As part of external quality assessment procedures, every year the National Reference Laboratory (NRL) sends unknown isolates to the clinical laboratories in its network for identification and antimicrobial susceptibility testing and collects the results. Based on these results, the NRL evaluates three predefined quality indicators presented below and develop an improvement strategy accordingly.
Efficiency in microbial identification
Bacterial Identification is evaluated according to one of these categories:
- “Completely correct identification” of the microorganism: correctly identified genus and species
- “Incomplete, correct identification”: correctly identified genus but no species specified
- “Partial, incorrect identification”: correct genus and incorrect species
- “Completely incorrect identification”: incorrect genus
For eg. in the case of an isolate of Klebsiella pneumoniae:
- a report of Klebsiella pneumoniae is classified as correct genus and species,
- a report of Klebsiella spp. is interpreted as correct genus with no species specified,
- a report of Klebsiella oxytoca is interpreted as correct genus and incorrect species, and
- a report as Enterobacter cloacae is interpreted as incorrect genus.
Efficiency in Interpretation of Susceptibility Tests in accordance with current regulations
To categorize errors in the interpretation of susceptibility test results, global consensus definitions are used: “minor”, “major” or “very major” errors.
- No errors in interpretation: adequate interpretation of the susceptibility test result.
- Minor error (Mi): Microorganism shows intermediate sensitivity to a given antimicrobial but is reported as susceptible or resistant; microorganism is susceptible or resistant but reported as showing intermediate resistance.
- Major error (Ma): classification as resistant of a susceptible strain (false resistance).
- Very major error (Vma): classification as susceptible of a resistant strain (false susceptibility).
Values relative to a reference range
The reference ranges of the NRL for inhibition zone sizes are set as the average in mm of the zones obtained after 30 determinations ± 2 standard deviations (SD), with a minimum deviation of ± 3 mm from the average value (minimum range: 7 mm). The MICs of the strains are evaluated using different methods: broth dilution, agar dilution, automatized methods (Vitek2, Phoenix, MicroScan) or epsilometric method (E-test, MICE, CIM). In the case of ATCC strains, the reference ranges established by CLSI (for inhibition zones and MICs) are used and in the case of strains without inhibition zone (6 mm) no range is used.
“Within the reference range”: the value obtained is within the established range
“Outside the reference range”: the value obtained is outside the established range