Interpretation of MICs
TECHNICAL UPDATE

DESCRIPTION/BACKGROUND INFORMATION:

When susceptibilities are reported for an organism that has been isolated on a culture, the antibiotics are given a qualitative interpretation of susceptible, intermediate, or resistant.

• Susceptible – implies that infections may be treated appropriately with the dosage of antibiotic recommended for the type of infection and infecting species, unless otherwise indicated.
• Intermediate – implies that infections may be treated if the antibiotic is able to reach specific tissues where the drug will be concentrated (for example quinolones in the urine) or when the drug can be used in higher than usual doses without adverse effects. This category also includes a “buffer zone” which should prevent small, uncontrolled technical factors from causing major discrepancies in interpretation.
• Resistant – isolates are not inhibited by the usually achievable systemic concentrations of the drug in normal dosage and/or fall in the range where specific microbial resistance mechanisms are likely (for example, beta lactamases) and clinical efficacy has not been reliable in treatment studies.

If the susceptibility is performed using broth microdilution there will also be an MIC result reported in ug/ml. This result is a quantitation of the minimum concentration of a antimicrobial agent that will inhibit growth of a organism in-vitro.

The concentrations of antibiotic to be tested for the MIC result are chosen to fall within the calling range of the susceptible, intermediate, or resistant interpretations as determined by NCCLS (National Committee for Clinical Laboratory Standards). Thus different antibiotics may be tested at different concentrations and the MIC numbers should not be directly compared.

CLINICAL APPLICATION:

The sensitivity report comes with an interpretation of the MIC – listing each antibiotic as sensitive, intermediate or resistant. Generally knowledge of the actual MIC value is not needed, except in complicated cases such as immunosuppressed patients, multiple resistant organisms or recurrent infections.
A general rule of antimicrobial therapy using MIC values is that the drug concentration achieved in-vivo should be two to four times the in-vitro MIC value. Knowing the achievable level of the drug being considered is necessary if the MIC value is to be a factor in the choice of antibiotic. Information on achievable levels is available in the Physicians Desk Reference.

Other factors include:

1. The mode of action of the antimicrobial.
2. Protein binding characteristics of the drug.
3. Age, weight, and general health of the patient.
4. The potential for toxicity of the drug.
5. Interaction with other medications.
6. Site of the infection.
7. Route of administration.
8. Pathogenicity of the infective organism.
9. Clinical efficacy of the drug.

REFERENCES:

1. Performance Standard for Antimicrobial Disk Susceptibility Tests; Approved Standard – Eighth Edition, NCCLS (The National Committee for Clinical Laboratory Standards.) January, 2003
2. Laboratory Guide to MICs, Technical Bulletin, bioMerieux Vitek, Inc, Vitek – R07.02, 007166-5 REV 0502