Mixed Cultures, Unusual Phenotypes, and Confirmatory Testing

Distinguish a likely technical error from a plausible unusual resistance phenotype by checking culture purity, identification–AST concordance, intrinsic resistance, QC status, and expected phenotype patterns.

Select confirmatory methods for flagged Gram-positive resistance phenotypes such as methicillin resistance in staphylococci and vancomycin resistance in enterococci.

Interpret a D-test result by applying correct disk placement, incubation, and D-zone criteria.

Select phenotypic or molecular follow-up for suspected ESBL or carbapenemase production in Gram-negative bacteria according to the purpose of testing and current laboratory policy.

Explain why automated colistin results may be unreliable and when broth microdilution confirmation is required.