Frequently
Asked
Questions

The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antibiotic that inhibits the growth of an organism. MIC susceptibility testing is considered the ‘gold standard’ against which all other susceptibility testing methods are assessed.

MICs are reported as one of the following serial doubling dilution of antibacterial agent. Note: All concentrations are micrograms/ml (µg/ml).

Note: This reference range is different for each antibiotic Each antibiotic has a different reference range based on various pharmacokinetic factors such as achievable concentrations, routes of excretion, and volume of distribution.

When you receive a report from IDEXX, you will see the name of the organism, followed by a list of the antibiotics tested against that organism. Following each antibiotic listed, you will find one of three results: Sensitive, Intermediate or Resistant. This is followed by a number – the Minimum Inhibitory Concentration that is expressed in µg/ml. On the right hand side of the report is the reference range for the antibiotic tested.

This reference range indicates the upper and lower concentrations of the antibiotic tested. Between these limits there is a series of letters that represent the number of dilutions between the upper and lower concentrations tested. These are represented by a ' s ' (sensitive), ' i ' (intermediate) or ' r ' (resistant). The place within the reference range which corresponds with the result of the organism isolated from the submitted sample is capitalised.

The MIC result for one antibiotic can NOT be compared to the MIC number for another antibiotic. It is vital that the reference ranges are known. MIC interpretations are based on plasma concentrations of the antibiotic.

If the organism is reported as sensitive, it implies that the infection due to the strain may be appropriately treated with the dosage of antimicrobial agent recommended for that type of infection.

Intermediate indicates that the MIC is approaching attainable blood and tissue levels and response rates may be lower than for sensitive strains. There is a high likelihood of therapeutic success if exposure to the antibiotic is increased by adjusting the dosing regimen or by its concentration at the site of infection.

A result of resistant indicates that the strain is not inhibited by the usually attainable concentrations of the agent with normal dosage schedules.

We follow guidelines set by the Clinical Laboratory Standards Insititutes (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) combined with our years of experience in performing susceptibility testing. As such the following is an outline of our antibiotic testing policy:

• Susceptibilities are not performed on normal flora and non-pathogenic organisms.
• Pathogens with predictable susceptibility patterns or where routine testing is not available will be reported with a recommended list of antibiotics. Examples include anaerobes and Campylobacter spp.
• We do not test inappropriate organism/antibiotic combinations, such as Clindamycin against Gram-negative organisms or amoxicillin-clavulanic acid against Pseudomonas aeruginosa (both examples have intrinsic resistance mechanisms against these antibiotics).

The classifications for tiered antibiotics are:

1. First-line antimicrobial – this should be considered a first-line antimicrobial where antimicrobial treatment is required.
2. Second-line antimicrobials – this is a second-line antimicrobial and should be reserved for when first-line antimicrobials are ineffective or inappropriate for the clinical case.
3. Third-line antimicrobial - this is a third-line antimicrobial and should ideally be reserved for human use.

We have refined the order in which antibiotics are listed on our reports. The order is a combination of antibiotic class (beta-lactams, fluoroquinolones etc.) and the category (first/second/third-line) that the antibiotics are assigned to. The general approach is first-line antimicrobials appearing first, then second-line, and finally third-line antimicrobials.

Our overall approach to antibiotic reporting remains unchanged. We construct our panels based on the principle of providing you with relevant antibiotics, based on the organism, site, and host species. We do not routinely report antibiotics that may not be suitable at the site of infection, or where organisms have known intrinsic resistance. However, we cannot account for all patient particularities (e.g., age, renal and hepatic function, risk of dysbiosis, pharmacokinetics, etc) and reported antimicrobials should be viewed as a guidance only. We recognise that antimicrobial prescription considers many patient and clinical factors that cannot be restricted to what is listed here. These changes in the reporting intend solely to make it easier to identify which class and antimicrobial category each antimicrobial represents.