error. The categorical agreement is:
1. A92.5%
2. B97.5%
3. C98.8%
4. D7.5%
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Question 21 · Easy
By CLSI, disk diffusion plates for Enterobacterales are incubated at:
1. A35 °C in ambient air for 16-18 hours
2. B42 °C in ambient air for 24 hours
3. C30 °C in ambient air for 48 hours
4. D35 °C in 5% CO2 for 16-18 hours
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Question 22 · Easy
By CLSI, disk diffusion zones for most non-fastidious bacteria on plain Mueller-Hinton agar are measured:
1. AFrom the back of the plate with reflected light
2. BAlong one radius on the side nearest the edge
3. CFrom the top of the plate with the lid in place
4. DAfter staining the plate with crystal violet
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Question 23 · Medium
By CLSI, disk diffusion for Streptococcus pneumoniae is performed on:
1. APlain Mueller-Hinton agar in ambient air
2. BMueller-Hinton agar with 5% sheep blood in 5% CO2
3. CHaemophilus Test Medium in ambient air
4. DChocolate agar in an anaerobic atmosphere
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Question 24 · Medium
Why is broth microdilution, not disk diffusion, recommended for colistin susceptibility testing of carbapenem-resistant Klebsiella pneumoniae?
1. AColistin is destroyed by Mueller-Hinton agar
2. BKlebsiella does not grow well on agar plates
3. CColistin diffuses poorly through agar
4. DColistin disks lose potency within hours
Answer and explanation in LearnFree account

Susceptibility testing methods and interpretation

A meropenem gradient strip was tested against an Acinetobacter baumannii complex isolate, as shown. What MIC is read?

The broth microdilution panel shown is for a Pseudomonas aeruginosa isolate from sputum. CLSI breakpoints (µg/mL, S / I / R): piperacillin-tazobactam ≤16 / 32 / ≥64; ceftazidime ≤8 / 16 / ≥32; meropenem ≤2 / 4 / ≥8; ciprofloxacin ≤0.5 / 1 / ≥2. The drug reported as intermediate is:

How should the gentamicin result in the broth microdilution panel shown be reported for this Klebsiella pneumoniae?

Blood cultures from a 66-year-old woman with pyelonephritis grow Escherichia coli, and the cefepime gradient strip is shown. CLSI cefepime breakpoints for Enterobacterales (µg/mL): S ≤2, SDD 4-8, R ≥16. How should cefepime be reported?

A Streptococcus pneumoniae from the blood culture of an adult with pneumonia was screened with the oxacillin disk shown, on Mueller-Hinton agar with 5% sheep blood. By CLSI, a zone of 20 mm or more predicts penicillin susceptibility. What should the laboratory do next for penicillin?

A disk diffusion inoculum is prepared at about a 2 McFarland density instead of 0.5 McFarland. The zones of inhibition will be:

Why is Escherichia coli ATCC 35218 included in QC when amoxicillin-clavulanate or piperacillin-tazobactam is tested?

Weekly disk QC with Escherichia coli ATCC 25922 shows imipenem and amoxicillin-clavulanate zones below range, while all other drugs are in range. The disk cartridges had been kept on the bench between runs. The most likely cause is:

Which quality control strain is used for disk diffusion testing of Enterobacterales with most antimicrobials?

Under EUCAST, an isolate reported as 'I' for an antibiotic is described as:

A laboratory uses EUCAST rapid AST (RAST) directly from positive blood culture bottles and reads the plates at 6 hours. How must the zones be interpreted?

Disk diffusion plates for Pseudomonas aeruginosa are mistakenly incubated in 5% CO2. Which drug class is most likely to show falsely small zones?

On a disk diffusion plate of a Klebsiella pneumoniae blood isolate from an ICU patient, several discrete colonies grow inside the clear meropenem zone. What should be done?

Daily disk QC with Escherichia coli ATCC 25922 gives zones 3-5 mm below range for every drug. S. aureus ATCC 25923, tested the same day with the same plate lot and disk cartridges, is in range. The most likely cause is:

A gradient diffusion strip gives an MIC reading of 0.19 µg/mL where the ellipse meets the strip. Which value is used for interpretation?

A laboratory changing from CLSI to EUCAST disk diffusion keeps using its 30-µg cefotaxime disks. Why would EUCAST zone breakpoints applied to these zones be invalid?

A verification study tests 100 isolates; the reference method classifies 25 as resistant. The new method calls 2 of these 25 susceptible. What is the very major error rate?

A new automated AST system is verified against broth microdilution with 80 isolates for one drug. Results: 74 in the same category, 4 minor errors, 1 major error and 1 very major error. The categorical agreement is:

By CLSI, disk diffusion plates for Enterobacterales are incubated at:

By CLSI, disk diffusion zones for most non-fastidious bacteria on plain Mueller-Hinton agar are measured:

By CLSI, disk diffusion for Streptococcus pneumoniae is performed on:

Why is broth microdilution, not disk diffusion, recommended for colistin susceptibility testing of carbapenem-resistant Klebsiella pneumoniae?