How to evaluate microbiological resistance?

Comprehensive Approach to Evaluating Microbiological Resistance

Microbiological resistance should be evaluated using standardized quantitative methods that include both phenotypic and genotypic testing approaches, with results interpreted using established epidemiological cut-off values and clinical breakpoints to determine susceptibility categories. 1, 2

Phenotypic Testing Methods

Minimum Inhibitory Concentration (MIC) Determination

• Use standardized quantitative methods to determine MICs, which remain the gold standard for antimicrobial susceptibility testing 1
• Ensure proper quality control with appropriate reference strains before and during testing 1
• For studies lasting more than 3 days, perform quality control testing each day 1

Interpretation of Results

• Categorize results into three classifications according to EUCAST guidelines 2:
  1. Susceptible (S): First choice for therapy
  2. Intermediate (I): Consider only in specific situations
  3. Resistant (R): Avoid use as infection is unlikely to respond

Reporting Results

Results should be expressed in terms of 1:

1. Percentage of strains belonging to wild-type distribution (MIC ≤ epidemiological cut-off value)
2. Percentage of strains exhibiting clinical resistance (above clinical R breakpoint)
3. Percentage of strains showing high-level resistance (when defined breakpoints exist)

Genotypic Testing Methods

Detection of Resistance Mechanisms

Target the following for surveillance 1:

1. Genes or gene combinations encoding resistance mechanisms
2. Specific products coded by resistance genes (e.g., PBP2a, β-lactamases)
3. Resistance phenotypes characteristic of specific mechanisms

Clonal Analysis

When tracking resistant clones, evaluate 1:

1. Phenotype: Resistance patterns that characterize specific clones
2. Genotypic traits: Genetic polymorphisms that identify particular clones

Special Considerations for Testing

Populations with Non-Wild-Type Distribution

• When testing bacterial populations enriched with non-wild-type isolates, use the error-rate-bound method for evaluation 1
• Calculate error rates by dividing the number of errors by the total number of isolates with MICs at intermediate value plus/minus 1 log₂ dilution 1

Handling Discrepant Results

When discrepancies occur between test methods 1:

1. Repeat testing in duplicate or triplicate
2. If repeat results match original, the error stands
3. If results are variable, select the predominant MIC (mode)
4. For very distinct results, ensure pure culture and confirm organism identification

Surveillance and Monitoring

Tracking Resistance Trends

• Regular surveillance of antibiotic susceptibility patterns is essential for guiding empiric therapy 2
• Monitor for evolving trends in low-level resistance as a predictor of high-level resistance emergence 1

Data Stratification

Include minimal stratification of data according to 1:

• Specimen type
• Inpatient/outpatient status
• Ward type for inpatients

Pitfalls and Caveats

1. Breakpoint Challenges: Breakpoints that bisect wild-type MIC distributions (e.g., cefazolin for Enterobacteriaceae) will inherently result in higher error rates 1

2. Method Limitations: Phenotypic methods may miss emerging resistance mechanisms, while genotypic methods may detect resistance genes that are not expressed phenotypically 3

3. Quality Control Failures: Ensure all QC results are in range before performing evaluations; if any results are out of range, perform additional testing until 95% of results are in range 1

4. Population Distribution Effects: The error rate is directly proportional to the percentage of isolates that hover around the breakpoint by 1 log₂ dilution 1

By implementing this comprehensive approach to evaluating microbiological resistance, clinicians can make more informed decisions about antimicrobial therapy, potentially reducing morbidity and mortality associated with resistant infections.