Overview
Antimicrobial Susceptibility Testing (AST) is a laboratory process used to determine how effectively specific antimicrobial agents—such as antibiotics—act against microorganisms like bacteria or fungi. By exposing an isolate to a range of drugs, AST helps determine whether the organism is susceptible, intermediate, or resistant to each agent.
AST results are essential for guiding clinical treatment, supporting antimicrobial stewardship, and contributing to wider public-health surveillance efforts, especially in the context of rising antimicrobial resistance.
Results Handling
Within SEDRI-LIMS, susceptibility results can be associated with an isolate in two ways:
- Automated analyser integration
If your laboratory uses an automated AST system—such as a bioMérieux Vitek 2 or BD Phoenix M50—SEDRI-LIMS can automatically receive and assign analyser results to the correct isolate, provided the interface has been configured.
- Manual or non-integrated testing
If AST is performed manually, or your automated system is not yet interfaced, SEDRI-LIMS supports the entry of both quantitative and qualitative results from any combination of disk diffusion, MIC determination, or gradient diffusion strip tests, all captured within a single AST form for each isolate.
The remainder of this section is focussed on the second case, manual entry of AST results.
Test Patterns and Automated Row Selection
AST is typically performed as a panel of antibiotics chosen according to the organism or organism group being tested. To streamline manual entry, SEDRI-LIMS provides configurable test patterns, each of which defines a set of antibiotic test rows relevant to a particular scope of organisms.
A test pattern may be associated with:
- Taxonomic groups, such as Enterobacterales, Salmonella spp., E. coli, etc.
- Non-taxonomic categories, such as “coagulase-negative staphylococci”.
Each row in a test pattern specifies:
- The antibiotic
- The test method (disk diffusion or MIC)
- Disk potency (if applicable)
- The guideline used (EUCAST or CLSI)
- Other method-specific details (see ??????)
On entry to the AST form, SEDRI-LIMS searches for matching test patterns based on the organism identity (if known to the system). If there is a clear, unambiguous match, the system will render the test pattern antibiotic rows on the form, adding measurement (quantitative) and interpretation (qualitative) susceptibility fields per row, to be filled out as results become available.
Automatic Interpretation of Results
As measurements are entered for disk diffusion, MIC, or gradient diffusion tests, SEDRI-LIMS attempts to interpret each result automatically. Interpretation is based on:
- The organism
- The antibiotic
- The test method
- The guideline selected
- The breakpoints configured in the system
Where valid breakpoints exist, the system assigns the appropriate susceptibility category automatically.
AST Entry Before Organism Identification
It is possible to begin AST entry before an organism has been identified—for example, when identification and susceptibility testing run in parallel. However:
- Without an organism ID, test patterns cannot be auto-selected, so users must manually choose a pattern or create test rows individually.
- Qualitative interpretations cannot be generated until the organism is known.
- Once the organism is identified, the AST form can be re-opened so the system can apply the correct breakpoints and interpret the results.
Availability of AST Results
Whether entered manually or imported from an analyser, once AST results are stored, they can be:
- Viewed directly within SEDRI-LIMS
- Included in specimen reports
- Exported for AMR surveillance or any other downstream purpose