Susceptibility testing. Phenotypic and genotypic tests for bacteria and mycobacteria.

Genotypic-based methods hold promise for the rapid and accurate detection or confirmation of antimicrobial resistance; however, phenotypic methods will continue to have an advantage when resistance to the same antimicrobial agent may be caused by several different mechanisms. The diversity of genetic mechanisms may exceed the capabilities of current molecular technology. Genotypic assays have the ability to detect resistance but not susceptibility. Although resutls can be obtained rapidly, many molecular methods are labor-intensive, expensive, and lack standardization. Clinical studies will be required to validate the genotypic approach to detection of antimicrobial resistance. Molecular assays are also at risk for false-positive results because of contamination of specimens by other specimens that carry the DNA targeted for the assay, or carryover of amplified target DNA (amplicons) from a previous PCR assay during sample preparation. Detection of certain genetic resistance loci in clinical specimens must be interpreted with caution, because organisms in normal flora may also harbor the same loci. All these factors must be taken into consideration when introducing a genotypic method in the clinical laboratory. Other considerations include cost, turnaround time, and assay performance. It must be emphasized that the bedside assessment of the patient should always be considered in addition to the results of antimicrobial susceptibility tests (whether phenotypic or genotypic) so that the best outcome is assured for the patient.