Sensitivity and Susceptibility Testing

  Table of Contents...                     

Use Case. AST, Antibiotic Susceptibility Testing

A la Wikipedia, Antibiotic Sensitivity as the susceptibility of bacteria to antibiotics

Antibiotic susceptibility testing, AST, is carried out to determine which antibiotic will be most successful in treating a bacterial infection.

Small wafers containing antibiotics are placed onto a plate upon which bacteria are growing. If the bacteria are sensitive to the antibiotic, a clear ring, or zone of inhibition, is seen around the wafer indicating poor growth.

The same configuration can also be used for Minimum Inhibitory Concentration, MIC. testing. Still with the Wikipedia: MIC tests for the lowest concentration of an antimicrobial that will inhibit the visible growth of a micro-organism after overnight incubation

Minimum inhibitory concentrations are important to confirm resistance of micro-organisms to an antimicrobial agent and also to monitor the activity of new ones

Clinically, minimum inhibitory concentrations are used to determine the amount of antibiotic that a patient will receive and also the type of antibiotic used

Configuration for AST 

In this example we are looking at an AST Analysis Service, for which organism sizes will be measured and reported as:

This sounds the wrong way round...

AST Calculation

Since the calculation will be used for more than one organism or drug, and re-captured for each, create Interim values in a wide band of values, say Lower and Upper Limits of 0 and 100 mm, and Diameter 0

AST Formula

This formula, using the Interim Keywords, resolves the logic, interpreting organism size as Resistant, Intermediate or Sensitive:

[DIAMETER] <= [LOWER_LIMIT]
and
"{0:0.0f} mm - Resistant ( Resistant: <= {1} )".format([DIAMETER], [LOWER_LIMIT]) or ([DIAMETER] > [LOWER_LIMIT] and [DIAMETER] < [UPPER_LIMIT])
and
"{0:0.0f} mm - Intermediate ( Intermediate: >{1}, <{2} )".format([DIAMETER], [LOWER_LIMIT], [UPPER_LIMIT]) or [DIAMETER] >= [UPPER_LIMIT]
and
"{0:0.0f} mm - Sensitive ( Sensitive: >= {1} )".format([DIAMETER], [UPPER_LIMIT])

Click [Save]

AST Method

Select the Calculation for an AST Method

Create the AST Analysis Service

The Test that'll be using the Calculation. Let's go with Enterococcus' Penisillin sensitivity

Set it's standard configuration up as is explained in the Analysis Services section of the LIMS setup and configuration chapter and select the Method that is using the AST Calculation

Then overwrite the default diameter, upper and lower limit values if required, in this case Lower and Upper Limits of 14 and 15, and Diameter 0

Capturing AST Results

All Analyses created on ARs using the Enterrococcus Analysis Service will then use the calculation formula to report captured measurements as such: 12.2 mm, Resistant