Post on 25-Feb-2016
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Interpretation/Evaluation of Laboratory Data
CLS 552 Human Microbiology & Immunology Laboratory
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MIC Interpretation• MIC = minimum antibiotic concentration tested
that visibly inhibits growth• First determine if sterility and growth controls
have appropriate resultsGrowth Control =
Growth Sterility Control =
No growth
• MIC wells are compared to the above quality control wells
MIC Interpretation – Escherichia coli
4.0 micrograms/ml
> 64.0 micrograms/mlMIC must be greater than 64.0 micrograms/ml or the highest concentration tested
MIC =
MIC =
MIC = < 4.0 micrograms/mlMIC could = 4.0, but could also be at a much lower dilution
MIC Interpretation – Escherichia coliGentamicin = 4.0 micrograms/ml
Ampicillin = > 64.0 micrograms/mlImipenem = < 4.0 micrograms/ml
• After MICs have been determined, the next step is to determine if the values indicate the organism tested is susceptible (S), intermediate (I) or resistant (R) to a particular antibiotic
• S, I or R status is determined by using universal, published standardized criteria
MIC Interpretation – Escherichia coliStandardized Interpretation Criteria
• Select the appropriate Minimum Inhibitory Concentration (MIC) Interpretive Standards to use
• Category examples:– Staphylococcus sp.– Enterococcus sp.– Streptococcus pneumoniae– Streptococcus sp. other than Streptococcus pneumoniae– Enterobacteriaceae (e.g. Escherichia coli)– Pseudomonas aeruginosa and Other Non-
Enterobacteriaceae
MIC Interpretation – Escherichia coliStandardized Interpretation Criteria
• E. coli = Enterobacteriaceae (gram-negative rod)
Previously determined:Ampicillin MIC = >64 micrograms/mlPreviously determined:Ampicillin MIC = >64 micrograms/ml = Resistant (R)
MIC Interpretation – Escherichia coliStandardized Interpretation Criteria
• E. coli = Enterobacteriaceae (gram-negative rod)
Previously determined:Imipenem MIC = < 4.0 micrograms/mlGentamicin = 4.0 micrograms /ml
Previously determined:Imipenem MIC = < 4.0 micrograms/ml = Susceptible (S)Gentamicin = 4.0 micrograms /ml
Previously determined:Imipenem MIC = < 4.0 micrograms/ml = Susceptible (S)Gentamicin = 4.0 micrograms /ml = Susceptible (S)
MIC Data Evaluation – E. coli isolateGentamicin = 4.0 micrograms/ml = S
Ampicillin = > 64.0 micrograms/ml = RImipenem = < 4.0 micrograms/ml = S
• The next step: evaluate the MIC data with cumulative antibiogram data• Clinical microbiology laboratories usually publish susceptibility data at six month or year intervals
Cumulative Antibiogram Data
Reported as the % susceptible for a defined period
Total isolate number tested of a particular organism for the defined period listed
MIC Data Evaluation – E. coli isolate
Isolate tested: Imipenem = <4.0 micrograms/ml = S
Antibiogram data = 99% of E. coli isolates tested were susceptible 1% were not susceptibleTherefore, this isolate does follow predictable patterns of susceptibility.Based upon the cumulative antibiogram, we would expect E. coli to be (S).
MIC Data Evaluation – E. coli isolate
Isolate tested: Gentamicin = 4.0 micrograms/ml = S
Antibiogram data = 93% of E. coli isolates tested were susceptible 7% were not susceptibleTherefore, this isolate does follow predictable patterns of susceptibility.Based upon the cumulative antibiogram, we would expect E. coli to be (S).
MIC Data Evaluation – E. coli isolate
Isolate tested: Ampicillin = > 64.0 micrograms/ml = R
Antibiogram data = 53% of E. coli isolates tested were susceptible 47% were not susceptibleTherefore, no predictive value prior to susceptibility data
Predictable Patterns of Susceptibility
• There are also general predictable patterns of susceptibility.
• See Unit 3, pg. 5 (orange color)• Read as if, then statements• For example, item 7:
If isolate is Klebsiella pneumoniae or Citrobacter koseri, then should be (R) to ampicillin
Overview1. Interpret the MIC values of the susceptibility
test2. Determine if the MIC values are S, I or R for each
antibiotic tested using published standardized criteria charts
3. Evaluate the data by comparing to the:Cumulative antibiogramKnown predictable patterns of susceptibility pg. 5, Unit 3