Enhanced Data Reports

Traditionally methods of antimicrobial susceptibility testing in the diagnostic microbiology laboratory are performed by either disc diffusion or by MIC determinations. MIC testing provides the clinician with more information than standard disc diffusion testing. Outlined below is some background information to help in interpreting the enhanced data available on our reports.

What is a MIC?

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.

What do the numbers mean?

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.

How is the MIC reported?

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.

What do the interpretations mean?

The MIC number 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 for which response rates may be lower than for sensitive strains. A report of resistant indicates that the strain is not inhibited by the usually attainable concentrations of the agent with normal dosage schedules.

Why don’t I get sensitivity tests on some of my reports but just a list of suggested agents?

We follow guidelines set by the Clinical Laboratory Standards Insititutes (CLSI) and the British Society for Antimicrobial Chemotherapy (BSAC) 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 will be reported with a recommended list of antibiotics. Examples include beta-haemolytic streptococci 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).