microbiologic lab teests in ID and rapid diagnostic tests

Question 1

MIC/MBC testing - broth dilution

Answer 1

micro v macro dilution - difference in volume of broth used
two-fold (2,4,8,16, etc) dilutions of drug concentration in liquid media - only difference between tubes is drug conc
standard bacterial inoculum (10^5-6 CFU/ml) added to each tude; incubated at 35 C for 18-24 hours
MIC - lowest conc that prevents VISIBLE growth (unaided eye)
MBC - lowest conc resulting in over 99.9% decrease in initial inoculum

Question 2

MIC - agar dilution

Answer 2

“drug in plate”
two fold dilutions of AB incorporated into molten agar, poured into petri dishes and allowed to solidify
bacterial inoculum applied to surface of agar; incubate at 35 C for 18-24 hours, inspect for growth
MIC - agar plate with lowest conc of drug and no growth of organisms
dots = different organisms

Question 3

MIC - epsilometer test (Etest)

Answer 3

antibiotic impregnated plastic strips containing a continuous conc gradient of drug
mircoorganism streaked onto surface of agar plate; etest strip applied to surface of plate, incubate for 24 hours, elliptical zone of inhibition formed
MIC - conc on strip where inhibition ellipse intersects the scale of the strip
more “precise” MIC values compared to conventional two-fold dilutions

Question 4

Automated AST systems

Answer 4

Vitek 2 - uses reagent cards
microscan - algorithm-dervied MIC
BD phoenix - algorithm-derived MIC

Question 5

susceptible

Answer 5

isolates are inhibited by usually achievable conc when normal dosing regimens are used

Question 6

susceptible-dose dependent

Answer 6

S-DD

susceptibility is dependent on the dosing regimen used (higher dosing)

Question 7

intermediate

Answer 7

may be useful when max doses are used or if drug is conc at the site of infection

Question 8

resistance

Answer 8

never use

Question 9

how are breakpoints esablished?

Answer 9

clinical pharmacology of the drug
clinical and bacteriologic response from human studies
distribution of MICs within a bacterial population
phenotypic detection of bacterial isolated with certain resistance mechanisms

Question 10

inoculum effect

Answer 10

increase in MIC when a higher than standard inoculum of bacteria is used in the susceptibility test (i.e. inoculum is over 10^5 CFU/ml)
seen more frequently with B-lactams
important because in the lab our conc is always 10^5 but what is it in a patient???

Question 11

tolerance

Answer 11

MBC greater than or equal to 32 x MIC
clinical significance not well known - has been associated w poor outcome
rarely identified clinically because MBCs are not routinely determined

Question 12

MIC stats

Answer 12

in a population of bacteria (same strain, different samples) in which the MIC for each isolate has been determined
MIC50: antibiotic conc that inhibits 50% of the bacteria tested
MIC 90: AB conc that inhibits 90% of bacteria tested
geometric mean MIC - antilog of the mean of the log MICs
modal MIC - most frequently occurring MIC

Question 13

disc diffusion

Answer 13

fixed amount of AB in commerically supplied disc
placed onto agar plate streaked w organism and incubated
zone of inhibition created
big zone doesnt necessarily mean it’s our best drug clinically - i.e. vanc won’t move that quickly
growth within a zone is considered resistance

Question 14

synergy

Answer 14

the activity of an antimicrobial combination is greater than that expected from the additive activity of the individual agents
1+1=5

Question 15

antagonism

Answer 15

the activity of an antibicrobial combination is less than that expected from the additive activity of the individual agents
2+2=3

Question 16

additivity or indifference

Answer 16

neither synergy not antagonism

Question 17

synergy testing - time-kill curves

Answer 17

measures rate and extent of bactericidal activity
synergy - over 100-fold increase in bacterial killing at 24 hours with the combination, compared with the most active single agent
antagonism - over 100-fold decrease in bacterial killing at 24 hours with the combination, compared with the most active single agent

Question 18

post antibiotic effect (PAE)

Answer 18

persistent effect of the antibiotic on bacterial growth after brief exposure of organism to drug

Question 19

timeframe for routine culture and susc testing

Answer 19

24 h: culture
36 h: preliminary report
48 h: susc report
72 h: susc report (fastidious(
96 h: final C&S report

Question 20

National action plan for combating antibiotic resistant bacteria (CARB)

Answer 20

goal 1: slow emergence of resistant bacteria and prevent spread of resistant infections*
goal 2: strengthen national surveillance efforts to combat resistance
goal 3: advance development and use of rapid and innovative diagnostic test for identification of resistant bacteria*
goal 4: accelerate R and D for new ABs
goal 5: improve international collaboration and capacities for prevention, surveillance and antibiotic research and development

Question 21

7 steps to preserve the miracle of antibiotics

Answer 21

-
-aggressively promote antimicrobial stewardship
-promote use of new diagnostics with emphasis on point-of-care molecular methods
-
-

Question 22

chromogenic medium

Answer 22

only rapid test that you don’t need a positive culture for 1st

Question 23

PNA-FISH differentiating various pathogens

Answer 23

GNR traffic light:
R: P. aeruginosa
Y: K. pneumoniae
G: E. coli
yeast traffic light:
R: C. glabrate and/or C. krusei
Y: C. tropicalis
G: C. albicans and/or C. parapsilosis

Question 24

Verigene gram-positive blood culture tests

Answer 24

resistance: mecA, vanA, vanB

Question 25

verigene gram-negative blood-culture test

Answer 25

resistance: CTX-M, IMP, KPC, NDM, OXA, VIM

Question 26

consideration for RDTs in ASPs

Answer 26

provides collarborative opportunities for pharmacists with physicians and microbiology personnel
ASPs must have an active role with implementation of RDTs
RDTs have little value if not coupled with ASPs and acted upon in timely fashion
sensitivity and specificity varies among RDTs
purchase price or lease agreement
costs of supplies, reagents, etc
test complexity