8 Screening for resistance mechanisms

8.1 Theory behind screening and confirmatory testing

Many of the phenotypic and genotypic tests used to identify the resistance mechanism are time-consuming and expensive. Therefore, to reduce the total number of tests performed it is customary to screen for potential resistance mechanisms first and then confirm the mechanism on those isolates that screen positive.

The initial screening tests are done to identify organisms which may have a particular mechanism. This could involve either:

  • a.routine screening of samples for asymptomatic carriage of resistance by AST, for example using samples from healthy food animals, or from asymptomatic hospital patients (for infection control purposes)
  • b.testing a human or other animal clinical isolate for resistance.

Confirmatory tests then prove whether the organism has the resistance mechanism or not (EUCAST, 2017).

  • Why is the approach of screening followed by confirmation useful?

  • Screening for organisms with a resistant phenotype first followed by confirmatory tests for the resistant ones saves time and resources. You can screen large numbers quickly then focus on the resistant ones for the confirmatory tests.

Confirmation is a necessary step to avoid over-reporting of the organisms which do show resistance but do not have the mechanism of concern.

Figure 13 AST in a clinical microbiology laboratory. Screening samples saves time and reduces workload.

7 Genotypic versus phenotypic methods

8.2 Screening for resistance mechanism in practice