Microbiology Quiz: Interpreting Susceptibility Results
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Interpreting Susceptibility ResultsQuestion 1 of 20

A final susceptibility report on an Enterococcus faecium isolate from a peritoneal fluid culture is reviewed by a laboratory supervisor. The report lists the organism as 'Susceptible' to ceftriaxone. What is the most appropriate action for the supervisor to take?

Release the report, as automated systems are highly accurate.
Call the physician to recommend ceftriaxone based on the favorable result.
Verify the organism identification and repeat the susceptibility test.
Add a comment that high-dose ceftriaxone therapy is required for efficacy.
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Microbiology Quiz

Microbiology Quiz: Interpreting Susceptibility Results

Practice Interpreting Susceptibility Results in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Interpreting Susceptibility Results, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A final susceptibility report on an Enterococcus faecium isolate from a peritoneal fluid culture is reviewed by a laboratory supervisor. The report lists the organism as 'Susceptible' to ceftriaxone. What is the most appropriate action for the supervisor to take?

  1. Release the report, as automated systems are highly accurate.
  2. Call the physician to recommend ceftriaxone based on the favorable result.
  3. Verify the organism identification and repeat the susceptibility test. (correct answer)
  4. Add a comment that high-dose ceftriaxone therapy is required for efficacy.
Explanation: Enterococcus species, particularly E. faecium, are intrinsically resistant to cephalosporins due to low-affinity penicillin-binding proteins. A result of 'Susceptible' to ceftriaxone is biologically implausible and strongly suggests either a misidentification of the organism (e.g., it may be a streptococcus) or a technical error in susceptibility testing. The correct action is to investigate the discrepancy by confirming the ID and repeating the test.

Question 2

A Staphylococcus aureus isolate from a patient with persistent bacteremia despite vancomycin therapy has a vancomycin MIC of 2 µg/mL by broth microdilution. A subsequent screen using a brain heart infusion agar plate containing 6 µg/mL of vancomycin shows growth of numerous colonies after a 48-hour incubation. Which of the following best describes this isolate?

  1. Vancomycin-resistant S. aureus (VRSA) with the vanA gene.
  2. A typical vancomycin-susceptible S. aureus (VSSA).
  3. Heteroresistant vancomycin-intermediate S. aureus (hVISA). (correct answer)
  4. A mixed culture of S. aureus and vancomycin-resistant Enterococcus (VRE).
Explanation: This scenario is classic for hVISA or VISA. The isolate's MIC is within the susceptible range (≤2 µg/mL), but a subpopulation of cells is able to grow at higher vancomycin concentrations (6 µg/mL), as shown by the agar screen test. This heteroresistance can lead to clinical failure. VRSA would have a much higher MIC (≥16 µg/mL). A typical VSSA would not grow on the screen plate. While a mixed culture is possible, hVISA is a well-described phenomenon that perfectly fits these laboratory findings in the context of treatment failure.

Question 3

A Klebsiella pneumoniae isolate from a bloodstream infection provides the following MIC results:

  • Meropenem: 16 µg/mL (Resistant)
  • Ertapenem: >8 µg/mL (Resistant)
  • Ceftazidime-avibactam: 1 µg/mL (Susceptible)
  • Colistin: 0.5 µg/mL (Susceptible)

Based on this resistance pattern, which of the following is the most likely mechanism of carbapenem resistance in this isolate?

  1. Production of a metallo-β-lactamase (MBL) such as NDM or VIM.
  2. Production of a KPC-type serine carbapenemase. (correct answer)
  3. Porin channel loss combined with production of an AmpC β-lactamase.
  4. Hyperproduction of an OXA-48-like carbapenemase.
Explanation: The key finding is the susceptibility to ceftazidime-avibactam. Avibactam is a β-lactamase inhibitor that effectively inhibits Class A serine carbapenemases, such as Klebsiella pneumoniae carbapenemase (KPC). Metallo-β-lactamases (MBLs) are not inhibited by avibactam. Porin loss combined with AmpC would likely result in lower-level carbapenem resistance and would not be reversed by avibactam. While some OXA-48-like enzymes are inhibited by avibactam, the combination of high-level resistance to meropenem and susceptibility to ceftazidime-avibactam is the classic phenotype for a KPC producer.

Question 4

An Escherichia coli isolate from a urine culture has a positive phenotypic test for Extended-Spectrum β-Lactamase (ESBL) production. The automated susceptibility panel shows a cefepime MIC of 4 µg/mL, which interprets as 'Susceptible' according to current CLSI guidelines. How should the laboratory report the cefepime result?

  1. The cefepime interpretation should be edited to 'Resistant' because the isolate is an ESBL producer.
  2. The cefepime interpretation should be edited to 'Intermediate' to warn the clinician of potential failure.
  3. The cefepime result should be reported as 'Susceptible' but with a comment indicating that the organism is an ESBL producer. (correct answer)
  4. The cefepime result should be suppressed entirely, and the clinician should be guided to use a carbapenem.
Explanation: Current CLSI guidelines have moved away from automatically editing cephalosporin results to 'Resistant' for ESBL producers. Instead, results should be reported as measured. However, because therapeutic outcomes with agents like cefepime can be unpredictable for ESBL producers, even with susceptible MICs (due to inoculum effects, etc.), a comment should be added to the report indicating ESBL production. This provides the clinician with all relevant information to make an informed decision.

Question 5

During a Kirby-Bauer susceptibility test of an Enterobacter cloacae isolate, a microbiologist observes a flattening of the ceftazidime inhibition zone adjacent to a cefoxitin disk. What is the most likely mechanism responsible for this observation?

  1. Synergy between cefoxitin and ceftazidime against the isolate.
  2. Antagonism due to competitive binding to penicillin-binding proteins.
  3. Inhibition of an ESBL by the clavulanic acid present in the cefoxitin disk.
  4. Induction of chromosomal AmpC β-lactamase expression by cefoxitin. (correct answer)
Explanation: When you encounter zone distortion patterns in Kirby-Bauer testing, especially with Enterobacter species and β-lactam antibiotics, think about inducible resistance mechanisms. The flattening of the ceftazidime zone near the cefoxitin disk is a classic indicator of AmpC β-lactamase induction. Enterobacter cloacae carries a chromosomal AmpC gene that's normally expressed at low levels. Cefoxitin is a potent inducer of this enzyme—when the organism is exposed to cefoxitin, it dramatically upregulates AmpC production. This increased enzyme expression creates a gradient effect that extends beyond the cefoxitin disk itself, reaching the adjacent ceftazidime disk area and degrading the ceftazidime there. The result is the characteristic "flattening" or "blunting" of the ceftazidime inhibition zone on the side closest to cefoxitin. Option A is incorrect because synergy would create enhanced inhibition, not zone flattening. Option B misidentifies the mechanism—this isn't about competitive PBP binding but rather enzymatic drug degradation. Option C is wrong because standard cefoxitin disks don't contain clavulanic acid, and this pattern isn't related to ESBL inhibition. The correct answer is D: cefoxitin induces chromosomal AmpC expression, leading to increased β-lactamase production that degrades ceftazidime in the overlapping zone. Study tip: Remember the "CRIES" organisms (Citrobacter, Enterobacter, Serratia, and others) that have inducible chromosomal AmpC. When you see zone distortion patterns involving these organisms and cefoxitin, think AmpC induction rather than ESBL mechanisms.

Question 6

A urine culture from a patient with an uncomplicated urinary tract infection (UTI) grows >100,000 CFU/mL of Escherichia coli. The automated susceptibility report shows susceptibility to ampicillin, cephalexin, and cefazolin. According to the principle of selective or cascade reporting, which result is most appropriate to suppress from the final report sent to an outpatient clinic?

  1. Ampicillin, because general resistance rates are high in the community.
  2. Cephalexin, because it is a first-generation cephalosporin with a narrow spectrum.
  3. Cefazolin, as it is an injectable-only agent and may confuse oral therapy choices. (correct answer)
  4. No results should be suppressed; all data should be provided to the clinician.
Explanation: Selective (or cascade) reporting is a strategy to guide clinicians towards narrower-spectrum, safer, and more cost-effective antimicrobial choices. For an uncomplicated UTI treated in an outpatient setting, oral antibiotics are standard. Cephalexin is an appropriate oral cephalosporin, and its result should be reported. Cefazolin is an intravenous cephalosporin. Reporting its susceptibility might confuse the clinician or lead to inappropriate use of an IV agent. Therefore, it is standard practice to suppress results for parenteral-only agents when appropriate oral options are available for a given infection type.

Question 7

A hospital's cumulative antibiogram from 2018 showed 80% of E. coli isolates were susceptible to ciprofloxacin using the CLSI breakpoint of ≤1 µg/mL. In 2022, after the lab adopted the revised, more stringent CLSI breakpoint of ≤0.25 µg/mL, the antibiogram shows only 65% susceptibility. What is the most methodologically sound interpretation of this change?

  1. The true change in resistance cannot be determined without analyzing the MIC distributions from both time periods. (correct answer)
  2. The apparent increase in resistance is an artifact of the breakpoint change and does not reflect a true biological shift.
  3. The change in susceptibility is a direct measure of increasing fluoroquinolone resistance in the bacterial population.
  4. The newer breakpoint is overly sensitive, and the older 2018 data is a more accurate reflection of clinical efficacy.
Explanation: When interpreting changes in antibiogram data over time, you must carefully consider whether observed differences reflect true biological changes or methodological artifacts. Antibiograms report susceptibility percentages based on breakpoints—the MIC values that distinguish susceptible from resistant isolates. The correct interpretation requires examining the underlying MIC distributions from both time periods. Consider what happens when breakpoints change: isolates with MICs between 0.25-1 µg/mL would be classified as susceptible under the old breakpoint but resistant under the new one. Without knowing how many isolates fall into this range, you cannot determine whether the population's actual resistance profile changed or if the decrease simply reflects reclassification of existing isolates. Option B incorrectly assumes the change is purely artifactual. While breakpoint changes do affect classifications, true resistance could have increased simultaneously—you need the MIC data to separate these effects. Option C makes the opposite error, assuming the change directly reflects biological resistance without accounting for the methodological change. This ignores the confounding effect of different breakpoints. Option D makes a value judgment about breakpoint appropriateness, but CLSI revisions typically reflect improved correlation with clinical outcomes, not arbitrary changes. Remember that antibiogram interpretation requires understanding both the biological and methodological factors that influence susceptibility reporting. When breakpoints change between time periods, always look for MIC distribution data to distinguish true resistance trends from classification artifacts. This principle applies whenever comparing antimicrobial susceptibility data across different testing standards or time periods.

Question 8

A Pseudomonas aeruginosa isolate displays a paradoxical susceptibility pattern: resistant to piperacillin-tazobactam (MIC > 128 µg/mL) but susceptible to ticarcillin-clavulanate (MIC = 32 µg/mL). What is a plausible enzymatic basis for this phenotype?

  1. The isolate produces a PER-type extended-spectrum β-lactamase. (correct answer)
  2. The clavulanate in the test was more potent due to a lower inoculum size.
  3. The isolate produces an inhibitor-resistant TEM (IRT) β-lactamase.
  4. The isolate has a derepressed AmpC β-lactamase.
Explanation: When you encounter unusual antibiotic susceptibility patterns in Pseudomonas aeruginosa, think about the specific β-lactamases involved and how different inhibitors interact with them. This paradoxical pattern—resistance to piperacillin-tazobactam but susceptibility to ticarcillin-clavulanate—points to the type of β-lactamase enzyme present. PER-type extended-spectrum β-lactamases (ESBLs) have a unique characteristic: they're effectively inhibited by clavulanate but poorly inhibited by tazobactam. This explains why the isolate remains susceptible to ticarcillin-clavulanate (MIC = 32 µg/mL) while being highly resistant to piperacillin-tazobactam (MIC > 128 µg/mL). The clavulanate successfully blocks the PER enzyme, restoring ticarcillin activity, but tazobactam fails to inhibit it effectively. Option B is incorrect because inoculum effects wouldn't create such a dramatic, selective pattern between two β-lactam/inhibitor combinations. Option C describes inhibitor-resistant TEM (IRT) β-lactamases, but these are resistant to both clavulanate and tazobactam, so you'd see resistance to both combinations. Option D suggests derepressed AmpC, but AmpC enzymes aren't significantly inhibited by either clavulanate or tazobactam, so this wouldn't explain the differential susceptibility pattern. Study tip: Remember that different β-lactamase inhibitors have varying effectiveness against specific enzyme types. PER-type ESBLs are the classic example of preferential clavulanate sensitivity over tazobactam—this pattern is almost pathognomonic for PER enzymes in Pseudomonas.

Question 9

An isolate of Bacteroides fragilis from an intra-abdominal abscess is found to be resistant to imipenem with an MIC >32 µg/mL. Which of the following is the most likely mechanism mediating this resistance?

  1. Production of a CfiA-type metallo-β-lactamase. (correct answer)
  2. Modification of the ribosomal target site by an Erm methylase.
  3. Alteration of penicillin-binding protein 2a (PBP2a).
  4. Upregulation of a multidrug efflux pump system.
Explanation: High-level carbapenem resistance in Bacteroides fragilis is most commonly mediated by the production of a chromosomal metallo-β-lactamase encoded by the cfiA gene (also known as ccrA). This enzyme efficiently hydrolyzes carbapenems like imipenem. Erm methylases confer resistance to macrolides and clindamycin. PBP2a is the mechanism of methicillin resistance in staphylococci. While efflux pumps contribute to resistance in B. fragilis, they do not typically cause such high-level carbapenem resistance alone.

Question 10

An Enterococcus faecalis isolate from a patient with endocarditis is found to be susceptible to ampicillin (MIC = 2 µg/mL) and vancomycin (MIC = 1 µg/mL). However, it demonstrates high-level resistance to gentamicin (MIC > 500 µg/mL). What is the clinical implication for treating this patient's infection?

  1. A synergistic combination of ampicillin and gentamicin is the recommended therapy for this infection.
  2. The combination of ampicillin and gentamicin is unlikely to be synergistic and should be avoided. (correct answer)
  3. Vancomycin should be combined with gentamicin for a synergistic bactericidal effect.
  4. Gentamicin monotherapy at a high dose should be effective due to the low ampicillin MIC.
Explanation: For serious enterococcal infections like endocarditis, synergistic bactericidal therapy is desired. This is typically achieved by combining a cell wall agent (like ampicillin or vancomycin) with an aminoglycoside (like gentamicin). However, this synergy is abrogated if the isolate possesses high-level aminoglycoside resistance. The mechanism of synergy requires aminoglycoside entry into the cell, which is blocked by this resistance. Therefore, the combination would not be effective.

Question 11

A disk diffusion test (D-test) is performed on a Staphylococcus aureus isolate from a wound infection that is resistant to erythromycin but appears susceptible to clindamycin. The test shows a flattening of the clindamycin inhibition zone adjacent to the erythromycin disk. How should the clindamycin result be reported to the clinician?

  1. Report as Susceptible, as the zone size is within the susceptible range.
  2. Report as Resistant, because the positive D-test indicates inducible resistance that may lead to clinical failure. (correct answer)
  3. Report as Intermediate, and recommend using a higher dose of clindamycin.
  4. Request a new specimen, as this result pattern indicates a mixed culture of two different staphylococcal strains.
Explanation: A positive D-test indicates the presence of an erm gene, which confers inducible resistance to clindamycin. Although the isolate appears susceptible in vitro without the presence of an inducer (erythromycin), the resistance mechanism can be activated in vivo, leading to therapeutic failure. Therefore, standard laboratory practice is to edit the interpretation for clindamycin from Susceptible to Resistant.

Question 12

A time-kill curve analysis is performed for a bactericidal antibiotic against a bacterial isolate. The results show that the rate of killing is maximal at 4 times the MIC, but the killing rate significantly decreases at concentrations of 16 and 32 times the MIC, approaching the rate of the no-antibiotic control. This phenomenon is best described as:

  1. Tolerance
  2. Post-antibiotic effect
  3. Heteroresistance
  4. The Eagle effect (correct answer)
Explanation: The Eagle effect, also known as a paradoxical effect, is a phenomenon where the bactericidal activity of an antibiotic decreases at very high concentrations. This is observed with some β-lactams and other agents. Tolerance is defined by a high MBC/MIC ratio, not by reduced killing at high concentrations. The post-antibiotic effect is the suppression of bacterial growth after brief exposure to an antibiotic. Heteroresistance involves a subpopulation of resistant cells, which would manifest differently in a time-kill assay.

Question 13

A clinical laboratory identifies Staphylococcus aureus from a patient's blood culture. The cefoxitin disk diffusion screen test yields a zone of inhibition of 18 mm (Resistant). However, the automated broth microdilution test reports an oxacillin MIC of 1 µg/mL (Susceptible). What is the most appropriate course of action?

  1. Report the isolate as methicillin-susceptible based on the quantitative oxacillin MIC result.
  2. Report the isolate as methicillin-resistant based on the cefoxitin screen result. (correct answer)
  3. Report both contradictory results and ask the physician to make a clinical judgment.
  4. Conclude the test is invalid due to a technical error and request a new specimen from the patient.
Explanation: The cefoxitin disk screen is a more reliable predictor of mecA-mediated methicillin resistance than the oxacillin MIC, especially with some automated systems. Cefoxitin is a better inducer of the mecA gene. In cases of discrepant results between cefoxitin and oxacillin, the cefoxitin result should take precedence. The isolate should be reported as methicillin-resistant S. aureus (MRSA).

Question 14

A Pseudomonas aeruginosa isolate from a sputum culture shows the following susceptibility pattern: resistant to ceftazidime, susceptible to cefepime, resistant to piperacillin-tazobactam, and susceptible to meropenem. Which phenotype best describes this resistance pattern?

  1. Extended-spectrum β-lactamase (ESBL) production
  2. KPC-type carbapenemase production
  3. Derepressed chromosomal AmpC β-lactamase (correct answer)
  4. OprD porin loss with efflux pump upregulation
Explanation: This pattern is classic for the derepression of the intrinsic chromosomal AmpC β-lactamase in P. aeruginosa. AmpC enzymes efficiently hydrolyze penicillins (including piperacillin-tazobactam) and most cephalosporins, but cefepime is relatively stable. Carbapenems are also stable to AmpC hydrolysis unless there is concurrent porin loss. ESBLs are typically inhibited by tazobactam, so piperacillin-tazobactam would be susceptible. KPC production would confer resistance to cefepime and meropenem.

Question 15

A Carba NP test is performed on a clinical isolate of Klebsiella pneumoniae. The test solution in the well containing the bacterial lysate and imipenem changes color from red to yellow within 30 minutes. What does this result indicate?

  1. The isolate produces a carbapenem-hydrolyzing β-lactamase. (correct answer)
  2. The isolate is negative for carbapenemase production.
  3. The isolate possesses a porin mutation that prevents imipenem entry.
  4. The test is inconclusive and should be repeated after 2 hours of incubation.
Explanation: The Carba NP test is a biochemical assay for detecting carbapenemase activity. The test relies on the hydrolysis of imipenem, which is a β-lactam antibiotic. When the β-lactam ring is hydrolyzed by a carbapenemase, it produces an acidic byproduct. This acid production lowers the pH of the solution, causing the pH indicator (phenol red) to change from red (neutral) to yellow (acidic). Therefore, a yellow color is a positive result indicating the presence of a carbapenemase.

Question 16

A Klebsiella pneumoniae isolate from a bloodstream infection provides the following MIC results:

  • Meropenem: 16 µg/mL (Resistant)
  • Ertapenem: >8 µg/mL (Resistant)
  • Ceftazidime-avibactam: 1 µg/mL (Susceptible)
  • Colistin: 0.5 µg/mL (Susceptible)

Based on this resistance pattern, which of the following is the most likely mechanism of carbapenem resistance in this isolate?

  1. Production of a metallo-β-lactamase (MBL) such as NDM or VIM.
  2. Production of a KPC-type serine carbapenemase. (correct answer)
  3. Porin channel loss combined with production of an AmpC β-lactamase.
  4. Hyperproduction of an OXA-48-like carbapenemase.
Explanation: The key finding is the susceptibility to ceftazidime-avibactam. Avibactam is a β-lactamase inhibitor that effectively inhibits Class A serine carbapenemases, such as Klebsiella pneumoniae carbapenemase (KPC). Metallo-β-lactamases (MBLs) are not inhibited by avibactam. Porin loss combined with AmpC would likely result in lower-level carbapenem resistance and would not be reversed by avibactam. While some OXA-48-like enzymes are inhibited by avibactam, the combination of high-level resistance to meropenem and susceptibility to ceftazidime-avibactam is the classic phenotype for a KPC producer.

Question 17

An Escherichia coli isolate from a urine culture has a positive phenotypic test for Extended-Spectrum β-Lactamase (ESBL) production. The automated susceptibility panel shows a cefepime MIC of 4 µg/mL, which interprets as 'Susceptible' according to current CLSI guidelines. How should the laboratory report the cefepime result?

  1. The cefepime interpretation should be edited to 'Resistant' because the isolate is an ESBL producer.
  2. The cefepime interpretation should be edited to 'Intermediate' to warn the clinician of potential failure.
  3. The cefepime result should be reported as 'Susceptible' but with a comment indicating that the organism is an ESBL producer. (correct answer)
  4. The cefepime result should be suppressed entirely, and the clinician should be guided to use a carbapenem.
Explanation: Current CLSI guidelines have moved away from automatically editing cephalosporin results to 'Resistant' for ESBL producers. Instead, results should be reported as measured. However, because therapeutic outcomes with agents like cefepime can be unpredictable for ESBL producers, even with susceptible MICs (due to inoculum effects, etc.), a comment should be added to the report indicating ESBL production. This provides the clinician with all relevant information to make an informed decision.

Question 18

A final susceptibility report on an Enterococcus faecium isolate from a peritoneal fluid culture is reviewed by a laboratory supervisor. The report lists the organism as 'Susceptible' to ceftriaxone. What is the most appropriate action for the supervisor to take?

  1. Release the report, as automated systems are highly accurate.
  2. Call the physician to recommend ceftriaxone based on the favorable result.
  3. Verify the organism identification and repeat the susceptibility test. (correct answer)
  4. Add a comment that high-dose ceftriaxone therapy is required for efficacy.
Explanation: Enterococcus species, particularly E. faecium, are intrinsically resistant to cephalosporins due to low-affinity penicillin-binding proteins. A result of 'Susceptible' to ceftriaxone is biologically implausible and strongly suggests either a misidentification of the organism (e.g., it may be a streptococcus) or a technical error in susceptibility testing. The correct action is to investigate the discrepancy by confirming the ID and repeating the test.

Question 19

An isolate of Pseudomonas aeruginosa is tested against tobramycin. The minimum inhibitory concentration (MIC) is determined to be 2 µg/mL. A subsequent test finds the minimum bactericidal concentration (MBC) to be 128 µg/mL. How should the interaction between the organism and this antibiotic be characterized?

  1. The isolate is resistant to tobramycin.
  2. The MIC and MBC results are discordant and likely represent a testing error.
  3. The antibiotic has a paradoxical (Eagle) effect on the isolate.
  4. The isolate exhibits tolerance to tobramycin. (correct answer)
Explanation: When you encounter MIC and MBC values in microbiology, you're looking at two key measurements of antibiotic effectiveness. The MIC (minimum inhibitory concentration) is the lowest concentration that stops visible bacterial growth, while the MBC (minimum bactericidal concentration) is the lowest concentration that actually kills the bacteria. Normally, these values are close together—typically within a 4-fold difference. The correct answer is D because this isolate shows a classic pattern of tolerance. With an MIC of 2 µg/mL and MBC of 128 µg/mL, there's a 64-fold difference between stopping growth and killing the bacteria. This dramatic gap defines antibiotic tolerance: the organism is susceptible to growth inhibition but requires much higher concentrations for killing. Option A is incorrect because the MIC of 2 µg/mL likely falls within the susceptible range for tobramycin against P. aeruginosa (typically ≤4 µg/mL). Option B misinterprets the results—while the values seem discordant, this large MIC/MBC ratio is a real biological phenomenon, not a testing error. Option C refers to the Eagle effect, which describes decreased bacterial killing at higher antibiotic concentrations compared to lower ones, particularly seen with beta-lactams against gram-positive bacteria—not the scenario described here. Remember: when you see MIC and MBC values with a large gap (typically >32-fold difference), think tolerance. This is clinically significant because standard dosing may suppress symptoms without clearing the infection.

Question 20

An Etest is performed for imipenem against an Enterobacter cloacae isolate. The resulting ellipse of inhibition is well-defined with an apparent MIC of 2 µg/mL. However, a faint, hazy zone of growth is observed within the ellipse at the lower end of the strip. What is the most appropriate interpretation of this result?

  1. The MIC should be read as the lowest concentration that completely inhibits growth, ignoring the haze.
  2. The MIC should be reported as >256 µg/mL, as any growth within the ellipse indicates high-level resistance.
  3. The test is invalid due to contamination with a second organism and must be repeated.
  4. The hazy growth indicates the presence of a carbapenemase, and this should be confirmed with a specific test. (correct answer)
Explanation: When interpreting antibiotic susceptibility testing, particularly with carbapenems like imipenem, you need to recognize specific growth patterns that indicate resistance mechanisms rather than just reading the MIC value at face value. The key finding here is the "hazy zone of growth within the ellipse at the lower end of the strip." This characteristic pattern is a classic indicator of carbapenemase production in Enterobacteriaceae. Carbapenemases are enzymes that can hydrolyze carbapenems, and when present, they create this distinctive hazy growth pattern within what would otherwise be a clear zone of inhibition. The haze represents a subpopulation of bacteria that are expressing the carbapenemase and can survive in the presence of the antibiotic. Option A is incorrect because ignoring the haze would miss a critical resistance mechanism. The hazy growth is diagnostically significant, not an artifact to dismiss. Option B is wrong because the presence of hazy growth doesn't automatically indicate the highest resistance category - the pattern itself is more important than assuming maximum resistance. Option C misinterprets the finding as contamination when this is actually a recognized phenotypic pattern associated with enzyme production. Option D correctly identifies that this hazy growth pattern suggests carbapenemase production and appropriately calls for confirmatory testing. This is crucial because carbapenemase-producing organisms require special infection control measures and alternative treatment approaches. Remember: In carbapenem susceptibility testing, always look beyond the MIC number. Hazy growth patterns within the inhibition zone are red flags for carbapenemase production and require immediate follow-up testing for proper patient management and infection control.