DD - Clinical Use of Antibiotics

Question 1

Principles determining empirical antibiotic use

Answer 1

A. What is the infectious syndrome that I will be treating/where is infection located?
- influences choice and dose of antimicrobials (ex. meningitis vs. ear infection)
B. What are the common pathogens that might be causing the syndrome?
- depends on the possible exposures/social history
C. What is the likely antibiotic resistance of the organisms being considered?
- need to consider MRSA if S. aureus is reasonable
- if Pseudomonas is involved, make sure to include anti-pseudomonal coverage
- if anaerobes are involved (unlikely to have resistance) but make sure to cover it
D. What is the most appropriate antibiotic to use?

Question 2

When are antibiotics bactericidal?

Answer 2

When MBC/MIC < or = 4

Question 3

When are antibiotics bacteriostatic?

Answer 3

When MBC/MIC > 4

Question 4

MIC

Answer 4

Minimal inhibitory concentration - lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism following incubation (expressed mg/L or ug/L)

Question 5

3 methodologies to determine MIC

Answer 5

1) Tube dilution (broth dilution): concentration of antibiotic at which broth becomes translucent
2) Disk diffusion (Kirby-Bauer): mm inhibition around disk translates to MIC
3) E test: use strip that has graded antibiotic concentration to create an ellipse on the plate. Point at which ellipse meets the strip is generally regarded as MIC.

Question 6

MBC

Answer 6

Minimal bactericidal concentration - minimal concentration of an antibiotic that KILLS 99.9% of bacteria (less than 10 colonies on a plate)

Question 7

Principles involved in setting antimicrobial breakpoints

Answer 7

Pharmacokinetics/dynamics of the antimicrobial
Pattern of susceptibility of a population of bacteria
Clinical experience (e.g. Vancomycin and Pip/Tazo)

Question 8

Potential pitfalls of setting/interpreting antimicrobial breakpoints

Answer 8

Can and do change over the years, usually because clinical data accumulates suggesting a problem. Ex. vancomycin against S. aureus was lowered from 4mcg/mL to 2mcg/mL to be considered sensitive
Breakpoints are not adjusted for site of infections, number of organisms in an infection, conditions in the host, or to reflect patient’s host defenses

Question 9

Antimicrobial breakpoints

Answer 9

MICs of antibiotics at which a particular bacterium is determined to be: susceptible, intermediately susceptible, or resistant

Question 10

Relationship between maximal Cp and MIC in order for bacteria to be considered suscpetible

Answer 10

At normal dosages the antibiotic should have a maximal serum concentration that exceeds the MIC of the bacteria. (Cp > MIC) Most commonly via oral or IV route