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Antimicrobial susceptibility testing of Haemophilus influenzae. Improvement of accuracy of the disc diffusion test
S Ringertz1, B Olsson-Liljequist, G Kronvall
1Department of Clinical Microbiology, Karolinska Institute, Karolinska Hospital, Stockholm, Sweden.
Abstract:
A national quality control study was performed in 1986 to investigate the standard of performance of the disc diffusion antimicrobial susceptibility testing of Haemophilus influenzae in Sweden. The accuracy of susceptibility interpretations was unacceptably low. A new standardized method for susceptibility testing of H. influenzae was then worked out, and a new method of setting interpretive zone breakpoints was introduced. The susceptibility category of the main population of clinical isolates was determined according to the MIC50 of the strains. The zone histograms of clinical isolates from five reference laboratories were used for the calculation of interpretive breakpoints. For instance, for susceptible strains the mean of the combined zone values from these laboratories +/- 2 S.D. covered the zone range of the susceptible group, and one more S.D. below covered the intermediate/indeterminate group. The new zone breakpoints would place the main population of clinical isolates in the correct susceptibility group, and even make it possible to detect strains with different degrees of reduced susceptibility in the routine test. In a follow-up quality control study in 1988 the interpretive errors for clinical isolates were eliminated for all antibiotics except doxycycline in some laboratories. Laboratory-related zone breakpoints for doxycycline calculated by the single strain regression analysis method led to correct susceptibility interpretations also in these cases.
Insights
A 1986 study revealed inaccurate antimicrobial susceptibility testing for Haemophilus influenzae in Sweden. A new standardized method and breakpoint system implemented in 1988 significantly improved accuracy, virtually eliminating interpretive errors.
Area of Science:
- Medical Microbiology
- Clinical Diagnostics
- Pharmaceutical Science
Background:
- The disc diffusion method is crucial for antimicrobial susceptibility testing.
- In 1986, a national quality control study in Sweden identified unacceptably low accuracy in interpreting Haemophilus influenzae susceptibility results.
- This highlighted a need for improved standardization and interpretive criteria.
Purpose of the Study:
- To develop and evaluate a new standardized method for antimicrobial susceptibility testing of Haemophilus influenzae.
- To establish a revised system for setting interpretive zone breakpoints.
- To improve the accuracy of susceptibility interpretations in clinical laboratories.
Main Methods:
- A national quality control study was conducted in 1986.
- A new standardized disc diffusion method was developed.
- Interpretive zone breakpoints were recalculated using MIC50 values and zone histograms from five reference laboratories.
- A follow-up quality control study was performed in 1988 to assess the impact of the new methods.
Main Results:
- The initial 1986 study showed unacceptably low accuracy in susceptibility interpretations.
- The newly developed standardized method and breakpoint system significantly improved accuracy in the 1988 follow-up study.
- Interpretive errors were virtually eliminated for most antibiotics, with specific adjustments for doxycycline improving accuracy in all laboratories.
Conclusions:
- The revised standardized method and breakpoint setting strategy effectively improved the accuracy of antimicrobial susceptibility testing for Haemophilus influenzae.
- The new system allows for correct categorization of bacterial susceptibility and detection of reduced susceptibility.
- Further refinement, including laboratory-specific breakpoints for certain antibiotics like doxycycline, ensures robust diagnostic performance.