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Microbiology: Antimicrobial Susceptibility Testing Flashcards

7 cards from real AP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. Which parameter describes the relationship between an antibiotic's pharmacokinetics and its MIC to predict efficacy, specifically for concentration-dependent drugs?

    Answer: Area under the curve to MIC ratio (AUC/MIC)

    For concentration-dependent antibiotics like aminoglycosides and fluoroquinolones, the AUC/MIC ratio is the primary pharmacodynamic parameter predicting efficacy.

  2. A laboratory receives a Klebsiella pneumoniae isolate with elevated MICs to all carbapenems. Which test would best distinguish KPC from MBL carbapenemase production?

    Answer: Phenotypic inhibitor testing using EDTA (MBL inhibitor) and boronic acid (KPC inhibitor)

    EDTA chelates zinc and inhibits metallo-beta-lactamases (MBLs), while boronic acid inhibits KPC; using both inhibitors in combination disk tests differentiates the two carbapenemase types.

  3. What happens to antibiotic disk potency if disks are stored improperly at room temperature for extended periods?

    Answer: Disk potency degrades, leading to falsely small zones of inhibition and incorrect susceptible classifications

    Antibiotic disks stored outside of recommended refrigeration conditions lose potency over time, producing smaller zones that may incorrectly classify susceptible organisms as intermediate or resistant.

  4. Which organism requires supplementation of Mueller-Hinton agar with sheep blood and NAD for reliable disk diffusion susceptibility testing?

    Answer: Streptococcus pneumoniae

    Streptococcus pneumoniae and other streptococci require 5% sheep blood added to Mueller-Hinton agar because they are fastidious and will not grow reliably on plain Mueller-Hinton.

  5. In synergy testing, a fractional inhibitory concentration index (FICI) of ≤0.5 indicates:

    Answer: Synergy between the two drugs

    An FICI ≤0.5 indicates synergy, meaning the combination of two drugs is significantly more effective than either drug alone.

  6. What is the primary limitation of the disk diffusion method compared to broth microdilution?

    Answer: Disk diffusion provides categorical results (S/I/R) but not a specific MIC value

    Disk diffusion categorizes organisms as susceptible, intermediate, or resistant but does not generate a specific MIC needed for advanced pharmacodynamic dosing strategies.

  7. Which CLSI interpretive category was introduced to replace 'intermediate' for some drug-organism combinations, indicating susceptibility dependent on dose escalation or infection site?

    Answer: Susceptible-dose dependent (SDD)

    The 'Susceptible-dose dependent' (SDD) category indicates that susceptibility of the isolate depends on the dosing regimen used, requiring higher or more frequent doses for efficacy.