What types of susceptibility testing are there?
- Disc Diffusion, Vitek 2, Etest, Agar Dilution, Broth Dilution
What is the importance about the Broth Dilution in Susceptibility Testing?
- Two fold dilution of drug conc. liquid media [Broth] with no proteins
- GOLD STANDARD
- Looks at MIC and MBC
What is the difference between MIC and MBC?
- Minimum Inhibitory Conc. [MIC]: lowest drug conc. that prevents visable growth
- Minimum Bactericidal Conc. [MBC]: lowest drug conc. resulting in> 99.9% decreased in inoculum (not done often)
What is the importance of Agar Dilution in Susceptibility Testing?
- Two fold dilution of antibiotics incorporated in molten agar, poured into petro dishes, and then allowed to solidify
What is important to know about Epsilometer Test [Etest] in Susceptibility Testing?
- Plastic strips containing a continuous conc. gradient of drug where microorganism are streaked onto
- More “Precise”
What is the importacne of Vitvek 2 in Susceptibility Testing?
- Small plastic reagent cards containing 64 wall for testing
- Looks at turbidity
- Computerized
What are the two automated AST systems and what is important to know about them?
- Mircoscan & BD Phoneix
- ALGORITHM-DERIVED MIC
What is important to know about Disc Diffusion [Kirby Bauer] in Susceptibility Testing?
- Fixed amount of antibiotics on disc creating zone of inhibition
- Results are S, I, R; NO MIC
What is the definition of a breakpoint?
- MIC or zone diameter used to categorize an organism into 1 of 5 criteria [Susceptible, Susceptible-dose Dependent, Intermiediate, Resistant, Nonsusceptible]
What is Susceptibile in Interpretice Criteria?
- Isolates with MIC at or below or zone diameter at or great than susceptibility breakpoint are inhibited by normal dosing.
What is Susceptibile-Dose Dependent in Interpretive Criteria?
- Susceptibility is dependent on the dosing regimen that is used
What is Intermediate in Interpretive Criteria?
- Isolates with MICs approaches conc. and response rate maybe lower then for susceptible isolates UNLESS it is concentrated
What is Resistant in Interpretive Criteria?
- Isolates with MIC at or above or zone diameter at or below the resistant breakpoint are not inhibited
What is Nonsusceptible in Interpretive Criteria?
- Isolates for which only susceptable breakpoint is designated
How are Breakpoints/interpretive criteria established?
- Pharmacology [PK/PD]
- Outcomes [Clinical or Bacteriologic]
- Phenotypes
What is selective and cascade reporting in Susceptibilty testing and what is there rationale?
- Selective: Based on defined criteria unrelated to AST
- Cascade: Based on AST
- They want to encourage appropriate use of antimicrobial agents
What is the Inoculum Effect within Susceptibility Testing?
- Increased in MIC when higher inoculum is used will increase the activity
- More common in b-lactams
What is Tolerance in Susceptibility Testing?
- MBC > 32 x MIC
- Has poor outcomes because MBC is rarely found
What are some of the important terms to know about when talking about MIC Statistics in Susceptibilty Testing?
MIC50, MIC90, Range, Mean, Mode??
- MIC50: antibiotic that inhibits 50%
- MIC90: antibiotic that inhibits 90%
- Range: lowest MIC to highest MIC
- Geometric Mean MIC: antilog of the mean of the log MIC [better measurement]
- Modal Mean: most common MIC
What are the 3 general definition to know about when discussing Synergy Testing?
- Synergy, Antagonism, Additivity
What is the definition of Synergy in Synergy Testing?
- The activity of an antimicrobial combination is greater then that expected from the additive activity on the individual agents
What is the definition of Antagonism in Synergy Testing?
- The activitiy of an antimircobial combination is less than that expected from the additive activity of the individual agents
What is the definition of Additivity in Synergy Testing?
- Neither synergy or antagonism
What is the Checkerboard method of synergy testing?
- Measures inhibitory activity of the combination of cross dilution of 2 antibiotics [does not mean killing]
