Clinical interpretations of antibiotic susceptibility results rely on them being analytically reliable. Part of quality management of susceptibility results is to compare results of a test method with results from a reference method. One comparison involves the reporting of qualitative susceptibility categories.
Possible results for each method:
(1) susceptible
(2) intermediate or moderately susceptible
(3) resistant
This gives 9 possible combinations (3 * 3) for reference to test results.
|
Reference Method
|
Test Method
|
Agreement Category
|
|
resistant
|
resistant
|
agreement
|
|
resistant
|
intermediate
|
minor error
|
|
resistant
|
susceptible
|
very major error
|
|
intermediate
|
resistant
|
minor error
|
|
intermediate
|
intermediate
|
agreement
|
|
intermediate
|
susceptible
|
minor error
|
|
susceptible
|
resistant
|
major error
|
|
susceptible
|
intermediate
|
minor error
|
|
susceptible
|
susceptible
|
agreement
|
percent very major errors =
= (number of very major errors) / (number of organism susceptibility test results) * 100%
percent major errors =
= (number of major errors) / (number of organism susceptibility test results) * 100%
percent minor errors =
= (number of minor errors) / (number of organism susceptibility test results) * 100%
percent category agreement =
= (number of agreements) / (number of organism susceptibility test results) * 100%
These can be calculated for:
(1) all organisms and all antibiotics
(2) all organisms for a given antibiotic
(3) a given organism for all antibiotics
(4) a given organism for a given antibiotic
Interpretation:
• Ideally there should be complete agreement between the reference and test results.
• Major and very major errors should be < 5% with very major errors <= 1.5% (FDA, Thornsberry).
