Antimicrobial Susceptibility Flashcards
when should you NOT perform susceptibility testing? (3)
- probable pathogen not identified
- systemic antimicrobial therapy will not be used
- significant pathogen has a predicatble susceptibility pattern
Which 9 organisms are predictably susceptible?
- beta hemolytic streps
- clostridium
- erysipelothrix rhusiopathiae
- listeria monocytogenes
- bacillus anthracis
- actinomyces spp
- haemophilus spp.
- pastuerella spp
- fusobacterium spp
T/F: if a bacterial pathogen is predictably susceptible, treatment will be easy
false, it just means that in vitro resistance to first line therapies for that organism has not been reported.
exs of predictably susceptible athogens that are NOT easy to tx are bacillus anthracis, clostridium, and actinomyces bovis
when is susceptibility indicated?
- sample collected appropriately
- sample transported appropriately to lab
- clinical evidence of inflammation
- significant growth isolated and not consistent with contamination.
- you want to use systemic antibiotics
what happens when you run susceptibiliy on an organism with questionable clinical signfiicance?
you may get skewed results. It may say that the organism is resistant to antimicrobials that it is not.
_____________ means bacterial growth is inhibited by a concentration of antibacterial associated with high likelihood of therapeutic success.
susceptible
__________ means bacterial growth is inhibited by a concentration of an antibacterial associated with an uncertain therapeutic effect.
intermediate
_________ means bacterial growth is inhibited by a concentration of antibacterial associated with high likelihood or therapeutic failure
resistant
T/F: susceptibility category intermediate drugs may have efficacy in body sites where the drug is physiologically concentrated.
true
Describe the 90-60 rule
if a patient has a monobacterial infection and is immunocompetent, then…
90% pts will have a positive therapeutic response using ‘susceptible’ drugs
60% pts will have a will have a positive therapeutic response using ‘resistant’ drugs
This is due to the help of the immune system and proves that in most cases, even if we do nothing at all, the patient will have a positive outcome. Our goal is to close the 90-60 gap.
If you started empirical antibiotic treatment on a patient with a UTI and their culture results inform you that the current antibiotic is “intermediate”, what do you do?
Call the client and get an update on the patient’s condition
If they are improving, you can finish the course of current antibiotics.
If they are not doing well, you can switch to a drug listed as “susceptible”
In what scenario would using “intermediate” drugs be MOST appropriate and therefore unecessary to switch to a “susceptible” drug?
- young, healthy animal
- uncomplicated infection
- infection is clinically responding well to therapy
does it matter if a lab uses microbroth dilution vs K-B disk diffusion?
no
________ is the minimum concentration of drug required to inhibit the organism’s growth and can only be dervied from broth micro dilution or gradient strip testing.
MIC
why is it important that you indicate the site from which the sample was collected when submitting c/s?
The amount of drug achievable at different sites of the body is variable. The lab needs to know the site so that they can use the correct breakpoint for determining susceptible, intermediate, or resistance.
________ is the maximum MIC that predicts sucessful therapy and tells you if an organism is susceptible, intermediate, or resistant.
breakpoint
what 3 things do breakpoints take into account?
- the normal range of MIC values of wild type bacteria
- PK/PD properties of the drug in the species of interest
- the site of infection
T/F: there is a breakpoint for each source species, drug, organism, and site combination
true
T/F: picking a drug off the susceptibility results list requires picking the drug with the lowest MIC value
false
this is inaccurate because each source species, drug, organism, and site has its own breakpoint, so choosing a drug based on its MIC value alone is not significant. A MIC for doxycycline may be 0.12, but perhaps thats right at the breakpoint, whereas the MIC for cephalexin may be 0.5 and the breakpoint is 2. In general, a susceptible drug is a susceptible drug.
You must consider other things like side effects, penetration to site of infection, cost, dosing convenience, owner compliance, etc.
what do we do when there is no breakpoint?
use breakpoints from other species or from similar organisms.
T/F: breakpoints stay consistent over time
false
T/F: MIC breakpoints are based on topical and systemic concentrations of antimicrobials
false – only systemic concentrations.
Topical antimicrobials have much higher concentrations than can be achieved through oral or parenteral dosing.
why is the susceptibility list of drugs so short for organisms like pseudomonas aeruginosa and enterococcus faecium?
high level of intrinsic resistance! labs do not want you to mistake it for some highly resistant pathogen.
P. aeruginosa is intrinsically resistant to beta-lactams, most cephalosporins, tetracyclines, chloramphenicol, trimethoprim-sulfa, and easily develops resistance to fluoroquinolones.
So, the list of drugs is always shorter.
E. faecium is intrinsically resistant to aminoglycosides, cephalosporins, clindamycin, trimethoprim-sulfa
