Interpretation/Evaluation of Laboratory Data

CLS 552 Human Microbiology & Immunology Laboratory

Note: View this PowerPoint as a ‘Full Screen Slide Show’. In the right hand corner of the screen, click on the ‘Full Screen Slide Show’ icon. To advance, perform left-mouse click.

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