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 results Growth 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/ml MIC must be greater than 64.0 micrograms/ml or the highest concentration tested MIC = < 4.0 micrograms/ml MIC could = 4.0, but could also be at a much lower dilution

MIC Interpretation – Escherichia coli Gentamicin = 4.0 micrograms/ml Ampicillin = > 64.0 micrograms/ml Imipenem = < 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 coli Standardized 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 coli Standardized Interpretation Criteria E. coli = Enterobacteriaceae (gram-negative rod) Previously determined: Ampicillin MIC = >64 micrograms/ml Previously determined: Ampicillin MIC = >64 micrograms/ml = Resistant (R)

MIC Interpretation – Escherichia coli Standardized Interpretation Criteria E. coli = Enterobacteriaceae (gram-negative rod) Previously determined: Imipenem MIC = < 4.0 micrograms/ml Gentamicin = 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 isolate Gentamicin = 4.0 micrograms/ml = S Ampicillin = > 64.0 micrograms/ml = R Imipenem = < 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 susceptible Therefore, 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 susceptible Therefore, 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 susceptible Therefore, 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

Overview 1.Interpret the MIC values of the susceptibility test 2.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 antibiogram Known predictable patterns of susceptibility pg. 5, Unit 3