Interpretation/Evaluation of Laboratory Data

1. 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.

2. MIC Interpretation Sterility Control = No growth Growth Control = Growth • MIC wells are compared to the above quality control wells MIC = minimum antibiotic concentration tested that visibly inhibits growth First determine if sterility and growth controls have appropriate results

3. MIC Interpretation – Escherichia coli MIC = 4.0 micrograms/ml > 64.0 micrograms/ml MIC must be greater than 64.0 micrograms/ml or the highest concentration tested MIC = MIC = < 4.0 micrograms/ml MIC could = 4.0, but could also be at a much lower dilution

4. 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

5. 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 aeruginosaand Other Non-Enterobacteriaceae

6. MIC Interpretation – Escherichia coliStandardized Interpretation Criteria Previously determined: Ampicillin MIC = >64 micrograms/ml Previously determined: Ampicillin MIC = >64 micrograms/ml = Resistant (R) E. coli = Enterobacteriaceae (gram-negative rod)

7. MIC Interpretation – Escherichia coliStandardized Interpretation Criteria 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) E. coli = Enterobacteriaceae (gram-negative rod)

8. MIC Data Evaluation – E. coli isolate • The next step: evaluate the MIC data with cumulative antibiogram data • Clinical microbiology laboratories usually publish susceptibility data at six month or year intervals Gentamicin = 4.0 micrograms/ml = S Ampicillin = > 64.0 micrograms/ml = R Imipenem = < 4.0 micrograms/ml = S

9. Cumulative Antibiogram Data Reported as the % susceptible for a defined period Total isolate number tested of a particular organism for the defined period listed

10. 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).

11. 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).

12. 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

13. 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

14. Overview • Interpret the MIC values of the susceptibility test • Determine if the MIC values are S, I or R for each antibiotic tested using published standardized criteria charts • Evaluate the data by comparing to the:Cumulative antibiogramKnown predictable patterns of susceptibility pg. 5, Unit 3