Qualitative susceptibility tests versus quantitative MIC tests.

Qualitative susceptibility categories show reasonable, but incomplete, correlation with therapeutic outcome. Studies using quantitative MIC tests have demonstrated that treatment failures within the susceptible category are associated with higher minimum inhibitory concentrations (MICs) than with therapeutic successes. Other trials have exhibited enhanced response for increasing ratios of a pharmacokinetic parameter to MIC (for example, peak level to MIC ratio for aminoglycosides). Dose-response studies in animal infection models also demonstrate an excellent correlation between the dose of drug required for a given response and the infecting organism MIC. These studies suggest that the use of quantitative MIC tests may enable more individualization of the therapeutic regimen, especially in regards to dose and dosing frequency, than provided by qualitative category susceptibility tests. However, there are only rare studies that have used MIC results or pharmacokinetic parameters to improve efficacy. Furthermore, these studies have not consistently documented enhanced clinical efficacy. MICs can also be used to reduce drug dosage and cost of antimicrobial therapy for very susceptible organisms. Additional studies are clearly needed to define the full potential of the quantitative MIC test result.