From the Guidelines
When interpreting a susceptibility report from a urinalysis (UA), it is crucial to focus on the lower Minimum Inhibitory Concentration (MIC) value, as this indicates greater susceptibility of the bacteria to the antibiotic. The lower the MIC value, the less antibiotic is needed to inhibit bacterial growth, meaning the antibiotic is more effective against that particular organism. For example, if a report shows an MIC of 0.5 μg/mL for one antibiotic and 8 μg/mL for another, the first antibiotic (with the 0.5 value) would be the more effective option, as supported by the guidelines for the diagnosis, treatment, prevention, and control of infections caused by carbapenem-resistant gram-negative bacilli 1. Sensitivity reports typically include interpretive categories like "Susceptible," "Intermediate," or "Resistant" alongside these numerical values to help guide treatment decisions. When multiple antibiotics show susceptibility, other factors should influence your choice, including:
- The antibiotic spectrum (preferring narrow over broad when possible)
- Potential side effects
- Patient allergies
- Cost
- Local resistance patterns, as highlighted in the optimization of the treatment with beta-lactam antibiotics in critically ill patients-guidelines from the French Society of Pharmacology and Therapeutics (Société Française de Pharmacologie et Thérapeutique-SFPT) and the French Society of Anaesthesia and Intensive Care Medicine (Société Française d'Anesthésie et Réanimation-SFAR) 1. Remember that sensitivity testing is performed under laboratory conditions, so clinical response may sometimes differ from what the report predicts, and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) specifies the antibiotic–bacteria pairs for which the MIC should be reported 1.
From the FDA Drug Label
The MIC values should be interpreted according to the following criteria: For testing aerobic microorganisms other than Haemophilus influenzae , Haemophilus parainfluenzae, and Neisseria gonorrhoeaea: MIC (μg/mL)Interpretation ≤ 1 Susceptible (S) 2 Intermediate (I) ≥ 4 Resistant (R)
To determine susceptibility from a urinalysis (UA) report, lower minimum inhibitory concentration (MIC) values indicate susceptibility.
- A MIC value of ≤ 1 μg/mL is considered Susceptible (S).
- A MIC value of 2 μg/mL is considered Intermediate (I).
- A MIC value of ≥ 4 μg/mL is considered Resistant (R) 2
From the Research
Interpreting Susceptibility Reports from Urinalysis
To determine susceptibility from a urinalysis (UA) report, it is essential to understand the minimum inhibitory concentration (MIC) values.
- The MIC is the lowest concentration of an antibiotic that can inhibit the growth of a microorganism.
- A lower MIC value indicates that the microorganism is more susceptible to the antibiotic, whereas a higher MIC value indicates resistance 3, 4.
- When interpreting a susceptibility report, the lower MIC value should be used to determine susceptibility, as it represents the most effective concentration of the antibiotic against the microorganism.
Factors Influencing MIC Values
Several factors can influence MIC values, including:
- The type of microorganism and its resistance patterns 5, 6
- The specific antibiotic being tested 7
- The testing method used, such as disk diffusion or Etests 3, 4
Clinical Significance of MIC Values
Understanding MIC values is crucial in guiding antimicrobial treatment and reducing the use of broad-spectrum antibiotics.
- MIC values can help clinicians choose the most effective antibiotic for a particular infection, taking into account the microorganism's resistance patterns and the antibiotic's efficacy 3, 7
- By using MIC values to guide treatment, clinicians can help reduce the development of antimicrobial resistance and improve patient outcomes 5, 6