Table 1.
Advantages and disadvantages of different approaches to accelerated antimicrobial susceptibility testing using blood cultures as an example.
| Method | Time-to-resulta | Advantages | Disadvantages |
|---|---|---|---|
| Conventional AST (disk diffusion, manual broth microdilution, gradient tests) | 38-48 hours | Reference method; Established breakpoints; MIC distributions and epidemiological cut-off values availableb; High-throughput testing possible; Widely used methods, large amount of validation data published; Full flexibility regarding choice of tested antimicrobials; Detection of inducible resistance, e.g. D test for clindamycin or keyhole effect for ESBL in disk diffusion testing; Relatively cheap | Relatively slow; Expert knowledge on data interpretation required at all times; Laboratory is fully responsibility for quality control |
| Automated AST | 22 48 hours | Widely used method, large amount of validation data published; Pre-defined set of rules to support data interpretation (expert system); Detection of inducible resistance; Manufacturers responsible for product quality | Higher cost; No direct visual feedback, e.g. on incremental; development of drug resistance in follow-up samples |
| Direct AST | 14-24 hours | High-throughput testing possible; Free choice of compounds (unless an automated system is used); No additional equipment required | No systematically evaluated breakpoints; No standardized inoculum; Not controlled for polymicrobial infections; Not controlled for substances in the specimens that affect results |
| Rapid conventional AST | 18 32 hours | High-throughput testing possible; Free choice of compounds (unless an automated system is used) | No valid breakpoints; Inducible resistance may be missed; Additional equipment may be required (e.g. high-resolution cameras) |
| Novel approaches to phenotypic AST | 3-8 hours | Very short time-to-result (same working day); Some systems offer simultaneous species identification | Few commercial products available, additional equipment may be required; High throughput testing may be limited; Flexibility regarding choice of tested compounds may be limited; Breakpoint validation required |
| Genotypic AST | 1-5 hoursc | Very short time-to-result (same working day); Full flexibility and relatively low cost for in-house systems | Unambiguous genotype-phenotype relation required; No detection of novel resistance mechanisms; Little flexibility and relatively high cost for commercial systems; Laboratory is fully responsible for validation and quality control of in-house systems |
a Time-to-result was defined as the time required from blood culture positivity until the generation of an AST report. For estimates of the time-to-result, standard weekday working hours of 8 a.m. until 6 p.m. were taken as a basis.
b Breakpoint distributions for both minimal inhibitory concentrations and disc diffusion zone diameters available through the EUCAST website (httpyAvww.eucast.org/mic_distributions_and_ecoffs/).
c Depending on the assay format.